CHANGELOG.md

# Change Log

## [23.05] - 2023-04-18

Support for Python 2 has been removed in this release.

### Build

- Changes and fixes for build_usd.py:
  - MaterialX support is now enabled by default.
  - Updated to use libpng version v1.6.38.
    (PR: [#1946](https://github.com/PixarAnimationStudios/USD/pull/1946))
  - Fixed error when using Visual Studio 2019 or later with 
    `--generator Ninja`. 
    (PR: [#2192](https://github.com/PixarAnimationStudios/USD/pull/2192))
  - Cleanup unused code. 
    (PR: [#2267](https://github.com/PixarAnimationStudios/USD/pull/2267))
  - NASM is no longer a required dependency when enabling OpenImageIO support 
    on Windows.
  - Updated to use TurboJPEG 2.0.1 on all platforms for OpenImageIO.

- Various CMake fixes
  (PR: [#2275](https://github.com/PixarAnimationStudios/USD/pull/2275)
   [#2030](https://github.com/PixarAnimationStudios/USD/pull/2030), 
   [#2179](https://github.com/PixarAnimationStudios/USD/pull/2179))

- Bumped CMake minimum to 3.14 on linux to support vfx2020 dependencies. Other 
  platforms require 3.20 or greater.
  (Issue: [#1620](https://github.com/PixarAnimationStudios/USD/issues/1620))

- Updated CMake to use PROJECT_SOURCE_DIR and PROJECT_BINARY_DIR instead of 
  CMAKE_SOURCE_DIR and CMAKE_BINARY_DIR respectively, in order to allow building 
  USD as a subdirectory.
  (PR: [#1971](https://github.com/PixarAnimationStudios/USD/pull/1971))

- Removed uic as alternative to pysideX-uic from FindPySide.cmake as uic will 
  produce a py file containing C++ code.

- Added ability to build docstrings for the USD Python modules. This can be 
  enabled by specifying the `PXR_BUILD_DOCUMENTATION`, 
  `PXR_ENABLE_PYTHON_SUPPORT`, and `PXR_BUILD_PYTHON_DOCUMENTATION` options 
  when running CMake, or `--docs`, `--python`, and `--python-docs` when running 
  build_usd.py. See BUILDING.md for more details.
  (Issue: [#196](https://github.com/PixarAnimationStudios/USD/issues/196))

- Fixed issue when PXR_PY_UNDEFINED_DYNAMIC_LOOKUP is explicitly set when
  packaging wheels, or when cross compiling to a Python environment that is not 
  the current interpreter environment. The value will now be set automatically 
  during cmake configuration.
  (Issue: [#1620](https://github.com/PixarAnimationStudios/USD/issues/1620))

- Various fixes for build issues, including missing headers on different 
  platforms and compilers. 
  (PR: [#2194](https://github.com/PixarAnimationStudios/USD/pull/2194), 
   [#2215](https://github.com/PixarAnimationStudios/USD/pull/2215))

- Various fixes for compiler warnings from clang. This has reduced the warning 
  count on MacOS builds from several hundred to roughly 60. 
  (PR: [#1684](https://github.com/PixarAnimationStudios/USD/pull/1684), 
   [#1997](https://github.com/PixarAnimationStudios/USD/pull/1997))

- Removed PXR_USE_PYTHON_3 CMake option.

### USD

- Fixed a bug where undefined behavior could be invoked when reading a corrupted 
  .usdz file.

- Fixed a bug where UsdStage::LoadAndUnload() could over invalidate and
  recompose.
  (Issue: [#2286](https://github.com/PixarAnimationStudios/USD/issues/2286))

- Fixed deadlock when opening and closing layers from multiple threads in 
  Python. 
  (Issue: [#2306](https://github.com/PixarAnimationStudios/USD/issues/2306))

- Fixed deadlock in TfType::GetAllDerivedTypes(), which could appear when 
  opening and closing layers from multiple threads. 
  (Issue: [#2282](https://github.com/PixarAnimationStudios/USD/issues/2282))

- VtArray now uses operator new (including global replacements) to allocate 
  memory.

- Fixed a bug where VtArray could read from freed memory when inserting copies 
  of elements from within the container.

- Fixed various safety issues for reading from corrupted .usdc files.

- The composition engine now supports internal references expressed by a 
  variant. Internal references also now retain visibility to namespaces outside 
  the referenced scope.

- The composition engine now prohibits scene paths with embedded variant 
  selection components (such as </A{x=y}B>) when used for composition arc 
  targets. This is enforced both at the Sdf schema level and composition 
  subsystem level. Schema enforcement means that invalid paths cannot be 
  authored via runtime API, nor will they be accepted by the text parser. As a 
  deployment aid, the environment setting 
  SDF_SCHEMA_PROHIBIT_INVALID_VARIANT_SELECTIONS may be set to '0' to continue 
  to allow these paths at the schema & parser level. Regardless of the setting, 
  however, the composition system will always prohibit such paths where invalid 
  for composition purposes.

- UsdCollectionAPI now handles the combination of includeRoot=true and
  expansionRule=explicitOnly without throwing a coding error.  
  (Issue: [#2258](https://github.com/PixarAnimationStudios/USD/issues/2258)

- Updated RapidJSON used by pxr/base/js to the latest available version as of 
  January 2023. 
  (Issue: [#2219](https://github.com/PixarAnimationStudios/USD/issues/2219))

- Completed initial support for Schema Versioning
  - usdGenSchema now allows API schemas with version suffixes to be generated.
  - usdGenSchema does not allow a schema to inherit from another version of a 
    schema in the same family.
  - usdGenSchema generates schema identifier and family tokens in the library 
    tokens class.
  - UsdPrimDefinitons now enforce the rule that more than one version of the 
    same API schema family cannot be applied at the same time.
  - If more than one version of the same API schema family is auto-applied to a 
    schema only the latest version of the API schema is applied.
  - Added a templated static overload of FindSchemaInfo to UsdSchemaRegistry.

- Improved UsdStage update performance when applying or removing an API schema 
  on prim.
  (Issue: [#2343](https://github.com/PixarAnimationStudios/USD/issues/2343))

- Added "collection:<name>" to the CollectionAPI schema as a real, opaque 
  valued, attribute for allowing collections to be targeted by a relationship.

- usdGenSchema now allows multiple-apply schemas to declare a property with no 
  base name via the special property name "__INSTANCE_NAME__".

- usdGenSchema generates literal token identifiers (instead of camel cased 
  identifiers) by default. Some usdLux and usdVol token identifiers have been 
  updated.

- Added SdfFileFormat::FindAllDerivedFileFormatExtensions to determine all 
  extensions supported by a subset of file formats without loading the plugins 
  for those formats. 

- Updated usdRi schema to include RiRenderPassAPI, which provides 
  cameraVisibility and matte collection attributes to provide collections of 
  objects relevant to the renderer for respective concepts. Note by default 
  cameraVisibility collection includes all objects, and matte collection 
  excludes all objects.

- Updated RenderPass schema to include a renderVisibility collection property, 
  which provides a collection of objects relevant to the renderer. Note by 
  default all objects are included in the collection.

- Fixed race condition in ArFilesystemWritableAsset that could cause writing 
  to the same directory from multiple threads to fail if that directory had 
  not yet been created.

- Added support for array vstruct sdrShaderPropery types.

- Resolved UDIM tiles when calculating asset dependencies.
  (PR: [#2133](https://github.com/PixarAnimationStudios/USD/pull/2133))

- File format plugins can now specify read / write / edit capabilities in their 
  plugInfo.json file.
  (Issue: [#2147](https://github.com/PixarAnimationStudios/USD/issues/2147))

- Added pointWeights attribute to NurbsCurves in usdGeom.
  (Issue: [#1554](https://github.com/PixarAnimationStudios/USD/issues/1554))

- Introduced GetAppliedItems for SdfListOp API. Deprecates 
  GetAddedOrExplicitItems.

- SdfSpec::ClearInfo now errors for unknown metadata fields.

- UsdRenderComputeSpec() now skips schema properties when aggregating authored 
  settings.

- Replaced uses of boost::hash with TfHash throughout the base and usd packages 
  of pxr. 
  (PR: [#2173](https://github.com/PixarAnimationStudios/USD/pull/2173),
   [#2176](https://github.com/PixarAnimationStudios/USD/pull/2176), 
   [#2181](https://github.com/PixarAnimationStudios/USD/pull/2181), 
   [#2182](https://github.com/PixarAnimationStudios/USD/pull/2182), 
   [#2184](https://github.com/PixarAnimationStudios/USD/pull/2184), 
   [#2185](https://github.com/PixarAnimationStudios/USD/pull/2185), 
   [#2186](https://github.com/PixarAnimationStudios/USD/pull/2186), 
   [#2187](https://github.com/PixarAnimationStudios/USD/pull/2187), 
   [#2190](https://github.com/PixarAnimationStudios/USD/pull/2190), 
   [#2191](https://github.com/PixarAnimationStudios/USD/pull/2191), 
   [#2289](https://github.com/PixarAnimationStudios/USD/pull/2289), 
   [#2298](https://github.com/PixarAnimationStudios/USD/pull/2298), 
   [#2304](https://github.com/PixarAnimationStudios/USD/pull/2304), 
   [#2329](https://github.com/PixarAnimationStudios/USD/pull/2329))

- Removed use of "register" keyword, which is no longer supported as of C++17. 
  (PR: [#2204](https://github.com/PixarAnimationStudios/USD/pull/2204))

- Various changes to remove uses of boost. 
  (Issue: [#2203](https://github.com/PixarAnimationStudios/USD/issues/2203), 
   [#2211](https://github.com/PixarAnimationStudios/USD/issues/2211),
   [#2217](https://github.com/PixarAnimationStudios/USD/issues/2217),
   [#2221](https://github.com/PixarAnimationStudios/USD/issues/2221), 
   [#2247](https://github.com/PixarAnimationStudios/USD/issues/2247), 
   [#2290](https://github.com/PixarAnimationStudios/USD/issues/2290) 
   PR: 
   [#2210](https://github.com/PixarAnimationStudios/USD/pull/2210), 
   [#2213](https://github.com/PixarAnimationStudios/USD/pull/2213),
   [#2213](https://github.com/PixarAnimationStudios/USD/pull/2224), 
   [#2249](https://github.com/PixarAnimationStudios/USD/pull/2249), 
   [#2291](https://github.com/PixarAnimationStudios/USD/pull/2291))

- Fixed a bug where UsdUtilsCreateNewUsdzPackage would not correctly handle 
  relative filesystem paths pointing to a directory outside the directory 
  containing the root layer.

- Fixed a bug where using a Usd.PrimRange from an expired Usd.Stage in Python 
  would crash or throw an unexpected exception. 
  (Issue: [#2160](https://github.com/PixarAnimationStudios/USD/issues/2160))

- Updated USD compliance checker to error instead of warn when the following 
  issues are found:
  - Shader prims are not encapsulated within a container type prim.
  - For 8bit normal maps, if sourceColorSpace is not "raw" and if bias and 
    scale values are not provided.

- Updated deprecation logic for USD_SHADE_COORD_SYS_IS_MULTI_APPLY, such that
  warnings are reported when the environment variable is set to Warn and usdPrim 
  is using any old style coordSys property. 

- Updated testUsdBugs to explicitly use integer division, so as to avoid passing 
  a float to truncate method. 
  (Issue: [#2170](https://github.com/PixarAnimationStudios/USD/issues/2170), 
   PR: [#2171](https://github.com/PixarAnimationStudios/USD/pull/2171))

- Updated UsdProperty.GetPropertyStack and 
  UsdProperty.GetPropertyStackWithLayerOffsets python APIs to take a 
  `UsdTimeCode` as an optional argument, to match their respective C++
  APIs. 
  (PR: [#2317](https://github.com/PixarAnimationStudios/USD/pull/2317))

- Added compliance tests to make sure UsdSkelBindingAPI is applied on prims 
  having properties provided by UsdSkelBindingAPI. Also added a compliance test 
  to ensure that UsdSkelBindingAPI has only been applied on SkelRoot prims or 
  descendants of SkelRoot prims.

- Updated usdRiPxr codeless schemas with updated concepts from Renderman 24.5.

- Fixed ZipFileWriter context manager to discard any archived files if the root 
  USD file is non-compliant. This fixes usdzip and usdfixbrokenpixarschemas 
  usage with non-compliant usdz archives.

- Fixed various gf tests to resolve floating point precision issues on ARM 
  (aarch64) platforms.
  (PR: [#2115](https://github.com/PixarAnimationStudios/USD/pull/2115))

- Updated UsdShadeShaderDefUtils::GetPrimvarNamesMetadataString to only append 
  existing non-empty primvar strings into the list in order to avoid appending 
  of empty strings on aarch64 platforms.
  (PR: [#2131](https://github.com/PixarAnimationStudios/USD/pull/2131))

### UsdImaging

- Added UsdImaging::SetRootInstancerId as a method to set new member 
  `_rootInstancerId`, which is now used as the fallback case for 
  UsdImagingDelegate::GetInstancerId.

- Fixed crash in UsdImagingAdapterRegistry by comparing with the end iterator 
  of the same unordered_map.
  (PR: [#2197](https://github.com/PixarAnimationStudios/USD/pull/2197))

- Removed material prims under a point instancer when resyncing.
  (Issue: [#1960](https://github.com/PixarAnimationStudios/USD/issues/1960))

- Fixed shader compilation errors with usdImagingGL volume tests on macOS.

- Fixed errors and inconsistencies with usdImagingGL instancing tests related 
  to Ptex textures and handling of "--cullStyle" and "--shading" test command 
  line arguments.
  (Issue: [#2097](https://github.com/PixarAnimationStudios/USD/issues/2097))

- Updated PreviewSurface.glslfx to mirror changes made to simpleLighting.glslfx 
  to make sure the intensity and exposure on dome lights are properly affecting 
  PreviewSurface materials.
  
- Added GetRenderSettingsNamespaces to HdRenderDelegate to get namespaces for 
  render settings attributes that are render-specific.

- Updated HdRenderSettingsSchema and UsdImagingRenderSettingsAdapter.

- Add Scene Index support functions to the UsdImagingRenderSettingsAdapter.

- Added UsdImagingUsdSchema which combines data sources internal to UsdImaging.

- Fixed a cyclic reference (in UsdImaging_NiInstanceAggregationSceneIndex_Impl) 
  where a data source in a HdRetainedSceneIndex 
  (_InstanceObserver::_retainedSceneIndex) held on to a ref ptr to a data 
  structure (namely, _InstanceObserver) containing a ref ptr to the scene index. 
  This was causing a memory leak and a crash in _InstanceObserver::PrimsRemoved.

- Made various small fixes and clean-ups to the filtering scene indices required 
  for USD native and point instancing support.

- UsdImagingPiPrototypesPropagatingSceneIndex now lists the paths of the 
  propagated prototypes for a USD prototype. This is intended for clients that 
  need to translate a USD path to a Hydra scene path, e.g., to support 
  selection highlighting.

- UsdImagingNiPrototypesPropagatingSceneIndex populates the HdInstanceSchema 
  for native instances.

- Enabled selection highlighting of USD native instances when using scene 
  indices. The UsdImagingSelectionSceneIndex processes the path to a native USD 
  instance by reading its HdInstanceSchema and populating the HdSelectionSchema.

- Fixed purpose handling in the UsdSkelImagingAdapter.

- Added adapter support for transporting render settings scene description 
  (which includes render products and render vars, but not render passes).

- Fixed an issue where having multiple prim/API adapters could cause double 
  population in UsdImagingStageSceneIndex.

- Added support for "keyless" API schema adapters which have an opportunity to 
  contribute to any prim.

- GetInstanceCategories now works under nested instancing, returning categories 
  for all instances in the flattened hierarchy. 
  (Issue: [#2002](https://github.com/PixarAnimationStudios/USD/issues/2002))

- Fixed crash due to out of bounds access in 
  UsdImagingInstanceAdapter::GetScenePrimPaths.
  (Issue: [#2268](https://github.com/PixarAnimationStudios/USD/issues/2149), 
   PR: [#2278](https://github.com/PixarAnimationStudios/USD/pull/2167))

- Updated UsdImagingCoordSysAPIAdapter to provide access to coordSys hydra prim 
  (HdCoordSysSchema), which provides information about coordSys name and xform 
  values. Also added dependency of this coordSys prim to the bound target xform.

- Removed Populate() from the UsdImagingStageSceneIndex. "Population" is now 
  done automatically when the stage is set.

### Storm

- Fixed use-after-free bug in basis-curves helper function in hdSt.

- Implemented support for the remaining refined drawing modes for basisCurves 
  and meshes using HW tessellation with HgiMetal.

- Renamed HdBinding to HdStBinding and moved to hdSt since this is used only by 
  Storm.
  
- Improved performance when freeing argument buffers after command buffer 
  submission with HgiMetal.
  (PR: [#2046](https://github.com/PixarAnimationStudios/USD/pull/2046))

- Fixed crash when loading non-filterable textures with HgiMetal.
  (PR: [#2048](https://github.com/PixarAnimationStudios/USD/pull/2048))

- Changed the name of an internal debug class to avoid problems with Cocoa 
  View classes.
  (PR: [#2256](https://github.com/PixarAnimationStudios/USD/pull/2256))

- Added HorizontallyNormalizedFilmbackCoordinates to renderPass.glslfx for 
  material shaders needing to draw in image space.

- Fixed picking of semi-transparent fragments by using a small alpha threshold 
  value when discarding fragments. 

- Fixed a bug to avoid an infinite loop and issue a coding error when passing 
  relative paths to HdSystemSchema::Compose.

- Improved the performance of cards stand-ins in Storm. Edges of cutouts may 
  show slightly increased artifacts as a result.

### Hydra

- Fixed a bug in scene index emulation where removing an Rprim or Bprim 
  from HdRenderIndex would remove all of that prim's children as well.

- Moved UsdImaging_SceneIndexPrimView, an iterator to traverse all ancestors of 
  a prim in a scene index, to HdSceneIndexPrimView.

- HdInstanceSchema now includes the index of a native instance within the 
  instancer instancing the prototype.

- Added special case for sprim dirty bits translation for emulated mesh lights.

- Added fix that ensures that HdSceneIndexAdapterSceneDelegate uses unique names 
  with HdSceneIndexNameRegistry.

- Added optional argument needsPrefixing to HdRenderIndex::InsertSceneIndex 
  to indicate whether input scene index needs prefixing.

- Added missing .tiff extension to hioOiio
  (PR: [#2307](https://github.com/PixarAnimationStudios/USD/pull/2307))

- Fixed double transforms in Hydra when using the new scene index API.

- Added support for transporting render settings scene description in Hydra.

- Added scene globals schema to encapsulate global state necessary to 
  orchestrate a render.

- Fixed computed view transform for the shadow matrix.

- Made HdxTask::_hgi member private. Subclasses should use the protected 
  _GetHgi() method instead.

- Exported HdxTask::Sync to allow for custom render task creation and use. 
  (PR: [#2178](https://github.com/PixarAnimationStudios/USD/pull/2178))

- Added HdCoordSysSchema which provides "name" member field and its builder 
  interface.  

- Fixed a memory leak during framebuffer resize.
  (PR: [#2342](https://github.com/PixarAnimationStudios/USD/pull/2342))

### Renderman Hydra Plugin

- The RenderMan Hydra delegate now transfers attributes from material to 
  geometry that binds the material. This is useful in cases such as displacement
  bounds and subsurface trace groups.

- Removed a deprecated staging flag, HD_PRMAN_ENABLE_LIGHT_MATERIAL_NETWORKS

- Fixed hdPrman slowdown when rendering many AOVs.

- Fixed an issue where normal maps going through the RtxHioImage plug-in were 
  being linearized. HdPrman now provides a "sourceColorSpace=raw" argument when 
  the rtxplugin is being invoked for a mtlx image node that has type 
  "ND_image_vector[2,3,4]"; the type comes about based on the mtlx image node's 
  "signature" param. This also affects the MaterialX Chess Set scene.

- Fixed an issue with MaterialX texcoord nodes referring to texture coordinates 
  as "uv" instead of "st". Now, as the MaterialX network is translated we swap 
  out texcoord nodes for geompropvalue nodes that look up "st".

- Fixed a crash in RtxHioImage plugin used by hdPrman, as a result of passing 
  nullptr for RixContext. Explicit nullptr guards are now added for the same.

- Added improvements for rendering with Solaris's husk, including support for 
  RenderMan display drivers and command line arguments.

- Reworked and reorganized the testHdPrman test harness to use the 
  RenderSettings Prim by default, instead of the UsdRenderSpec.

- Updated riley geometry instances for rprims owned by hydra instancers 
  regardless of local dirty bit state to ensure that instance primvars are 
  updated.

- Made hdPrman invalidation more granular for improved performance  

- HdPrman now uses Renderman nested instancing for nested point instancer 
  hierarchies, which can provide significant performance and memory footprint 
  improvements in scenes with heavy nested instancing. This feature is enabled 
  by default, and can be disabled by setting 
  HD_PRMAN_DISABLE_NESTED_INSTANCING=1. Users should be aware that picking and 
  selection of individual instances may not work as expected when this feature 
  is enabled and HdPrman is in use. Users who need to use instance picking with 
  HdPrman should disable this feature.

### usdrecord

- Fixed crash on startup when using PySide6.

### usdview

- Fixed python exceptions in usdview when selecting relationship targets whose 
  path is either a property path or is a path to any object that doesn't exist 
  on the stage.

- Added a display menu item to toggle display of dome light background texture 
  images when dome lights are enabled.
  (PR: [#2123](https://github.com/PixarAnimationStudios/USD/pull/2123))

- Fixed an error from the incorrect use of hasattr() when using openColorIO 
  color management.
  (Issue: [#2254](https://github.com/PixarAnimationStudios/USD/issues/2254))

- Fixed usdview to hide viewport frame when --norender is set

- Updated usdview Hydra Scene Browser to be compatible with more PySide 
  versions and macOS

### MaterialX

- Fixed MaterialX materials with the name 'material' 
  (Issue: [#2076](https://github.com/PixarAnimationStudios/USD/issues/2076))

- Fixed issue for MaterialX namespaces when adding MaterialX nodes to a node 
  graph.

- Fixed MaterialX split string array parsing to split only on "," delimiter and 
  not "," and spaces.  
  (Issue: [#2268](https://github.com/PixarAnimationStudios/USD/issues/2268), 
   PR: [#2278](https://github.com/PixarAnimationStudios/USD/pull/2278))

- Updated usdMtlx parser to parse various MaterialX metadata into sdr 
  properties.
  (Issue: [#1874](https://github.com/PixarAnimationStudios/USD/issues/1874),
   PR: [#1895](https://github.com/PixarAnimationStudios/USD/pull/1895))

### Documentation

- Improved Python help() documentation for USD APIs when Python docstrings are
  built, as described in Build section above.

- Various documentation updates, including:
  - Improved TfError documentation
  - Renamed image source filenames in tutorial docs
  - Added thread-safety guidance docs for ArResolver::RefreshContext()
  - Updated contributing guidelines
  - Updated docs on GeomPointInstancer xform resolution
  - Fixed incorrect description of SkelAnimation binding example, cleanups to 
    UsdSkel documentation and examples
  - Updated UsdLuxShadowAPI properties docs
  - Added additional asset resources to "Downloads and Videos" page
  - Fixed the path to the basisCurves example asset in the documentation.

- Updated Windows build instructions example in README.md to build into 
  non-protected directory.
  (PR: [#1994](https://github.com/PixarAnimationStudios/USD/pull/1994))

- Updated "Environment Setup" section of Tutorials page to specify adding 
  USD_INSTALL_ROOT/lib to the PATH environment variable on Windows when 
  running the tutorials. 
  (PR: [#2198](https://github.com/PixarAnimationStudios/USD/pull/2198))

- Added general guidance on FAQ page on where to install USD DLLs and plugins 
  on Windows.
  (PR: [#1788](https://github.com/PixarAnimationStudios/USD/pull/1788))

- Updated UsdUVTexture and UsdPreview docs to clarify usages of scale, bias and 
  sourceColorSpace when using an 8bit Normal Map, in order to satisfy tangent 
  space requirement of the normals.

- Updated USD toolset documentation to include usdfixbrokenpixarschemas.

## [23.02] - 2023-01-24

Version 23.02 is the last release that supports Python 2. Support for Python 2 
will be removed in the next release.

### Build

- The usd-core PyPI package now supports Apple Silicon.
  (Issue: [#1718](https://github.com/PixarAnimationStudios/USD/issues/1718))

- Added support for Python 3.10.
  (PR: [#2007](https://github.com/PixarAnimationStudios/USD/pull/2007))

- Added support for Visual Studio 2022, and removed 2015. The supported versions
  are now 2017, 2019, and 2022.
  (PR: [#2007](https://github.com/PixarAnimationStudios/USD/pull/2007))

- The minimum supported Boost version is now 1.70.
  (PR: [#2007](https://github.com/PixarAnimationStudios/USD/pull/2007), 
   [#2150](https://github.com/PixarAnimationStudios/USD/pull/2150))

- Updated CMake build system to use modern FindPython modules and import
  targets. This fixes an issue on macOS where the correct Python framework
  directory would not be added as an RPATH. Clients that were specifying CMake
  arguments like PYTHON_EXECUTABLE or PYTHON_LIBRARY or PYTHON_INCLUDE_DIR to 
  control the behavior of the deprecated find modules will need to use the 
  corresponding arguments for the new modules. 
  (Issue: [#2082](https://github.com/PixarAnimationStudios/USD/issues/2082), 
   PR: [#2091](https://github.com/PixarAnimationStudios/USD/pull/2091))

- Fixed macOS code signing issue when using the Xcode CMake generator that would
  cause builds to be unusable on Apple Silicon.

- Various CMake build system cleanup. 
  (PR: [#1958](https://github.com/PixarAnimationStudios/USD/pull/1958), 
   [#2152](https://github.com/PixarAnimationStudios/USD/pull/2152))

- Fixed threading issue causing assertions in Windows debug builds. 
  (PR: [#2127](https://github.com/PixarAnimationStudios/USD/pull/2127))

- Fixed compiler warning when using GfVec3f::GetLength with -Wconversion 
  enabled. (PR: [#2060](https://github.com/PixarAnimationStudios/USD/pull/2060))

- Removed dependency on boost::program_options.
  (PR: [#2114](https://github.com/PixarAnimationStudios/USD/pull/2114))

- Fixed issue where C++ executables like sdfdump would fail to run on Linux and 
  macOS in certain configurations due to a missing RPATH entry for the USD core
  libraries.

### USD

- Numerous performance and memory optimizations.

- Updated UsdSkel plugin bounds computation to include nested skeletons in the 
  case where there are no bindings.

- UsdGeomModelAPI::ComputeExtentsHint now uses 
  UsdGeomBoundable::ComputeExtentFromPlugins for boundable models.

- Added malloc tag reporting to sdfdump when run with the TF_MALLOC_TAG 
  environment variable set.

- Fixed bug where handles to .usdc files were not being closed after the 
  associated SdfLayer object was destroyed. 
  (Issue: [#1766](https://github.com/PixarAnimationStudios/USD/issues/1766))

- SdfLayer is now case-insensitive for characters in 'A'-'Z' when looking up 
  layer file format from file extension. 
  (Issue: [#1833](https://github.com/PixarAnimationStudios/USD/issues/1833))

- Improved TfMallocTag performance for parallel applications. For example, time 
  to open one test asset on a UsdStage with tagging enabled decreased from ~40 
  seconds to ~5 seconds.

- Updated doc discouraging use of ArchGetFileName. 
  (Issue: [#1704](https://github.com/PixarAnimationStudios/USD/issues/1704))

- UsdPrim::ComputeExpandedPrimIndex and UsdPrimCompositionQuery can be used with 
  prototype prims.

- Fixed a composition error involving specializes and subroot references.

- Initial Schema Versioning support. Note that not all of the schema versioning
  work is complete. In particular, it is not yet possible to generate versioned
  API schemas through usdGenSchema even though the UsdPrim API for querying and
  authoring them has been added. Full schema versioning support is expected in
  a following release.
  - Added SchemaInfo structure to UsdSchemaRegistry for each schema type that 
    can be retrieved via the FindSchemaInfo method.
  - Added functions to parse a schema's family and version from its identifier;
    the family and version are included in each schema's SchemaInfo
  - Added overloads the existing UsdPrim schema type query methods and API 
    schema authoring methods that can take a schema identifier or a schema 
    family and version.
  - Added schema family and version based queries, IsInFamily and 
    HasAPIInFamily, to UsdPrim.

- Added missing displayGroup and displayName hints for UsdLux properties that 
  have display hints specified in the PRMan args files. Fixed documentation
  for CylinderLight's inputs:length property.

- Added "opaque" and "group" attribute types. Opaque attributes have no value
  but can be connected to other attributes or targeted by relationships. Group
  attributes are opaque attributes that are intended to provide a 
  connectable/targetable object that represents a property namespace.

- Minor clean ups to the UsdCollectionMembershipQuery.

- Support for URL-encoded characters in session layer name. 
  (PR: [#2022](https://github.com/PixarAnimationStudios/USD/pull/2022))

- Fixed varying primvar size computation in UsdGeomBasisCurves for pinned cubic
  curves.

- Made `displayName` metadata available on UsdPrim as well as UsdProperty, 
  and updated `usdview` to optionally use `displayName` in the Prim Browser, 
  when present.  Display Names can use the entire UTF-8 glyph set. 
  (PR: [#2055](https://github.com/PixarAnimationStudios/USD/pull/2055)) 

- Added UsdMediaAssetPreviewsAPI schema, an applied API for data in the 
  `assetInfo["previews"]` dictionary on prims.  Initially, this allows robust 
  encoding of (asset) default and per-prim _thumbnail_ previews, but may be 
  expanded to other forms of preview in the future.

- Modified sdffilter, sdfdump, and various tests to use CLI11 instead of 
  boost::program_options.
  (PR: [#2107](https://github.com/PixarAnimationStudios/USD/pull/2107),
   [#2109](https://github.com/PixarAnimationStudios/USD/pull/2109),
   [#2110](https://github.com/PixarAnimationStudios/USD/pull/2110),
   [#2111](https://github.com/PixarAnimationStudios/USD/pull/2111),
   [#2113](https://github.com/PixarAnimationStudios/USD/pull/2113))

- Ported usdtree and usdcat utilities from Python to C++.
  (PR: [#2108](https://github.com/PixarAnimationStudios/USD/pull/2108),
  [#2090](https://github.com/PixarAnimationStudios/USD/pull/2090))

- Fixed various intermittent unit test failures. 
  (PR: [#2068](https://github.com/PixarAnimationStudios/USD/pull/2068))

- Fixed bug where adding an empty sublayer could cause errors and incorrect 
  results with value clips. 
  (Issue: [#2014](https://github.com/PixarAnimationStudios/USD/issues/2014))

- Added Python bindings for ArAssetInfo. 
  (Issue: [#2065](https://github.com/PixarAnimationStudios/USD/issues/2065))

- Fixed crashes related to the dynamic library loader on macOS 12. 
  (Issue: [#2102](https://github.com/PixarAnimationStudios/USD/issues/2102))

- Fixed issue where incorrect baseline image specifications for unit tests 
  would be silently ignored. 
  (PR: [#1856](https://github.com/PixarAnimationStudios/USD/pull/1856))

- Fixed UsdUtilsModifyAssetPaths to remove duplicate entries if the 
  modification callback returns such entries, and to remove the original entry 
  if the callback returns an empty asset path. 
  (PR: [#1282](https://github.com/PixarAnimationStudios/USD/pull/1282))

- Fixed crash when creating VtArray from Python buffer with over 2^31 elements.
  (PR: [#2064](https://github.com/PixarAnimationStudios/USD/pull/2064))

- Updated usdchecker to accept references to textures with the ".jpeg" 
  extension. 
  (PR: [#1919](https://github.com/PixarAnimationStudios/USD/pull/1919))

- Changed UsdShadeCoordSysAPI from a non-applied API schema to multiple-apply 
  API schema. The previous non-applied APIs have been deprecated and will issue
  a warning when called. Setting the environment variable 
  USD_SHADE_COORD_SYS_IS_MULTI_APPLY to "False" will disable these warnings; 
  setting it to "True" will disable the deprecated functions.

- Fixed bug where using C++ keywords as token names in schema definitions would
  cause usdGenSchema to produce uncompilable C++ code. 
  (PR: [#2020](https://github.com/PixarAnimationStudios/USD/pull/2020))

- Removed deprecated UsdRiTextureAPI schema.

### Imaging

- HdRendererPlugin::IsSupported and HdRendererPluginRegistry::GetDefaultPluginId
  now take an argument indicating if the GPU is enabled to assist in determining
  what to return to the caller.

- Fixed bug where render settings were not being returned by 
  HdSceneIndexAdapterSceneDelegate.

- Added missing null pointer checks to HdxColorizeSelectionTask.
  (Issue: [#1991](https://github.com/PixarAnimationStudios/USD/issues/1991))

- Draw mode: cards will now use UsdPreviewSurface materials, expanding renderer
  compatibility. The draw mode adapter and scene index have accordingly been 
  relocated from UsdImagingGL to UsdImaging.

- Fixed a bug that prevented changes to ModelAPI properties from triggering 
  invalidation.

- Fixed a bug in Storm that prevented certain changes to array-valued primvars
  from being properly applied.

- Added a warning when a plugin that duplicates built-in prim-handling 
  functionality is detected. Note that overriding built-in prim-type handlers 
  via plugin is not officially supported. The prior behavior remains unchanged 
  (last-to-load wins).

- Updated HdCoordSys::GetName() to return coodSys name in accordance with 
  recently updated CoordSysAPI to be multi-applied API.  

### UsdImaging

- Updated the precedence order of `primvars:ri:attributes:*`, which will now 
  take precedence over `ri:attributes:*`.

- Added support for OCIO 2.1.1 with native Metal support. 
  (PR: [#1936](https://github.com/PixarAnimationStudios/USD/pull/1936))

- Updated UsdImagingGL tests to always render to AOVs and to remove most direct
  use of OpenGL. This improves consistency of test runs across different system 
  configurations and renderer implementations. Also, fixed minor discrepancies 
  in test command arguments and baseline compare metrics to more closely match 
  internal test suite runs.

- Enabled multisample for Storm AOV image outputs for improved consistency with
  usdview viewport display.

- Populated the "system" container with asset resolution data.

- Fixed UsdImagingBuildHdMaterialNetworkFromTerminal to support multiple input
  connections.

- Improved point instancing support of usdImaging when using scene indices. 
  This is implemented through a filtering scene index 
  UsdImagingPiPrototypePropagatingSceneIndex.

- Added native instancing support of usdImaging when using scene indices. This
  is implemented through a filtering scene index 
  UsdImagingNiPrototypePropagatingSceneIndex (doing the instance aggregation) 
  and some changes to the UsdImagingStageSceneIndex itself adding USD native 
  instances and prototypes with identifying data sources.

- Reworked usage of a flattened dependency cache to speed up dependency 
  gathering.

- Added methods for UsdImaging 2.0 prim adapters to represent descendants in 
  terms of population and invalidation. Implemented property change invalidation
  for material prims via this mechanism. This change also manages time-varying 
  invalidation by making sure that properties of USD shader prims know the hydra
  material path and locator.

- Moved UsdImaging 2.0 primvar handling from gprim to prim and implemented 
  primvar invalidation.

- Added vector types to dataSourceAttribute.

- Removed HdContainerDataSource::Has, as it is redundant with GetNames and Get,
  and doesn't in practice fulfill its original intent of being cheaper to query
  than Get.

- Added initial UsdImaging 2 adapter support for GenerativeProcedural.

- UsdImagingDataSourcePrimvars updated to lazily handle "primvars:" 
  relationships as VtArray<SdfPath> data sources.

- Added UsdImagingStageSceneIndex-specific HdDataSourceLocator convention for 
  triggering resync in response to property invalidation.

- Added UsdPrim const& argument to UsdImaging 2.0 adapter methods to enable 
  property-driven population and invalidation.

- Added UsdSkel Dual Quaternion skinning as both a GPU and CPU computation.

- Fixed a crash in UsdSkelBlendShapeQuery::ComputeFlattenedSubShapeWeights, and
  added asserts around array bounds violations.

- Fixed bug where changing an attribute connection would not update primvars in
  Hydra. 
  (Issue: [#2013](https://github.com/PixarAnimationStudios/USD/issues/2013), 
   PR: [#2017](https://github.com/PixarAnimationStudios/USD/pull/2017))

- Added CoordSysAPIAdapter to add coordsys support for multi-apply 
  UsdShadeCoordSysAPI instances in UsdImagingStageSceneIndex.

- Fixed UsdImagingStageSceneIndex functionality for subprims with non-empty 
  names, which were previously tripping an assert in SdfPath.

### Storm

- Added missing instancer handling for volumes.

- Renamed Hgi function MemoryBarrier to InsertMemoryBarrier to avoid potential 
  build issues on Windows.
  (PR: [#2124](https://github.com/PixarAnimationStudios/USD/pull/2124))

- Fixed bug in which dome light prefilter textures were not created for 1x1 
  input textures.

- Updated texture system to create samplers for udim and ptex layout textures.

- Changed handling of invalid or missing textures. When there is no valid 
  texture, bind a 1x1x1 black fallback texture. The value of TEXTURENAME_valid 
  is now always checked before returning the texture value in the shader. Ptex
  and udim textures now also bind a boolean TEXTURENAME_valid value.

- Converted mesh.glslfx geometry shader snippets to resource layouts, and 
  introduced HgiShaderFunctionGeometryDesc.

- Added bindIndex member to HgiShaderFunctionTextureDesc.

- Introduced shader variables "hd_VertexID", "hd_InstanceID", and 
  "hd_BaseInstance" to use instead of gl_VertexID, gl_InstanceID, and 
  gl_BaseInstance, respectively, with appropriate values determine by the Hgi 
  backend.

- Various fixes and improvements to HgiVulkan backend, including:
  - Fixed first vertex binding in vkCmdBindVertexBuffers.
  - Added check to avoid destroying null Hgi resources in HgiVulkan::TrashObject.
  - Fixed texture view creation to correctly specify first mip and first layer.
  - Updated VkPhysicalDevicesFeatures to include additional needed features.
  - Enabled Vulkan extension VK_EXT_vertex_attribute_divisor and fixed input 
    rate of per-draw command vertex attributes.
  - Improved shader generation, including proper tracking of location layout 
    qualifiers of shader stage ins and outs and in and out blocks, and binding 
    indices of buffers and textures.
  - Changed handling of binding indices in HgiVulkanResourceBindings to 
    correspond to those used in shadergen.
  - Added handling for shader interpolation qualifiers.

- Added fix to convert double mesh primvars to floats if doubles are not 
  supported by the Hgi backend. 
  (Issue: [#2070](https://github.com/PixarAnimationStudios/USD/issues/2070))

- ImageShaderRenderPass now sets a viewport prior to executing a draw. 

- Improved rendering of assets with multiple geom subsets by increasing the 
  size of the drawingCoord topology offset and instancePrimvar offset from 
  8-bits to 16-bits.
  (Issue: [#1749](https://github.com/PixarAnimationStudios/USD/issues/1749), 
   PR: [#2089](https://github.com/PixarAnimationStudios/USD/pull/2089))

- Fixed some potential memory leaks related to HgiMetal resources. 
  (PR: [#2085](https://github.com/PixarAnimationStudios/USD/pull/2085))

- Added StartFrame/EndFrame to HdxPickTask to improve Hgi resource cleanup. 
  (PR: [#2053](https://github.com/PixarAnimationStudios/USD/pull/2053))

- Implemented GPU view frustum culling using compute shaders for 
  HdSt_PipelineDrawBatch (Metal and Vulkan) including minor cleanups to 
  HdStCommandBuffer and HdSt_RenderPass. As part of this, added a way to access
  drawingCoord values from compute shaders. Also, added support for concurrent 
  dispatch of compute command encoders for Metal. 
  (PR: [#2053](https://github.com/PixarAnimationStudios/USD/pull/2053))

- Updated render pass shader sources to include support for post tessellation 
  control shader stages.

- Updated basis curves and mesh tessellation shader source to use resource 
  layouts for tessellation shaders. 
  (PR: [#2027](https://github.com/PixarAnimationStudios/USD/pull/2027))

- Fixed some cases of undefined Osd symbols for Storm when using Osd shader 
  source.

- Added fix for Dome Light not changing. 
  (PR: [#2125](https://github.com/PixarAnimationStudios/USD/pull/2125))

- Fixed basis curves index computation and validation. This changes the imaging
  behavior in that non-periodic curves with a vertex count of 2 or 3 are now 
  rendered instead of being ignored.

- Fixed cullstyle resolution for a corner case.

### Hydra

- Added a "system" schema.  The "system" container is meant to be inherited by 
  prims within a scene index.  Scene indices that prefix and re-root have been 
  updated to preserve the "system" container.

- Fixed small bug for how visibility/purpose is handled.

- HdInstancedBySchema: added prototype root which is populated by 
  UsdImagingPiPrototypePropagatingSceneIndex and consumed by 
  UsdImagingNiPrototypePropagatingSceneIndex to support interleaved point and 
  native instancing.

- Removed HdInstanceBySceneIndex in favor of new instancing implementation in 
  UsdImagingPiPrototypePropagatingSceneIndex and 
  UsdImagingNiPrototypePropagatingSceneIndex.

- HdMergingSceneIndex and HdFlatteningSceneIndex: correct initialization when 
  adding an already populated scene index or constructing from an already 
  populated scene index.

- Made data flattened by the HdFlatteningSceneIndex configurable.

- Added initial support of picking for scene indices through HdPrimOriginSchema
  and HdxPrimOriginInfo.

- Added initial selection support for scene indices through HdSelectionsSchema 
  and HdSelectionSchema which are picked up by the HdxSelectionTracker.

- Introduced HdTypedVectorSchema and HdSchemaBasedVectorSchema.

- Added HdRenderSettings, a Hydra Bprim backing scene description render 
  settings.

- Added scene index that presents computed primvars as authored ones.

- Fixed bug in `HdExtComputationUtils::_GenerateDependencyMap()` with multiple 
  input computations. 
  (PR: [#2058](https://github.com/PixarAnimationStudios/USD/pull/2058))

- Implemented optional scene index for transfer of primvars from material to 
  bound prims.

- Added protection against GIL deadlocks within 
  generativeProceduralResolvingSceneIndex.

- Added support for translation callbacks for custom sprim types within 
  HdDirtyBitsTranslator.

- For mesh lights, nack-end scene index emulation now checks 
  GetLightParamValue(id, isLight) when considering whether a prim whose type is
  not a light should be treated as one.

- Fixed compile and logic bugs in 
  HdRetainedTypedMultisampledDataSource<T>::GetTypedValue.

- Added source example for a C++ Qt version of Hydra Scene Browser. This is the
  recommended reference and starting point for integration into applications 
  (aside from usdview).

### Renderman Hydra Plugin

- Added fix to treat invisible faces in the mesh topology as holes.

- Added scene index plugin to present computed primvars as authored ones.

- Added hdPrman support for UsdLuxMeshLightAPI, via the 
  meshLightResolvingSceneIndexPlugin.

- Added support for Renderman Display Filters.

- Removed deprecated MatfiltFilterChain, and associated
  envvar HD_PRMAN_USE_SCENE_INDEX_FOR_MATFILT.

### usdview

- Fixed usdview exception when right-clicking for a context menu in the 
  Composition tab.

- Included Hydra Scene Browser within usdview.

- Added a function to usdviewApi to get the current renderer.

- Changed the PluginMenu addItem() function to return the added action, so 
  further customizations (such as making it checkable) can be applied to it.

### usdAppUtils

- UsdAppUtilsFrameRecorder now sleeps between render invocations if the frame 
  has not converged to avoid potentially slowing down the render progress.

### usdRecord

- usdrecord can now be run without requiring a GPU by passing the "--disableGpu"
  option on the command line 
  (Issue: [#1926](https://github.com/PixarAnimationStudios/USD/issues/1926)). 
  Note the following:
  - The GPU is enabled by default.
  - If no renderer is specified, an appropriate default will be chosen 
    depending on whether the GPU is enabled or not.
  - Disabling the GPU will prevent the HdxTaskController from creating any 
    tasks that require it.  In particular, this means that color correction is 
    disabled when the GPU is disabled. 

### MaterialX Plugin

- Fixed viewport calculation issue with full masking on both Mac and Vulkan
  MaterialX Plugin.

- Updated MaterialX Imaging Tests, adding more comparisons between the Native 
  and MaterialX implementations of UsdPreviewSurface materials. Note that these
  PreviewSurface test cases are commented out by default. 

### Alembic Plugin

- HDF5 support for the Alembic plugin is now disabled by default.

### Documentation

- Various documentation fixes and cleanup. 
  (PR: [#1995](https://github.com/PixarAnimationStudios/USD/pull/1995))

- Updated sphinx documentation pages to use system fonts. 
  (PR: [#2084](https://github.com/PixarAnimationStudios/USD/pull/2084))

- Added new documentation page showing products using USD. 
  (PR: [#2095](https://github.com/PixarAnimationStudios/USD/pull/2095))

## [22.11] - 2022-10-21

### Build
- Updated Azure Pipelines configurations used for CI and PyPI
  The active configurations are now:
    - Ubuntu 20.04, Python 3.8.10, PySide2 5.15.2.1
    - macOS 11.7, Python 3.10.6, PySide6 6.3.2
    - Windows 10, Visual Studio 2019, Python 3.7.9, PySide2 5.15.2.1

- Updated PXR_USE_PYTHON_3 to ON to enable USD Python 3 binding by default. We
  still anticipate dropping support for Python 2 after the first release in 2023.

- Fixed export of targets for usd monolithic builds (usd_ms).
  (PR: [#2026](https://github.com/PixarAnimationStudios/USD/pull/2026))

- Updated USD build to optionally use Imath library by using Imath's cmake 
  config package. USD build falls back to using OpenEXR if Imath is not FOUND. 
  Also updated USD's cmake config, pxrConfig.cmake.in to set Imath_DIR 
  accordingly to set the import targets which were used for the USD build.
  (PR: [#1829](https://github.com/PixarAnimationStudios/USD/pull/1829))

- Disable Ptex support by default when using CMake directly.

- Updated OpenSubdiv version to 3.5.0 to take advantage of build configuration
  improvements. The USD source remains compatible with OpenSubdiv 3.4.x.

- Updated dependencies for JPEG, OpenImageIO, and Alembic for macOS to improve
  compatibility with cross architecture and universal builds.

- Updated build_usd.py for macOS with additional options to support specifying a 
  build architecture target and enabling code signing:
    -build-target {native,x86_64,arm64,universal}
    -codesign

- Fixed issue where using pxrConfig.cmake from a USD build with MaterialX 
  support enabled would cause linker errors. 
  (Issue: [#1955](https://github.com/PixarAnimationStudios/USD/issues/1955))

### USD
- Updated gathering of geom subset prims to not use UsdPrimDefaultPredicate and
  instead use a custom predicate that checks if the prim has a defining 
  specifier, rather than is defined, allowing for correct gathering of geom 
  subsets that are children of Prototype prims.

- Updated ndr parser plugin instantiation to provide a stable ordering for 
  plugin loading.

- Added ability to UsdAttributeQuery to make queries about an attribute's value 
  that only consider opinions "up to" or "stronger than" a certain edit target 
  or composition arc (via the new UsdResolveTarget object).
  (Issue: [#1483](https://github.com/PixarAnimationStudios/USD/issues/1483))

- Fixed a bug with calling UsdAttribute::GetResolveInfo at default time where it 
  could erroneously resolve to specs with time samples.

- Updated UsdGeomImageable::GetPurposeVisibilityAttr and 
  ComputeEffectiveVisibility to check that UsdGeomVisibilityAPI is applied 
  before querying any purpose visibility attributes.

- Updated TF_ERROR macros now return void instead of bool like the other 
  diagnostic macros.

- Updated some of the errors that are reported during prim index composition to 
  be more informative.

- Added GetTargetLayer and GetTargetPrimPath convenience functions to
  UsdPrimCompositionQueryArc.

- Updated UsdPrimCompositionQueryArcs to be validly queried if even after the
  UsdPrimCompositionQuery that returned them is destroyed.

- Added UsdGeomBBoxCache::ComputeWorldBoundWithOverrides.

- Fixed a bug and updated docs for GfFrustum::ComputeNarrowedFrustum.

- Updated UsdShadeMaterialBindingAPI::UnbindCollectionBinding documentation to
  explicitly state to use instance name of the targeted collection for unbinding 
  if bindingName was not provided.

- Documented the use of special use of "@@@" for asset paths containing "@".
  (Issue: [#1832](https://github.com/PixarAnimationStudios/USD/issues/1832))

- Created a usdzUtils python module, which provides methods like 
  CreateUsdzPackage, ExtractUsdzPackage and UsdzAssetIterator.

- Added a script "usdfixbrokenpixarschemas" to update any usda, usdc or usdz 
  asset to respect updated pixar provides schemas. This utility currently fixes 
  MaterialBindingAPI, SkelBindingAPI and authoring of upAxis metadata.

- Updated UsdUtilsComplianceChecker to check if MaterialBindingAPI is applied on 
  the prim which provides a material:binding. Also updated usdchecker unit tests 
  for the same.

- Fixed UsdShadeMaterialBindingAPI to return an empty Material if binding 
  relationship is invalid. 
  (PR: [#2024](https://github.com/PixarAnimationStudios/USD/pull/2024))

- Updated computing bound material core logic to check if MaterialBindingAPI is 
  applied on a prim providing material:binding. Introduced
  USD_SHADE_MATERIAL_BINDING_API_CHECK environment variable for backward 
  compatibility and default to "warnOnMissingAPI". In a later release the 
  default will be updated to "strict", which will ignore prims which do not have 
  MaterialBindingAPI applied for bound material computation.

- Updated UsdShadeMaterialBinding::ComputeBoundMaterial logic such that bound
  material remains consistent irrespective of the load / active state of the 
  target prim on the material:binding relationship.

- Added UsdShadeMaterialBinding::GetResolvedTargetPathFromBindingRel to help
  clients get the path of the target of the winning binding from bound material 
  computation.

- Added support for merging of collision groups in UsdPhysics.
  (PR: [#1947](https://github.com/PixarAnimationStudios/USD/pull/1947))

- Updated UsdPhysics documentation to clarify mesh subdivision options which 
  could affect physics colliders.
  (PR: [#1948](https://github.com/PixarAnimationStudios/USD/pull/1948))

- Added color4 type to Sdr to allow differentiating between color4 and vector4 
  input types at Sdr level. 
  (PR: [#1894](https://github.com/PixarAnimationStudios/USD/pull/1894))

- Added "GetAll" method for multi-apply schemas in codegenTemplates.
  (PR: [#1773](https://github.com/PixarAnimationStudios/USD/pull/1773),
   Issue: [#1772](https://github.com/PixarAnimationStudios/USD/issues/1772))

- Added "renderingColorSpace" to UsdRenderSetting schema. This describes a
  renderer's working (linear) colorSpace where all renderer/shader math is 
  expected to happen. Renderer is expected to use its own default when 
  renderingColorSpace is not provided.

- Improved performance in clip manifest generation when no manifest is supplied.

- Improved performance in pcp prim indexing by using a heap rather than a sorted 
  array to handle tasks in priority order.  This improves worst case runtime for 
  task processing from O(n^2 log(n)) to O(n log(n)).

- Changed pcp prim index graph representation, simultaneously expanding size 
  limitations and reducing memory usage by 5-15% in production sets. Also 
  improving composition time performance, saving 35% on one production example. 

- Improved pcp composition thread scaling by letting indexing and index 
  publishing proceed concurrently.

- Added node-based API to SdfPathTable so callers can pre-reserve memory
  locations and insert them into tables later.

- Added VtVisitValue() utility for operating on held-types of VtValues.

- Updated pyInterpreter and pyModule to Account for Python GIL explicit 
  initialization deprecation from Python 3.7 to 3.9
  (PR: [#1909](https://github.com/PixarAnimationStudios/USD/pull/1909))

- Updated usddiff logic in an attempt to handle files with bogus extensions by 
  trying the usd file format, and copying the files to temporary extensioned 
  files if necessary.

- Updated crateFile core to sanity check spec requirements in 
  prefer-safety-over-speed mode.
  (Issue: [#1763](https://github.com/PixarAnimationStudios/USD/issues/1763))

- Fixed a bug where TfType::_FindByTypeid() could call FindByName() while still 
  holding the registry lock, resulting in an attempt to recursively require the 
  lock, and a hang.

- Updated USD changelog to indicate fixed but not-documented-as-such
  CVE-2020-13495.
  (Issue: [#1966](https://github.com/PixarAnimationStudios/USD/issues/1966))

- Fixed usdc files to use read-only mappings in mmap mode to avoid commit charge 
  on most of the file content, only changing page protections to read/write when 
  detaching outstanding zero-copy-array data.

- Fixed Python 3.11 incompatibility. 
  (PR: [#1928](https://github.com/PixarAnimationStudios/USD/pull/1928))

- Added "detached layer" feature to Sdf. This allows clients to specify layers 
  to load "detached" from their serialized backing store, isolating them from 
  external changes. In particular, loading .usdc layers as "detached" can help 
  avoid crashes or other issues if another process attempts to overwrite those 
  .usdc layers. 
  (Issue: [#1852](https://github.com/PixarAnimationStudios/USD/issues/1852))

- Added SdfLayer::DetachedLayerRules and related API for specifying detached 
  layers.

- Added API to ArAsset, SdfFileFormat and SdfAbstractData to support detached 
  layers.

- Small optimizations to SdfLayer change processing to avoid unnecessary 
  iteration and copies.

- Fixed bug where SdfLayer would not strip file format arguments from 
  identifiers before passing them to ArResolver::GetModificationTimestamp when 
  calling SdfLayer::Save.

- Fixed bug where calling SdfLayer::CreateNew with a file format target would 
  not include the target in the new layer's identifier, causing subsequent 
  lookups via SdfLayer::Find or FindOrOpen with the same arguments to fail.

- Fixed bug causing errors when deleting a variant set containing only empty 
  variants.

- Fixed bug where reloading a text-based .usd layer that was overwritten with a 
  crate-based layer would copy the contents of the crate layer into memory 
  instead of streaming data on-demand. 

- Fixed crashes when reading .usda files containing certain strings with escaped 
  quotes. .usda layers with incorrectly escaped/quoted strings (like "fo\\"o") 
  will no longer load. 
  (Issue: [#1630](https://github.com/PixarAnimationStudios/USD/issues/1630))

- Fixed UsdUtilsComputeAllDependencies to remove incorrect coding error if a 
  local reference or payload is encountered. 
  (Issue: [#1902](https://github.com/PixarAnimationStudios/USD/issues/1902))
  
- Fixed variety of missing symbol exports and header includes. 
  (Issue: [#1942](https://github.com/PixarAnimationStudios/USD/issues/1942), 
   [#1969](https://github.com/PixarAnimationStudios/USD/issues/1969), 
   PR: [#1950](https://github.com/PixarAnimationStudios/USD/pull/1950))

### Imaging
- Fixed a speculative issue with light invalidation sometimes using dirty bits
  from the wrong enum.
  (Issue: [#1719](https://github.com/PixarAnimationStudios/USD/issues/1719))

- Fixes an issue where material binding paths on geometry subsets were not
  correctly prefixed, leading to missing materials in certain circumstances.
  (Issue: [#1687](https://github.com/PixarAnimationStudios/USD/issues/1687))

- Updated conversion of UsdImagingGLRenderParams to HdxRenderTaskParams to
  default to 0.1 for alphaThreshold no matter the transparency mode.

- Added missing initialization of OpenVDB before creating a stream in Hio 
  OpenVDB Utils, preventing crash due to uncaught OpenVDB exception.

- Implemented GPU Indirect Command Buffer (ICB) encoding of draw commands for
  Storm on Metal.
  (PR: [#1945](https://github.com/PixarAnimationStudios/USD/pull/1945))

- Fixed GPU Skinning computations to not execute GL error checking for Storm on
  Metal. (PR: [#1909](https://github.com/PixarAnimationStudios/USD/pull/1963))

- Fixed CPU Frustum culling for Storm on Metal. (PR:
  [#1978](https://github.com/PixarAnimationStudios/USD/pull/1978))

- Minor optimization for CPU culling.
  (PR: [#1860](https://github.com/PixarAnimationStudios/USD/pull/1860),
   [#1956](https://github.com/PixarAnimationStudios/USD/pull/1956))

- Fixed highlighting of selected mesh edges when mesh edges are not otherwise 
  being displayed.

- Extended Hgi interface for buffer and texture bindings to specify which 
  bindings are writable. Non-writable bindings are declared const in shader gen 
  for correctness and potentially improved performance.
  (PR: [#2042](https://github.com/PixarAnimationStudios/USD/pull/2042))

- Storm draw buffer entries are now aligned to 32 bytes on Metal for improved
  performance on newer drivers and correctness on older drivers
  (PR: [#1980](https://github.com/PixarAnimationStudios/USD/pull/1980))

- Implemented a workaround for fragment shader barycentrics and primitive id on 
  older Intel GPU drivers for Storm on Metal.
  (PR: [#1980](https://github.com/PixarAnimationStudios/USD/pull/1980))

- Refactoring and small performance improvements for the Hydra Scene Index API.

- Fixes an issue with redundant shader compilation requests as a result of 
  rearranged HdBuffer elements.

- Updated HdxTaskController to allow for DomeLight textures to use textures 
  other than the shipped default dome light texture.
  (Issue: [#1708](https://github.com/PixarAnimationStudios/USD/issues/1708))

- Updated HdSceneIndexAdapterSceneDelegate::GetCameraParamValue to access
  data sources at nested data source locators. E.g. 
  GetCameraParamValue(cameraId, TfToken("lensDistortion:k1")) accesses the data 
  source at HdDataSourceLocator(TfToken("camera"), TfToken("lensDistortion"), 
  TfToken("k1")).

- Added geomUtil, a library housing mesh generators for common implicit 
  primitives.

- Fixed a bug in the computation of the number of vertex and varying primvars 
  for indexed and periodic cubic basis curves.

- Added support for "pinned" basis curves in Hydra, via a scene index plugin 
  render delegates can use to convert "pinned" to "unpinned" curves.

- Documented that Hydra only supports 100 tiles for UDIM
  (Issue: [#1846](https://github.com/PixarAnimationStudios/USD/issues/1846))

- Fixed performance issue in HdNoticeBatchingSceneIndex::_PrimsDirtied and
  _PrimsRemoved which was previously fixed only for _PrimsAdded.

- UpdatedHdRenderIndex to track depth of calls to 
  SceneIndexEmulationNoticeBatchBegin/End to allow for nested batching.

- Unified behavior of procedural resolving scene index's GetChildPrimPaths to 
  match PrimsAdded's forwarding of existing child prims.

- Implemented first pass of materialBinding support within 
  HdFlatteningSceneIndex.

- Added support for prefixing of path array data sources in 
  HdPrefixingSceneIndex.

- Updated HdMergingSceneIndex to allow inputs to sparsely provide either type or 
  data sources and confirms primType value in forwarded _PrimsAdded notices 
  matches the GetPrim behavior.

- Added support for delivering multiple renderContext nodeIdentifier values 
  within HdMaterialNodeSchema.

- Added fallback for HdSceneIndexAdapterSceneDelegate::GetLightParamValue to
  check against the light terminal node within the "material" data source for 
  better compatibility with legacy render delegates.

- Fixed a spurious dirty bit getting set on lights on visibility changes, which 
  could theoretically lead to over-invalidation.
  (Issue: [#1719](https://github.com/PixarAnimationStudios/USD/issues/1719))

- Fixed rendering issue in colorize selection task in which original contents 
  of render target (the rendered scene) were not properly loaded when 
  rendering and blending the selection color on top.

### UsdImaging
- Deleted UsdImagingGLLegacyEngine, i.e. usdview's "HydraDisabled" renderer.
  This class was a simple GL3-based USD renderer that was kept as a fallback for
  hydra, as Hydra Storm GL requires GL 4.5.  However, it was missing many
  foundational USD features (like instancing) and it's main use (on OSX) has 
  been deprecated by the Storm Metal port.
  This change doesn't affect any of the existing usdview Hydra backends.  If 
  Hydra can't find any supported backends (e.g. a variant of Storm, Embree, 
  Prman), usdview will now display an empty viewport.

- Added an API for UsdImagingDelegate adapters to request a callback to
  UpdateForTime. Adapters now invoke this API on relevant invalidation, but
  UsdImagingDelegate doesn't automatically call UpdateForTime on every
  invalidation. This can be a major performance benefit in some scenes. Custom
  adapters need to be updated to work correctly; however, you can revert to the
  old behavior by setting the env var USDIMAGING_LEGACY_UPDATE_FOR_TIME=1.

- Added a domeLightCameraVisibility render setting that allows turning off the 
  dome light texture rendering while still using the dome light texture for 
  lighting computations.  This is respected by both Storm and Prman.
  (Issue: [#1769](https://github.com/PixarAnimationStudios/USD/issues/1769))

- Added partial support for point instancer to the scene index implementation.

- Added UsdImagingDataSourceSchemaBased container data source to reduce
  boilerplate code when writing usd prim adapters that mostly just pass through 
  Usd attribute values.

- Fixed a case where UsdImaging would unnecessarily dirty prims from multiple 
  threads. This was breaking the _PrimsDirtied threading contract.

- Updated LightDataSource to exclude light/lightLink|shadowLink identifiers for 
  trivial collections which include everything within UsdStageSceneIndex.

- Added light/isLight bool and light/materialSyncMode data sources for 
  identifying presence and behavior of LightAPI on non-light types (i.e. mesh 
  light) within UsdStageSceneIndex.

- Implemented Hydra 2.0 interfaces on UsdImagingLightAdapter to return
  UsdImagingDataSourcePrim so that boundable contributions are present.

- Introduced UsdImagingLightAPIAdapter for UsdImagingStageSceneIndex.

- Updated UsdImagingDataSourceMaterial to answer for unauthored light param
  attributes (like intensity) to match expectations of legacy hdStorm render 
  delegate.

- Updated disk, distant, dome, plugin, rect and sphere light adapters to 
  implement Hydra 2.0 interfaces for defining prim type. This is combined with 
  the LightAPI schema adapter's contributions.

- Updated UsdImagingGLEngine::Render to release GIL at the top of to avoid 
  deadlocks caused by imaging population code, potentially running plug-ins 
  which indirectly interact with python from multiple threads.

- Fixed an issue where material binding paths on geometry subsets were not 
  correctly prefixed, leading to missing materials in certain circumstances. 
  Consumers of UsdImagingDelegate::GetMeshTopology() can now expect any geometry 
  subsets present on the topology to express full index paths for their id and 
  materialId properties.

- Fixed rendering of excluded geometries upon modification of stage in a session 
  layer. (PR: [#1949](https://github.com/PixarAnimationStudios/USD/pull/1949))

- Improved performance for UsdImagingDelegate::SetTime by pre-caching path
  dependencies in parallel.
  (PR: [#1815](https://github.com/PixarAnimationStudios/USD/pull/1815),
   Issue: [#1813](https://github.com/PixarAnimationStudios/USD/issues/1813))

- Improved performance of UsdImagingInstanceAdapter::_Populate by using 
  SdfPathSet instead of SdfPathVector for instance paths.
  (PR: [#1823](https://github.com/PixarAnimationStudios/USD/pull/1823))

- Updated usdImaging tests to detect differences between visibly distinct 
  reference images by tweaking idiff failure values.
  (PR: [#1866](https://github.com/PixarAnimationStudios/USD/pull/1866))

### Storm
- Added new TF_DEBUG code HDX_DEBUG_DUMP_SHADOW_TEXTURES allowing
  users to save the shadow textures produced by HdxShadowTask to image files.

- Improved handling of oversized buffers in HdStInterleavedMemoryManager and
  HdBufferArrayRegistry.

- Added overload of HdInstanceRegistry::GarbageCollect that takes in a callback
  function to be called when the resource instance is removed, now used to 
  properly destroy Hgi resources in HdStResourceRegistry.

- Various fixes and improvements to HgiVulkan support.

- Updated HdxSimpleLightTask to warn about the need to truncate the number of 
  lights when changes are made to the light set, instead of every frame.

### Renderman Hydra Plugin
- Updated minimum RenderMan version required by hdPrman to 24.1 to avoid an
  issue where left-handed geometry may be shaded incorrectly.

- Updated HdPrman plugin to respect "vector" role for instance primvars of type
  VtArray<GfVec3> and pass an array of floats to rman.

- Added support for terminal SdrPropertyType in hdPrman.

- Updated MatFiltSceneIndexPlugin to allow scene index filters that interact 
  with materials to have access to terminal names in a renderer agnostic way by
  executing HdSiTerminalsResolvingSceneIndex earlier in the scene index stack.

- Added support for lens distortion. HdPrmanCamera::Sync reads the lens 
  distortion parameters, setting up the projection shader with the correct fov 
  and screen window. In order to render lens distortion, we aim to switch to the 
  PxrCamera projection shader in a later release (requiring RenderMan 25).

- Updated the default of HD_PRMAN_USE_SCENE_INDEX_FOR_MATFILT to true for
  better inspectability.

- Added support to respect the doubleSided parameter on USD meshes with
  UsdPreviewSurface materials applied to them. Also added support for cullStyle,
  enabling, e.g., GL-like backface culling.

- Updated UsdPreviewSurface OSL shader to improve appearance of transparency 
  when opacityThreshold is zero.

- Added gridGroup support to hdPrman for VDB grids. For now all grids provided 
  in the VDB are included.

### usdview
- Updated usdview's Bounding box dashed lines drawing to use Hgi interface 
  instead of glLineStipple.

- Moved the "View" menu in usdview into the Viewer panel as a reorganized set of 
  new menus.

- Added the ability to save a screengrab from the Viewer of current frame in 
  usdview.

- Added the guides column in the usdview Prim View to choose which cameras to 
  show oracles on.

- Fixed (on posix platforms only) an issue where the usdview state file could be
  corrupted when closing multiple instances of usdview simultaneously.

- Added ability to specify detached layers in usdview using the --detachLayers, 
  --detachLayersInclude, and --detachLayersExclude arguments. 
  (Issue: [#1852](https://github.com/PixarAnimationStudios/USD/issues/1852))

- Update usdview's layer stack view to be more informative of the effect of 
  cumulative layer offsets on each sublayer, including the auto-scaling that 
  occurs between layers with differing time codes per second.
  (Issue: [#1782](https://github.com/PixarAnimationStudios/USD/issues/1782))

- Fixed a bug in usdview where the property view would break when displaying an 
  array valued attribute whose resolved value is an unsupported or non-array 
  type.

### MaterialX Plugin
- Added support for locally defined custom nodes, and custom nodes that use
  textures/texture coordinates
  (Issue: [#1636](https://github.com/PixarAnimationStudios/USD/issues/1636))

- Added support for MaterialX materials that use nodes outside nodegraphs and
  nodegraph input/interface connections
  (Issue: [#1636](https://github.com/PixarAnimationStudios/USD/issues/1636))

- Added support for Custom Material Nodes that use textures.
  (Issue: [#1786](https://github.com/PixarAnimationStudios/USD/issues/1786))

- Added MaterialX tests to UsdImagingGL.

- Added initial material caching for simple MaterialX Materials in Storm.

- Updated HdStMaterialXShaderGen to use default geomprop/primvar value.
  (Issue: [#1880](https://github.com/PixarAnimationStudios/USD/issues/1880))

### Alembic Plugin
- Fixed Alembic plugin to not collapse Alembic geometries into their transform 
  parents if the parent has multiple children.
  (PR: [#1940](https://github.com/PixarAnimationStudios/USD/pull/1940))


## [22.08] - 2022-07-21

Support for Python 2 is sunsetted as of this release. We anticipate dropping
support after the first release in 2023, tentatively scheduled for January.

### Build
- Updated dependencies to align with VFX Reference Platform CY2020, with
  some exceptions to work around various build issues on different platforms.

- Added support for native builds on Apple Silicon.
  It is possible to cross compile for Intel systems by building within an
  x86_64 architecture environment, for example, by creating an x86_64 shell
  via "arch -x86_64 zsh" and then running the build within that shell.
  (PR: [#1875](https://github.com/PixarAnimationStudios/USD/pull/1875))

- Added support for PySide6. By default, the build will search first for PySide6
  then PySide2 then PySide.  Users may specify the following CMake variables to
  influence this search behavior:
    - PYSIDE_USE_PYSIDE2=TRUE  to force the use of PySide2 or
    - PYSIDE_USE_PYSIDE=TRUE   to force the use of PySide
  (PR: [#1898](https://github.com/PixarAnimationStudios/USD/pull/1898))

- Support for PySide is deprecated and will be removed in a future release.

- Changes and fixes for build_usd.py:
  - Removed special handling when being run from Maya's Python.
  - Fixed detection of Python library and include directories when run with a
    virtualenv-based Python.
    (PR: [#1692](https://github.com/PixarAnimationStudios/USD/pull/1692))
  - Fixed detection of Python libraries with ABI tags.
    (Issue: [#1724](https://github.com/PixarAnimationStudios/USD/issues/1724))

- Fixed build errors on Windows with OpenVDB v7.0.0+.

- Monolithic builds no longer add the "usd_" prefix to library names by default.

- Cleanup for miscellaneous build errors and compiler warnings.
  (Issue: [#806](https://github.com/PixarAnimationStudios/USD/issues/806),
  PR: [#1696](https://github.com/PixarAnimationStudios/USD/pull/1696))

- Unit tests will now extract files that fail baseline comparisons to
  ${CMAKE_BINARY_DIR}/Testing/Failed-Diffs/${TEST_NAME}
  (PR: [#1839](https://github.com/PixarAnimationStudios/USD/issues/1839))

### USD
- Automatically disable glibc malloc hooks support on glibc versions >= 2.34.

- Removed support for Ar 1.0, which was deprecated in v21.11.

- Added ability to disable Ndr plugins via the `PXR_NDR_DISABLE_PLUGINS` env var.

- Added NdrRegistry::AppendDiscoveryResult to allow clients to explicitly
  set additional discovery results that would not be found through the plugin
  system.
  (PR: [#1810](https://github.com/PixarAnimationStudios/USD/issues/1810))

- Reverted collaborative layer loading feature while investigating intermittent
  crashes.

- Fixed bug where authoring opinions at certain sites for prims using inherits
  and/or specializes arcs would not trigger change processing and notifications.

- Fixed bug with the composition of reference/payload list ops across
  sublayers with different time codes per second or layer offsets.
  (Issue: [#1778](https://github.com/PixarAnimationStudios/USD/issues/1778))

- Added the apiSchemaOverride feature to usdGenSchema that allows a schema
  definition to explicitly define sparse overrides to properties it expects to
  be included from a built-in API schema. See:
  https://graphics.pixar.com/usd/release/api/_usd__page__generating_schemas.html#Usd_APISchemaPropertyOverride

- Updated usdgenschemafromsdr to read Sdr node paths with environment variables.

- Fixed UsdUtilsUpdateSchemaFromSdrNode:
  - Non connectable nodes do not get any connectivity assigned.
  - Fixed pruning of properties when only typedSchemaForAttrPruning is set.

- Fixed issue where UsdInherits::GetAllDirectInherits might not have returned
  all direct inherit paths and might have returned paths that were not actually
  inherits.

- Fixed issue with removing references in sublayers with differing time codes
  per second.
  (Issue: [#1778](https://github.com/PixarAnimationStudios/USD/issues/1778))

- UsdAttribute::GetTimeSamplesInInterval will no longer open value clips
  outside the requested interval when interpolateMissingClipValues is disabled.

- Improved memory allocation for value clips by using a shared pointer to the
  array of time mapping in Usd_Clips.
  (Issue: [#1774](https://github.com/PixarAnimationStudios/USD/issues/1774),
  PR: [#1777](https://github.com/PixarAnimationStudios/USD/pull/1777))

- Improved performance of interpolating attribute time samples.
  (PR: [#1883](https://github.com/PixarAnimationStudios/USD/pull/1883))

- Improved UsdStage loading/repopulation performance and thread scalability.

- Improved .usdc write performance for integer arrays.

- Fixed UsdGeomImageable::ComputeProxyPrim to implement non-pruning semantics
  when computing the purpose value for the targeted prim.

- Removed Primvar API on UsdGeomImageable which was deprecated in v19.03.
  All remaining uses of this API have been updated to use UsdGeomPrimvarsAPI.

- Added UsdGeomPlane schema.
  (Issue: [#772](https://github.com/PixarAnimationStudios/USD/issues/772),
  PR: [#1819](https://github.com/PixarAnimationStudios/USD/pull/1819))

- Cleaned UsdShadeConnectableAPIBehavior code to not throw warnings when
  querying for incompatible prim type, which has no plugin registered.

- Updated OSL and Args parsers to use Sdr's nested page delimiter of ":",
  instead of ".". Updated documentation for the same in UsdShadeInput and
  SdrShaderProperty.

- Updated usdRiPxr schemas to reflect updates to RenderMan args files and to
  include other schema classes:
  - Added schemas for all RenderMan plugins/concepts, plus Options, Attributes,
    and Primvars.
  - Added RenderMan default plugins schema classes and display drivers schema
    classes.
  - Updated previously generated schemas to use ":" as the nested page delimiter
    in display groups.
  - A few vector3 attribute types were updated to use
    sdrUsdDefinitionType="float3", to provide appropriate SdfValueTypeName in
    the generated schema.

- Added UsdRenderPass and UsdRenderDenoisePass schemas to UsdRender.

- Added UsdProc schema domain for representing procedural data and an
  initial concrete UsdProcGenerativeProcedural schema.

- Added UsdHydraGenerativeProceduralAPI to the now-repurposed usdHydra schema
  domain.

- Miscellaneous small fixes.
  (PR: [#1775](https://github.com/PixarAnimationStudios/USD/pull/1775),
  PR: [#1842](https://github.com/PixarAnimationStudios/USD/pull/1842),
  PR: [#1845](https://github.com/PixarAnimationStudios/USD/pull/1845),
  PR: [#1850](https://github.com/PixarAnimationStudios/USD/issues/1850))

### Imaging
- Fixed crash in GlfSimpleShadowArray::GetShadowMapTexture, avoiding discrepancy
  between the lighting context/light data and the simple shadow array data.

- Added HdChangeTracker::GetInstanceIndicesChangeCount to detect changes to
  instance indexing.
  (PR: [#1754](https://github.com/PixarAnimationStudios/USD/pull/1754))

- Fixed bug in HdRenderIndex::RemoveSceneIndex where a client removes a scene
  index that was added with a non-trivial scene path prefix.

- Fixed double-creation of Bprims when using scene index emulation.
  (PR: [#1737](https://github.com/PixarAnimationStudios/USD/pull/1737))

- Various fixes to primvar sampling in scene index emulation and the usdImaging
  stage scene index.

- Fixed inconsistencies for clipping planes to work with scene index emulation.

- HdPrimvarSchema role tokens: changing capitalization for consistency with how
  these are used elsewhere in Hydra.

- GLSLFX shaders can now consume int- and int-array valued primvars.

- Fixed codegen of HdGetScalar_* for GLSL bindings to primvars.
  (Issue: [#1862](https://github.com/PixarAnimationStudios/USD/issues/1862),
  PR: [#1863](https://github.com/PixarAnimationStudios/USD/issues/1863))

- Exposed secondary selection highlight color as a settable value on
  HdxTaskController.
  (PR: [#1771](https://github.com/PixarAnimationStudios/USD/issues/1771))

- Added HdGp library for Hydra procedurals which have read access to the input
  scene and can generate (or update) prims beneath a given prim.
  HdGpGenerativeProcedural subclasses may be provided by plugins and are
  resolved/evaluated/updated via an HdGpGenerativeProceduralResolvingSceneIndex.

  HdGp procedurals are disabled by default but may be enabled by setting the
  HDGP_INCLUDE_DEFAULT_RESOLVER environment variable to 1.

- Added support for "includeDisconnectedNodes" within material network data
  source to trigger legacy behavior for renderers which rely on Hydra 1.0
  behavior.

- Fixed HdMergingSceneIndex::GetChildPrimPaths to incorporate ancestor paths
  of inputs with specified root paths.

### UsdImaging
- Added basic edit processing support for the USD scene index.

- Added basic support for SampleFilters in RenderSettings in UsdImaging scene
  delegate and HdPrman render delegate.

- Added hdSi library to support implicit shapes in UsdImagingStageSceneIndex.

- Added usdProcImaging library for imaging UsdProcGenerativeProcedural prims.

- Added initial support for API schema adapters in UsdImagingStageSceneIndex,
  along with an adapter for UsdShadeMaterialBindingAPI (direct bindings only).

- Implemented Hydra 2.0 methods on UsdImagingMaterialAdapter.

- Added API for accessing AOV render buffers to UsdImagingGLEngine.

- Removed remaining direct uses of OpenGL from UsdImagingGLEngine.

- Deprecated UsdImaging_GetUdimTiles.

- Fixed "model:drawMode" evaluation to apply to components without
  "GeomModelAPI" applied.

- Updated UsdImagingCameraAdapter::TrackVariability to consider all attributes
  on the camera prim.
  (PR: [#1797](https://github.com/PixarAnimationStudios/USD/pull/1797))

- Improved performance when handling large number of native instances.
  (Issue: [#1740](https://github.com/PixarAnimationStudios/USD/issues/1740),
  PR: [#1822](https://github.com/PixarAnimationStudios/USD/pull/1822),
  PR: [#1932](https://github.com/PixarAnimationStudios/USD/pull/1932))

- Fixed light collection change-tracking.
  (PR: [#1653](https://github.com/PixarAnimationStudios/USD/pull/1653),
  PR: [#1930](https://github.com/PixarAnimationStudios/USD/pull/1930))

- Fixed bug where coordSys Sprims were not dirtied on transform changes on the
  target prim.

- Updated UsdImagingPointInstancerAdapter to register velocity and
  acceleration primvars.
  (PR: [#1849](https://github.com/PixarAnimationStudios/USD/pull/1849))

- Fixed bug where changes to materials bound to geom subsets were not processed.
  (Issue: [#1837](https://github.com/PixarAnimationStudios/USD/issues/1837),
  PR: [#1838](https://github.com/PixarAnimationStudios/USD/issues/1838))

- Fixed UsdImagingPrimAdapter computation of dirty bits for primvars.
  (PR: [#1807](https://github.com/PixarAnimationStudios/USD/issues/1807))

- Fixed thresholds for testUsdImagingGLPurpose to be more accurate.
  (PR: [#1827](https://github.com/PixarAnimationStudios/USD/issues/1827))

### Storm
- Added a render setting for the maximum number of lights to support in
  Storm. This should be used with caution, as Storm hasn't been optimized for
  large numbers of lights.

- Added API export tags to HdStTextureIdentifier.
  (Issue: [#1758](https://github.com/PixarAnimationStudios/USD/issues/1758))

- Improved HgiGraphicsCmds interface for binding vertex buffers.
  (PR: [#1876](https://github.com/PixarAnimationStudios/USD/pull/1876))

- Stopped adding fallback/unconnected material param names to the list of a
  HdSt_MaterialNetworkShader's primvars.

- Changes and fixes for hgiVulkan backend:
  - Updated some missing HgiVulkanCapabilities and desired Vulkan extensions.
  - Fixed unsafe use of pointers to objects in temporary scopes.
    (Issue: [#1911](https://github.com/PixarAnimationStudios/USD/issues/1911))

- Fix crash when mesh prim has authored points and faceVertexCounts but
  not faceVertexIndices.

- Transform two channel (grayscale + alpha) PNG images into RGBA format.
  (Issue: [#1796](https://github.com/PixarAnimationStudios/USD/issues/1796))

- Storm volumes: making material final to avoid the volume shader being replaced
  by the default material network that does not work for volumes.

- Updated HdSt_DrawBatch::Prepare() to take a HgiGraphicsCmds instance to allow
  combined submission of GPU work.

- Improved organization of internal methods in HdSt_PipelineDrawBatch and
  HgiMetalGraphicsCommands and HgiGraphicsPipeline.
  (PR: [#1859](https://github.com/PixarAnimationStudios/USD/pull/1859))

- Fixed inconsistent naming of HgiDeviceCapabilitiesBitsBasePrimitiveOffset.
  (PR: [#1858](https://github.com/PixarAnimationStudios/USD/pull/1858))

- Removed redundant shader resource layout declarations from Storm shader
  source.

### RenderMan Hydra Plugin
- Added various implicit shapes support by using native Riley prims and the
  implicit surface scene index.

- Added support for SdfAssetPath-valued shader parameters.

- Added support for RenderSettings and Murk Sample Filter prims, with the
  ability to have multiple Sample Filters. These prims are supported with and
  without scene index emulation.

- Added support for MaterialX displacements and improved editing of
  displacements.

- Various improvements to MaterialX support.

- Improved diagnostic messages for materials with empty/wrongly typed material
  resources.

- Updated to look for material properties in the "primvars:ri:attributes"
  namespace. "ri:attributes:" is still supported for backwards compatibility.

- Fixed support for sharpness per crease edge with subdivs.

- Fixed "depth" AOV to return [0,1] instead of [-1,1], for consistency with
  Storm and the expectations of Hdx compositing code.

### usdview
- Added --mute command line option for muting layers prior to initial stage load.

- Improved muted layer support in the Layer Stack tab (previously muted layers
  were not listed) and added context menu support to interactively mute/unmute
  layers from the UI.

- Fixed Interpreter window to respect OS hover-focus/click-focus settings.

- Fixed Python 3 warning in usdview about not properly closing a file.
  (PR: [#1861](https://github.com/PixarAnimationStudios/USD/pull/1861))

- Fixed an error in usdview when framing with the root prim selected.
  (Issue: [#1619](https://github.com/PixarAnimationStudios/USD/issues/1619),
   PR: [#1843](https://github.com/PixarAnimationStudios/USD/issues/1843))

### MaterialX Plugin
- Added support for MaterialX v1.38.3+ and removed support for older versions.
  (PR: [#1792](https://github.com/PixarAnimationStudios/USD/pull/1792))

- Added support for usdBakeMaterialX baking script on Windows.

- Updated default texture node wrap values for MaterialX texture nodes.
  (Issue: [#1793](https://github.com/PixarAnimationStudios/USD/issues/1793))

- Fixed MaterialX shading without a Dome Light.
  (Issue: [#1877](https://github.com/PixarAnimationStudios/USD/issues/1877))

## [22.05b] - 2022-06-14

### Build
- Fixed broken URL in build_usd.py for downloading libtiff.
  (Issue: [#1901](https://github.com/PixarAnimationStudios/USD/issues/1901))

## [22.05a] - 2022-05-11

### USD
- Fixed a race condition that could lead to crashes during scene changes.

## [22.05] - 2022-04-22

This release enables Storm for macOS using Metal. Refer to notes under Storm for
details. Many thanks to our collaborators at Apple for all of their work to make
this happen!

### Build
- Fixed compilation issue on GCC11.
  (Issue: [#1721](https://github.com/PixarAnimationStudios/USD/pull/1721), 
   PR: [#1776](https://github.com/PixarAnimationStudios/USD/pull/1776))

- Deprecated support for Visual Studio 2015. Visual Studio 2017 will be the 
  minimum supported version as of the next release.

- Builds with Python enabled or disabled are now ABI-compatible.
  (PR: [#1729](https://github.com/PixarAnimationStudios/USD/pull/1729))

- Made 10.15.7 as the minimum macOS requirement (with compatible Xcode version 
  being 12.4) we test against. Minimum CMake requirement for macOS has also been 
  updated to 3.18.6.

- On Apple Silicon systems, it is required to use an x86_64 architecture
  environment (e.g. "arch -x86_64 zsh") to build and execute binaries.

- Updated build_usd.py to use OpenSubdiv 3.4.4.

### USD
- Added support for sdrUsdDefinitionType in UsdShadeShaderDef.

- Removed SDR_DEFAULT_VALUE_AS_SDF_DEFAULT_VALUE environment variable,
  SdrShaderProperty::GetDefaultValue and
  SdrShaderProperty::GetDefaultValueAsSdfType will provide default values
  appropriately.

- Added disableMotionBlur in favor of deprecated instantaneousShutter in
  UsdRender, to be more compliant of standard behavior of using only one sample
  at the current frame when motion blur is disabled.

- Deprecated motion:velocityScale in UsdGeomMotionAPI in favor of more general
  motion:blurScale. Also deprecated usage of velocityScale in 
  UsdGeomPointInstancer and UsdGeomPointBased. Refer to motion:blurScale 
  documentation for more details.

- Added GetMaterialPurposes() API to UsdShadeMaterialBindingAPI.

- Improved performance and correctness of TfDictionaryLess.

- Fixed performance regression due to thrashing of Sdf object identities.

- Numerous performance improvements in UsdPrim's property names computation.

- Changes for usdGenSchema:
  - Disabled inheritance for multiple apply API schemas, now that
    built-in API schemas are supported for multiple apply schemas.
  - Fixed issue where schema tokens with invalid C++ identifiers would be 
    generated. These tokens will now be converted into a valid identifier. 
    Tokens beginning with a numeral are now prefixed with '_'.
  - Added ability to generate C++ identifiers using property names and values 
    exactly as authored in schema.usda instead of camel-casing them by 
    specifying useLiteralIdentifier in the GLOBAL schema metadata. This is 
    helpful in scenarios where schema.usda is generated using utilities like 
    usdgenschemafromsdr instead of being hand authored.
  - Fixed Python 3 compatibility by explicitly included the None
    keyword in reserved list.

- Added error handling for corrupt .usdc files where VtValues claimed to 
  contain themselves.

- Fixed inconsistent behavior of UsdShadeConnectableAPI::HasConnectableAPI 
  between Python and C++.

- Fixed UsdShadeConnectableAPIBehavior registry that caused it to miss builtin 
  API schemas when querying for connectability. This caused incorrect result
  when querying for HasConnectableAPI on just the prim types.

- Numerous fixes and improvements for Python 3 compatibility.

- Instance proxies can now be used to author opinions to non-local edit targets.

- Added NdrFsHelperDiscoverFiles function to NdrDiscoveryPlugin which returns a
  list of raw and resolved URIs to the files found by walking the directory.
  This is similar to NdrFsHelperDiscoverNodes which returns a list of full
  discovery results.

- Updated UsdUtilsUpdateSchemaWithSdrNode to process nodes which do not exhibit
  UsdShade connectability. This is used to incorporate rman concepts like
  Options, etc, not connectable sdr nodes in the usdRiPxr schemas.

- Updated NdrFsHelperDiscoverNodes API to include a parameter to optionally 
  parse a shader identifier into its name, family and version.

- Added an optional parameter to specify the tolerance for
  UsdSkelNormalizeWeights.
  (PR: [#1667](https://github.com/PixarAnimationStudios/USD/pull/1667))

- Deprecated UsdLuxListAPI in favor of new UsdLuxLightListAPI.

- Added plugin mechanism for registering variantSets for 
  UsdUtilsGetRegisteredVariantSets.

- Fixed a critical bug with UsdPhysics Mass computation.
  (PR: [#1799](https://github.com/PixarAnimationStudios/USD/pull/1799))

- Restored fix for symlink reading on Windows.
  (PR: [#1804](https://github.com/PixarAnimationStudios/USD/pull/1804))

- Added UsdGeomBoundable::ComputeExtent API to sit alongside
  UsdGeomBoundable::ComputeExtentFromPlugins. This returns the extent on a
  boundable if one is explicitly authored, else it computes using the registered
  ComputeExtentFunction. Its recommended to use this API over extent getters,
  which might incorrectly return schema fallback extent.

- Added nonlinearSampleCount attribute to UsdGeomMotionAPI.

### MaterialX Plugin
- Make UsdMtlx plugin a library, moved to /usd/usdMtlx

- Added support for multi-output MaterialX nodes.
  (Issue: [#1581](https://github.com/PixarAnimationStudios/USD/issues/1581)

- Added support for MaterialX namespaces declared in stdLibs
  (PR: [#1631](https://github.com/PixarAnimationStudios/USD/pull/1631))

- Fixed MaterialX boolean inputs.
  (Issue: [#1784](https://github.com/PixarAnimationStudios/USD/pull/1784),
   PR: [#1789](https://github.com/PixarAnimationStudios/USD/pull/1789))

- Added support for using texcoord MaterialX node type for texture coordinates.
  (Issue: [#1636](https://github.com/PixarAnimationStudios/USD/issues/1636)

- Updated documentation to mention current known limitation of the MaterialX
  plugin.
  (Issue: [#1636](https://github.com/PixarAnimationStudios/USD/issues/1636)

- Added a UsdBakeMaterialX script to bake MaterialX materials, via 
  MaterialX::TextureBaker. Note this does not work on windows yet, and skips 
  volume materials.

- Improved discovery of MaterialX vector3 inputs
  (PR: [#1790](https://github.com/PixarAnimationStudios/USD/pull/1790))

### Alembic Plugin
- Pruned redundant "vals" namespace from the primvars namespace when reading 
  array primvars from alembic. 
  (PR: [#1635](https://github.com/PixarAnimationStudios/USD/pull/1635))

### Imaging
- Added HdNoticeBatchingSceneIndex and methods to begin and end batching of 
  notices for legacy scene delegates.

- Adjusted HdRenderIndex::RemoveSprim() to not remove hierarchical descendents 
  when scene index emulation is enabled.

- Added HdSceneDelegate::GetScenePrimPaths(), a vectorized version of 
  GetScenePrimPath(). Note that with this change, GetScenePrimPath() is 
  deprecated; use GetScenePrimPaths() instead. 
  (PR: [#1744](https://github.com/PixarAnimationStudios/USD/pull/1744))

### UsdImaging
- Added tests for usdImagingGL and testusdview. These are enabled on Linux but
  are currently disabled on macOS and Windows, while we investigate these
  further.
  (PR: [#1743](https://github.com/PixarAnimationStudios/USD/pull/1743))

- Support shapes with blendshapes but no skinning when using CPU computations
  (PR: [#1757](https://github.com/PixarAnimationStudios/USD/pull/1757))

- Fixed UDIM path resolving when no "1001" file exists
  (PR: [#1787](https://github.com/PixarAnimationStudios/USD/pull/1787))

- Made usdImaging stricter about only reading "model:drawMode" attributes from 
  prims with UsdGeomModelAPI applied.

- Added early support to UsdImagingEngine (and consequently usdview) for 
  loading USD stages into hydra with scene indices instead of scene delegates. 
  This is gated on an environment variable and is disabled by default.

- Added USD scene index support for cameras and volumes.

### Storm
- Enabled Storm for macOS using Metal. This is the first release with this 
  feature enabled and there are currently the following limitations:
  - Disabled for macOS releases before 10.15 (Catalina) since Metal Shading 
    Language 2.2 is required.
  - Disabled for systems using integrated Intel GPUs which do not yet support 
    fragment shader barycentric coordinates.
  - Curves primitives are always drawn as linear segments and do not refine 
    when complexity is changed.
  - Mesh primitives do support subdivision refinement but do not support 
    adaptive tessellation.
  - Frustum culling is enabled but only executes on the CPU, GPU frustum 
    culling is disabled.
  - MaterialX materials are not yet supported.

- Introduced several aspects to take advantage of full performance on Metal:
  - Post Tessellation Vertex Shaders (PTVS) used when executing displacement 
    shader terminals.
  - Vertex buffer step function used to support plumbing of drawing coordinate 
    attributes.
  - Argument buffers used for buffer and texture binding.
  - Support parallel encoding of render command buffers.

- Significant additions to the Hgi API and to the HgiMetal and HgiGL 
  implementations. The Hgi API is still under development, recent additions 
  include:
  - Extended shader parameter descriptor to include specification of 
    interpolation qualifiers, arraySize, and binding location.
  - Improved the distinction between UBO and SSBO bindings as needed for GL.
  - Added support for GLSL style interstage interface blocks.
  - Added a way to enable early_fragment_tests.
  - Added support for conservative rasterization for HgiGL
  - Added support for additional clipping distances.
  - Added support for declaring and accessing arrays of textures (and texture 
  arrays).
  - Added support to distinguish between [0,1] and [-1,1] depth ranges.
  - Added a BlitCmd to fill the contents of a GPU buffer.
  - Additional enumeration of GPU capabilities.
  - Improved consistency of naming for the input arguments to drawing commands.

- Added support for building against OCIO v2.0 with backup compatibility for 
  OCIO v1.x.

- Fixed domeLight computations and skyDome viewport rendering for Metal 
  including fixing the alpha value written by the skyDome
  (Issue: [#1656](https://github.com/PixarAnimationStudios/USD/issues/1656))

- Fixed management of framebuffer objects in HgiGL for applications with 
  multiple GL contexts.

- Added support for "widget" styled drawing in Storm.

- Added a display style flag "materialIsFinal" to let geometry (such as 
  widgets) opt out of material overrides.

- Added ImageToWorldMatrix to renderPassState and GetPositionInWorldSpace() 
  GLSL helper for fragment shaders.

- Updated Storm runtime shader code generation significantly to support 
  declaration of shader resources (textures, buffers, etc.) using Hgi.

- Shader code generation continues to support native GLSL declaration of shader 
  resources, the new method of using Hgi for this purpose is guarded by the 
  env setting HDST_ENABLE_HGI_RESOURCE_GENERATION which is enabled by default 
  only for Metal.

- Enabled declaration of resources (textures, buffers, interstage interface 
  blocks, etc.) as GLSLFX layouts for Storm's internal shader source files.

- Use the appropriate GLSL (for OpenGL) or MSL (for Metal) shader mixins from 
  OpenSubdiv.

- Updated shader source files and generated shader code to use Hgi abstractions 
  for accessing packed types, buffers, textures, atomic operators,

- Updated previewSurface.glslfx and simpleLighting.glslfx to use newly added 
  HdTextureLod_name() (consistent sampler access to specific mipmap levels) and 
  HdGetScalar_name() (single component, scalar value access of a parameter 
  without needing to guard against component swizzle of scalar data types) 
  accessors where necessary for compatibility with GLSL and MSL. Also updated 
  lighting shaders to use HdGet_name() for sampling shadow textures and arrays 
  of shadow textures.

- Shader programs compiled directly from Hgi (i.e. not going through Storm 
  codegen) now access textures using HgiGet_name() accessors, HdGet_name() and 
  similar accessors are provided by Storm codegen.

- Generated shader source now has organizational comments to indicate which 
  aspect of shader generation provided the following lines of shader source code.

- Enabled HdSt_PipelineDrawBatch for Metal, and deleted HdSt_ImmediateDrawBatch. 
  The existing env setting which enabled use of MDI drawing command buffers has 
  been repurposed to just control whether the CPU or GPU iterates over the 
  command buffer when executing drawing.

- Fixed HdStTextureUtils::HgiTextureReadback() to support readback into aligned 
  allocated buffers.

- Added guards to allow volume primitive shaders to run when double precision 
  values are not supported by the shader runtime.

- Improved calculation of the size of OIT buffers in shaders for compatibility 
  with Metal.

- Added an error message when a scene is rendered with more than 16 lights. 
  Changed the light selection to grab the first 16 lights, prioritizing dome 
  and simple lights (which tend to be used as camera lights). Filtered out 
  zero-intensity lights before light selection.

### RenderMan Hydra Plugin
- Removed use of __lightFilterParentShader as this is obsolete since R22, which 
  was generating unknown or mismatched input parameter errors.

- Changed light filter coordinate system name to be the full light filter path 
  as opposed to the leaf name. Since light filters affecting a light are 
  expressed via relationship, only the full path guarantees uniqueness.

- Added support for light filter combineMode.

- Updated the MaterialNetwork conversion function to return an 
  HdMaterialNetwork2 for a given HdMaterialNetworkMap, instead of using an out 
  param. Doing so avoids potential issues if the caller passes in an existing 
  HdMaterialNetwork2, in which case we would get an inaccurate final material 
  network.

- Fixed a hang in StopRender() when blocking = true. Fixed 
  HdPrmanRenderDelegate::IsStopped (and the Stop return value), which were 
  previously erroneously always returning true for offline mode, and always 
  returning false for interactive mode.

- Disabled MaterialX scene index plugin when MaterialX support is not enabled.

- Added scene index plugins for material filtering functionality.

- Improved hdPrman's motion blur support by adding blur scale and velocity and 
  acceleration motion blur implemented through a new scene index. 
  The instantaneousShutter setting has been deprecated in favor of 
  disableMotionBlur.

- Fixed a bug where toggling visibility of individual instances would lead to a 
  crash.

- Fixed an issue that was not correctly updating riley visibility on changing 
  rprim tags.

- Updated hdPrman display driver to correctly normalize render outputs based on 
  accumulation rule and type.

- Updated usdRiPxr schemas to reflect updates to renderman args files.

### usdview

- Fixed crash bugs when running usdview on macOS systems where Storm is not 
  enabled.
  (Issue: [#1780](https://github.com/PixarAnimationStudios/USD/issues/1780))

- Deprecated UsdImagingGLLegacyEngine, i.e. "HydraDisabled". This renderer 
  doesn't support most USD features, and will be removed in the next release.

- Deprecated UsdImagingGLEngine::_GetDelegate. This API will be removed in the 
  next release.

## [22.03] - 2022-02-18

### Build
- boost::program_options is now required only if PXR_BUILD_USD_TOOLS or
  PXR_BUILD_TESTS is enabled.
  (PR: [#1649](https://github.com/PixarAnimationStudios/USD/pull/1649),
   [#1703](https://github.com/PixarAnimationStudios/USD/pull/1703))

- Workaround issues with UTF-8 filenames when extracting boost in build_usd.py.
  (PR: [#1694](https://github.com/PixarAnimationStudios/USD/pull/1694))

- The source .rst files for documentation on https://openusd.org are
  now part of the repository under the docs/ subdirectory.

- Fixed warnings emitted when building with "-Wformat-security" and enabled
  this flag by default for gcc/clang builds.
  (Issue: [#1675](https://github.com/PixarAnimationStudios/USD/issues/1675))

- Fixed build failures with hioOpenVDB on Windows.

- Fixed build failures with precompiled headers disabled on Windows.
  (PR: [#1672](https://github.com/PixarAnimationStudios/USD/pull/1672))

- Fixed error when importing modules from the usd-core PyPI package
  in Conda environments on Windows.
  (Issue: [#1602](https://github.com/PixarAnimationStudios/USD/issues/1602),
   PR: [#1642](https://github.com/PixarAnimationStudios/USD/pull/1642))

### USD
- Additions to simplify porting client code to Python 3:
  - Support for "future division" in Python 2 to Vt.Array and Sdf.TimeCode.
  - Support for Unicode strings in Tf.Type and Tf.Type.FindByName.

- Fixed handling of file paths from mounted volumes on Windows.
  (Issue: [#1520](https://github.com/PixarAnimationStudios/USD/issues/1520),
   PR: [#1746](https://github.com/PixarAnimationStudios/USD/pull/1746))

- Improved accuracy of timing performed by Trace and TfStopwatch.

- Changes to Ndr and Sdr:
  - Improved performance of lookups by identifier and source type.
  - Removed aliasing functionality.
  - Switched environment variable `SDR_DEFAULT_VALUE_AS_SDF_DEFAULT_VALUE`
    off by default.

- Fixed bug where certain combinations of specializes and inherit arcs on
  a prim could lead to errors or incorrect strength ordering.

- Fixed regression where SdfSpec::IsInert would mark empty variant and
  variant set specs as not inert.
  (Issue: [#1652](https://github.com/PixarAnimationStudios/USD/issues/1652))

- Changes for applied API schemas:
  - Multiple-apply API schemas can now include other multiple-apply API schemas
    as built-ins.
  - "prepend apiSchemas" must be used in schema.usda to set built-in API schemas
    on another schema.
  - Applied API schemas authored on a prim can no longer change the type of
    a property defined by that prim's type or built-in API schemas.

- Fixed uninitialized fields in UsdZipFile.
  (Issue: [#1579](https://github.com/PixarAnimationStudios/USD/issues/1579),
   PR: [#1578](https://github.com/PixarAnimationStudios/USD/pull/1578))

- Cache .usdz archive traversal to improve performance for reading archives
  with many files and references. In one example, a .usdz file that previously
  took ~3 minutes to load improved to ~16 seconds with this change.
  (Issue: [#1577](https://github.com/PixarAnimationStudios/USD/issues/1577),
   PR: [#1578](https://github.com/PixarAnimationStudios/USD/pull/1578))

- Numerous changes to improve performance of various operations, including
  reading .usda and .usdc files and initializing the schema registry.

- Fixed bug where UsdUtilsComputeAllDependencies would inadvertently try to
  edit the given layer.

- Fixed UsdUtilsUpdateSchemaWithSdrNode to emit "allowedTokens" for
  token-valued attributes based on the Sdr node's options.

- Various documentation fixes.
  (Issue: [#1370](https://github.com/PixarAnimationStudios/USD/issues/1370))

- Fixed crash in UsdGeomCamera::SetFromCamera when the current edit target
  specifies a weaker layer than the layer defining xformOpList.
  (PR: [#1661](https://github.com/PixarAnimationStudios/USD/pull/1661))

- Added "guideRadius" attribute to UsdLuxDomeLight.

- Removed UsdRenderSettingsAPI.

- Changes to connectability in UsdShade:
  - Typeless prims can now be queried for connectability.
  - Multiple-apply schemas are now ignored when determining connectability.
    This previously would result in an error.

- UsdShadeShaderDef now treats an attribute's "allowedTokens" as options
  if no other "options" metadata has been specified in "sdrMetadata".

- Added support for skinning normals with faceVarying interpolation in UsdSkel.
  (PR: [#1695](https://github.com/PixarAnimationStudios/USD/pull/1695))

- Added UsdPhysicsRigidBodyAPI::ComputeMassProperties for computing the
  mass properties of the rigid body.
  (PR: [#1677](https://github.com/PixarAnimationStudios/USD/pull/1677))

- Changed UsdVolFieldBase to be an Xformable instead of a Boundable.

### Draco Plugin
- Fix compile error when building with Draco library that has disabled
  deduplication features.
  (Issue: [#1671](https://github.com/PixarAnimationStudios/USD/issues/1671))

### MaterialX Plugin
- Fixed issue with NodeDef discovery. 
  (Issue: [#1629](https://github.com/PixarAnimationStudios/USD/issues/1629),
   PR: [#1641](https://github.com/PixarAnimationStudios/USD/pull/1641))

- Fixed issue with computing a shader name for Sdr from the NodeDef.
  (PR: [#1641](https://github.com/PixarAnimationStudios/USD/pull/1641))

- Allow runtime adjustment of MaterialX search paths.
  (Issue: [#1586](https://github.com/PixarAnimationStudios/USD/issues/1586),
   PR: [#1628](https://github.com/PixarAnimationStudios/USD/pull/1628))

### Imaging
- Added HdMaterialNetworkInterface to let material processing operators abstract
  away data representation.

- Added API to merge application-provided scene indices with the Hydra emulation
  scene index.

- Added API to HdRenderDelegate to query pause and stop state.

- Augmented HdRenderDelegate::Stop to allow requests for blocking or
  non-blocking stop.

- For HdxTaskController users with non-Storm renderers, replaced the fallback
  sphere light with a distant light.

- Changed default dome light texture format from .exr to .hdr.
  (Issue: [#1481](https://github.com/PixarAnimationStudios/USD/issues/1481))  

- Added HdSceneIndexPluginRegistry and HdSceneIndexPlugin, to allow for external
  registration of filtering scene indices to be added for specified renderers.

- Improved HdMeshUtil Patch Param documentation.
  (Issue: [#1720](https://github.com/PixarAnimationStudios/USD/issues/1720))  

- Cache "renderTag" attribute in Hydra instead of pulling it at every use.

### UsdImaging
- Fixed instancing-related edit processing bugs: when prims are added to or
  removed from USD prototype roots; or when invalidations propagate from
  non-instanced prims to instanced prims.

- Fixed drawing of "unloaded-prim-as-bounds" when the unloaded prim is an
  instance.

- Improved performance of UsdImagingDelegate::PopulateSelection.
  (Issue: [#1689](https://github.com/PixarAnimationStudios/USD/issues/1689),
   PR: [#1691](https://github.com/PixarAnimationStudios/USD/pull/1691))

- Added UsdImagingStageSceneIndex, a scene-index native reader of Usd stages.
  Feature support is very limited as this is under development.

- Fixed a bug where disabling scene materials would also disable lights when
  lights were using material network representation.

### Storm
- Filled out Hgi interface for Storm rendering needs
  - Added a capability bit for double-precision support in shaders.
  - Added fields to sampler parameter, graphics pipeline, and attachment state.
  - Added HgiVertexBufferStepFunction, to support Hgi multidraw calls.
  - Started adding Hgi API support for tessellation shaders and
    post-tessellation vertex shaders.
  - Added support for Hgi compute pipelines to specify workgroup size.

- Refactored Storm to use Hgi for graphics API access
  - Augmented HdRenderPassState with further pipeline flags: conservative
    rasterization, depth clamp, depth range.
  - Added the Hgi-based PipelineDrawBatch, which is expected to deprecate the
    GL-specific Immediate- and IndirectDrawBatch classes.  PipelineDrawBatch is
    currently disabled behind a config flag for main render and image shader
    use.
  - Extended HdSt_TextureBinder, HdSt_ResourceBinder, and HdSt_RenderPassState
    to let them use Hgi instead of direct GL calls.
  - Ported HdxShadowTask shadow rendering to use hydra-managed shadow textures,
    rather than GlfSimpleShadowArray.
  - Ported HdxSimpleLightTask to pass in lighting data through the HdSt buffer
    management system, instead of using out-of-band GL uniform blocks. Note that
    as part of this change, there's a new shader API for accessing lighting
    data.
  - Added the "triangulated quad" primitive type, which draws quad geometry as
    triangles.  This is expected to deprecate the current quad rendering with
    adjacency lists, although the latter is supported in HgiGL at the moment.
  - Added support for built-in barycentric coordinates, based on
    NV_fragment_shader_barycentric in GL.
  - Removed geometry shader dependency from many rendering configurations: edge
    rendering & picking, patch coordinate evaluation, face-varying evaluation
    (only available in GL if barycentric coordinates are available).
  - Fixed codegen and shader snippet cross-compilation issues.

- Improved support for MaterialX imaging in Storm:
  - Moved MaterialX parameters out of generated shadercode and into parameter
    buffers to support faster updates.
  - Added support for reading from non-bindless textures.
    (PR: [#1634](https://github.com/PixarAnimationStudios/USD/pull/1634))
  - Added selection highlighting for MaterialX shaders.
    (PR: [#1647](https://github.com/PixarAnimationStudios/USD/pull/1647))
  - Fixed duplicate name error when processing network nodes.
    (PR: [#1659](https://github.com/PixarAnimationStudios/USD/pull/1659))
  - Added shadow support to MaterialX shaders.
    (PR: [#1680](https://github.com/PixarAnimationStudios/USD/pull/1680))

- Fixed typo in HgiVulkan backend.
  (Issue: [#1702](https://github.com/PixarAnimationStudios/USD/issues/1702))

- Extended GLSLFX file format to support resource layout configuration metadata.

- Added shader support for binding arrays of UV textures (as distinct from
  TEXTURE_2D_ARRAY).  Switched shadowmap binding to use arrays of textures, and
  removed bindless shadow texture code.

- Added basis curve builtin primvars: screenSpaceWidths (boolean), for rendering
  UI elements, and minScreenSpaceWidth, for reducing aliasing of thin curves.
  Note that these haven't been formalized in USD.

- Changed meshes with a present but invalid normals primvar to draw with
  shader-generated normals.

- Added support for bool array primvar data.

- Added support for widget rendering via the "widget" rendertag and
  surfaceShader implementation, which uses a draw/blend-on-top configuration.

- Sped up execution of empty render passes.

- Fixed leakage of vertex attrib binding state in HdxPickTask.

- Added shadow support for "distantLight" prim type.

- Fixed evaluation of dome lights and dome light parameters.

- Added support for depthStencil AOV output.

### RenderMan Hydra Plugin
- Added API to request a non-blocking stop of Prman in interactive mode.

- Removed fallback domelight when no lights are provided by the scene.

- Enabled rendering of in-memory VDB volumes in hdPrman. Previously, hdPrman
  only supported VDB volumes from disk.

- Added support (by scene index plugin) for velocity-based motion blur.

- Added a render setting for the Prman backend to use.

- Changed the shutter interval used for processing geometry deformation to come
  from the render camera, rather than render settings.

- Changed light prims to pull parameters from light material networks by
  default, instead of using GetLightParamValue.

- Improved support for offline rendering: e.g. writing results directly to file,
  configuring output via render settings, and rendering synchronously.

- Fixed cookie light filters in hdPrman to provide the appropriate coordinate
  system object.

- Updated hdPrman's fallback surface material to better match Storm, by
  respecting the "displayColor", "displayOpacity", "displayRoughness", and
  "displayMetallic" primvars.

- Improved performance blitting Prman output when running interactively.

- Reworked material filtering operations to use the material network interface.

### usdview
- Added usdviewApi functions to toggle viewer mode and set the renderer plugin.

- usdview plugins can now extend built-in menus like File or Window.

- Increased statistics timer resolution from millisecond to microsecond.

- Added light material network support to usdview builtin lights.

- Fixed uninitialized values when picking from Python.

- Fixed a bug with usdview's composition view when examining prims with
  specializes arcs.

- Added support to set the OCIO display, view, and colorspace.
  (PR: [#1491](https://github.com/PixarAnimationStudios/USD/pull/1491))

## [21.11] - 2021-11-01

Ar 2.0 is enabled by default as of this release. Ar 1.0 is deprecated and will
be removed in a subsequent release. During this transition, Ar 1.0 may still be
enabled by setting `PXR_USE_AR_2=OFF` when running CMake.

### Build
- Added support for building against debug versions of Python. Users must
  specify `PXR_USE_DEBUG_PYTHON=ON` when running CMake.
  (PR: [#1478](https://github.com/PixarAnimationStudios/USD/pull/1478))

- Libraries now use @loader_path instead of absolute paths for RPATHs on macOS.
  (PR: [#1478](https://github.com/PixarAnimationStudios/USD/pull/1478))

- Libraries are now prefixed with "libusd_" on Linux and macOS and "usd_" on
  Windows to avoid name conflicts with other packages. The previous behavior can
  be restored by setting `PXR_LIB_PREFIX="lib"` on Linux and macOS or 
  `PXR_LIB_PREFIX=""` on Windows when running CMake.
  (Issue: [#1615](https://github.com/PixarAnimationStudios/USD/issues/1615))

- Changes and fixes to build_usd.py:
  - Added `--debug-python` and `--build-python-info` options to enable builds
    against debug versions of Python and to control Python to build against.
    (PR: [#1478](https://github.com/PixarAnimationStudios/USD/pull/1478))
  - Added `--use-cpp11-abi` option on Linux to control defininition of
    `_GLIBCXX_USE_CXX11_ABI` when building USD and dependencies.
    (Issue: [#1609](https://github.com/PixarAnimationStudios/USD/issues/1609))
  - Replaced `--debug` option with `--build-variant`.
  - Updated download locations for boost, libtiff, and libpng to avoid
    certificate expiration issues on systems with OpenSSL older than 1.1.0.
  - Updated Blosc to v1.20.1 for compatibility with macOS Catalina (10.15).
  - Fixed issue where the OpenEXR libraries would be installed in the root of
    the filesystem.
    (PR: [#1575](https://github.com/PixarAnimationStudios/USD/pull/1575)

### USD
- Fixed deadlock in NdrRegistry::GetNodesByFamily.

- Changes for Ar 2.0:
  - ArResolver::CreateDefaultContextForAsset now gives all registered resolvers
    an opportunity to provide a default context for an asset.
  - ArResolver::GetModificationTimestamp now returns an ArTimestamp object
    instead of a VtValue. ArTimestamp requires the use of Unix time to
    facilitate asset comparisons across resolvers.
  - Added default implementations for ArResolver::GetExtension and
    GetModificationTimestamp to simplify resolver implementations.
  - Added UsdResolverExample, an example URI resolver implementation.
  - Added more documentation.

- Deprecated SdfLayer::GetExternalReferences and UpdateExternalReference in
  favor of GetCompositionAssetDependencies and UpdateCompositionAssetDependency.

- SdfLayer no longer treats the inability to retrieve a timestamp for a layer or
  its external asset dependencies as an error under Ar 2.0. Calls to
  SdfLayer::Reload will always reload layers with invalid timestamps.

- Updated SdfCreateVariantInLayer and SdfCreatePrimInLayer to use "prepend" when
  creating new variant sets.
  (Issue: [#1552](https://github.com/PixarAnimationStudios/USD/issues/1552))

- Added support for Unicode asset paths on Windows.
  (Issue: [#1443](https://github.com/PixarAnimationStudios/USD/issues/1443),
  PR: [#1580](https://github.com/PixarAnimationStudios/USD/pull/1580))

- Fixed bug that caused errors when reading large .usda files.

- Code invoked from Python that accesses invalid UsdPrim objects in C++ will now
  emit a Python exception instead of crashing.

- Fixed issue where ArResolver::OpenAsset would be called multiple times when
  opening an ASCII .usd file or a .usdz file.
  (Issue: [#1613](https://github.com/PixarAnimationStudios/USD/issues/1613))

- Fixed bugs in gfGenCode and usdGenSchema where file objects were not closed,
  leading to ResourceWarnings from Python.
  (PR: [#1557](https://github.com/PixarAnimationStudios/USD/pull/1557))

- Updated usdGenSchema to mark plugInfo as a resource instead of a library when
  codeless schemas are being generated via skipCodeGen option.

- Fixed bugs where changes to a layer's resolved path would not trigger
  recomposition of prims when needed.

- Fixed bug that caused crashes during composition.
  (PR: [#1568](https://github.com/PixarAnimationStudios/USD/pull/1568))

- Fixed bug where UsdNotice::ObjectsChanged notice emitted after call to
  UsdStage::Reload would contain pointers to destroyed objects.
  (Issue: [#1605](https://github.com/PixarAnimationStudios/USD/issues/1605)

- Fixed bug where UsdUtilsComplianceChecker returned a false error due to not
  recognizing the `UsdTransform2d` shader.
  (PR: [#1625](https://github.com/PixarAnimationStudios/USD/pull/1625))

- Fixed issue where UsdUtilsExtractExternalReferences would mutate the input
  layer unnecessarily.
  (Issue: [#1573](https://github.com/PixarAnimationStudios/USD/issues/1573)

- Added UsdGeomVisibilityAPI. This schema provides attributes to control prim
  visibility based on the "purpose" attribute from the UsdGeomImageable schema.
  It will eventually be the place where primary visibility and purpose are also
  housed and authored.

- Added UsdGeomImageable::ComputeEffectiveVisibility for computing purpose
  visibility.

- Removed unused UsdGeomImageable::ComputeVisibility overload that took a
  parentVisibility parameter.

- Fixed broken Python bindings for UsdPhysics on Windows under Python 3.8+.

- Fixed missing Python bindings and "kilogramsPerUnit" metadata in UsdPhysics.
  (PR: [#1606](https://github.com/PixarAnimationStudios/USD/pull/1606))

- Deleted many previously deprecated UsdRi schema classes, namely, UsdRiRisBxdf,
  UsdRiRisIntegrator, UsdRiRisObject, UsdRiRisOslPattern, UsdRiRisPattern,
  UsdRiRslShader and all UsdRiLight and UsdRiLightFilters which are now part of
  the usdRiPxr schema domain.

- Updated connectedAPIBehavior so that APISchemas can now provide
  connectableAPIBehavior by explicitly registering a new behavior associated
  with the APISchemaType or by providing plug metadata to configure the
  connectability behavior. Note that the latter can be used for codeless
  schemas.

- Fixed a couple of rare bugs associated with UsdShadeConnectableAPIBehavior and
  UsdGeomComputeExtentFunction, resulting in robust behavior when new plugins
  are registered.

- Updated UsdUtilsUpdateSchemaWithSdrNodes to include new concepts like addition
  of shaderId attribute to the generated schema classes, provide
  apiSchemaCanOnlyApplyTo, and provide ability to control
  UsdShadeConnectableAPIBehavior using metadata on the sdrNodes. Refer to the
  updated documentation for more details.

- Updated usdgenschemafromsdr to be able to provide sdr nodes via explicit asset
  paths and to parse and add renderContext related information from the json
  config.

- Added support for different Sdr to Sdf type mappings. Introducimg a new
  "sdrUsdDefinitionType" metadata field for sdrShaderProperty, which is used to
  distinguish between sdrDefaultValue and equivalent sdfTypeDefaultValue. Note
  that for backward compatibility SdrShaderProperty::GetDefaultValue currently
  returns the same value as SdrShaderProperty::GetSdfTypeDefaultValue, this is
  guarded by an environment variable "SDR_DEFAULT_VALUE_AS_SDF_DEFAULT_VALUE".

- Single apply API schemas can now include built-in API schemas and can auto
  apply to other single apply API schemas. API schemas are now only allowed to
  inherit directly from UsdAPISchemaBase.

- Applied API schemas can no longer inherit from other other applied API
  schemas, which allows UsdPrim::HasAPI() to be a much faster query. Non-applied
  schemas may still inherit from other non-applied schemas.

- Added UsdLuxLightAPI, which can be applied to geometry to create a light.
  UsdLuxLight has been deleted. Added the applied API schemas UsdLuxMeshLightAPI
  and UsdLuxVolumeLightAPI, which include UsdLuxLightAPI as a builtin.
  - The core lights UsdLuxCylinderLight, UsdLuxDiskLight, UsdLuxRectLight,
    UsdLuxSphereLight, and UsdLuxPortalLight all inherit from from
    UsdLuxBoundableLightBase, are now Boundable, and have extents/bbox
    computations.
  - The core lights UsdLuxDomeLight and UsdLuxDistantLight now inherit from
    UsdLuxNonboundableLightBase and are not Boundable.
  - Deprecated UsdLuxGeometryLight
  - Deleted deprecated UsdLuxLightPortal (use UsdLuxPortalLight instead).
  - Fixed issue where UsdLux would register default SdrShaderNodes with 'USD'
    source types for lights defined outside of UsdLux.
  - Default SdrShaderNodes registered for UsdLux lights now include shadow and
    shaping API properties.

### Imaging
- First iteration of the Scene Index and Data Source system is now on by default
  and compatible with existing scene and render delegates via emulation. No new
  functionality is yet exposed as this is thus far intended to validate
  correctness and performance. This can be disabled by setting an
  HD_ENABLE_SCENE_INDEX_EMULATION environmental variable to "0".

- Changed the return type of UsdImagingDelegate::Get() for the point instancer
  "rotate" attribute from GfVec4f (quaternion layout of <real, i, j, k>) to
  GfQuath (quaternion layout of <i, j, k, real>). This aligns with the result
  of SamplePrimvar(). In hdStorm, the quaternion data is now transported to the
  GPU as half-float data, instead of float.
  All custom scene delegates supplying the instancer "rotate" primvar via the
  Get() API must be updated for the new quaternion layout.

- Added half-float vec2 and vec4 primvar support to hdStorm.

- Added support to UsdImaging for new LightAPI features.

- Added sprim-sprim dependency map to HdChangeTracker so it can track the
  dependencies between lights and their light filters.

- UsdImagingMeshAdapter only keeps track of geom subsets that are using the
  materialBind family.

- UsdImaging internally manages which prims to call UpdateForTime on, instead of
  relying on hydra to maintain that list.

- Extended HdxTaskController to support built-in lights for backends like
  HdPrman.

- When filtering primvars by material in UsdImagingGprimAdapter, added filtering
  using materials bound to geom subsets.

- Improved dirty list management and performance including moving to a single
  dirty list managed by the render index.

- Deleted PrimvarDescCache::Extract and GarbageCollect, and
  UsdImagingDelegate::PostSyncCleanup.

- Deleted HdRenderPass::Prepare.

- Improved UsdImagingDelegate performance when editing many instances of the
  same prototype.
  (PR: [#1608](https://github.com/PixarAnimationStudios/USD/pull/1608))

- Updated stb_image code to latest versions, enable UTF-8 filename support for
  Windows.
  (PR: [#1580](https://github.com/PixarAnimationStudios/USD/pull/1580))

- Fixed sync valid repr tokens just once for Rprims.

- Fixed memory leak in HioOpenVDB.

- Fixed HioOpenVDB build on windows.
  (PR: [#1574](https://github.com/PixarAnimationStudios/USD/pull/1574))

- Fixed errors when trying to read udims from .usdz files.
  (Issue: [#1558](https://github.com/PixarAnimationStudios/USD/issues/1558))

- Fixed HdResampleRawTimeSamples() parameter bug and add test coverage.
  (Issue: [#1549](https://github.com/PixarAnimationStudios/USD/issues/1549))

- Fixed crash when processing multiple native instancer edits in the same frame.
  (Issue: [#1551](https://github.com/PixarAnimationStudios/USD/issues/1551))

- Fixed GprimAdapter::_AddRprim material resolution order to be consistent with
  other functions.
  (Issue: [#1626](https://github.com/PixarAnimationStudios/USD/issues/1626))

- Fixed to not normalize point instancer quaternions.
  (Issue: [#1505](https://github.com/PixarAnimationStudios/USD/issues/1505))

### Storm
- Added support for rendering meshes with geom subsets.

- Added support for rendering DomeLights as a skydome.
  (Issue: [#1385](https://github.com/PixarAnimationStudios/USD/issues/1385))

- Added support to rendering BasisCurves that don't completely cover the
  supplied points or other primvars.

- Added support for "constant shading" of basis curves wherein they're
  unaffected by lighting.

- Added support for screen-space flat normals.

- Improved 1st order approximations of the basic area lights (SphereLight,
  DiskLight, RectLight, DistantLight, and CylinderLight).

- Moved barycentric coord generation to codeGen.

- Consolidated edge param calculations.

- Implemented IsValid() for HdStUdimTextureObject.

- Sync visibility once per-Rprim sync rather than per draw item.

- Even if there are zero lights, prims should still use the lighting integrator,
  specifically the one specified by their shader key.

- Moved bindless address fetch out of HdStBufferResource.

- Changing the type storing the volume shader glslfx source code and material
  params in an HdStMaterial from HdStShaderCode to struct
  HdStMaterial::VolumeMaterialData.

- When a scene delegate provides a network without volume output for a material
  used by a volume, use the volume fallback shader.

- Removed IsFragmentOnPoint() and IsFragmentOnEdge() shader methods.

- Renamed HdStSurfaceShader to HdSt_MaterialNetworkShader.

- Fixed domelights to respond to visibility changes.

- Fixed crash when UDIM textures are missing.

- Fixed to support all sizes of domelight textures.

- Fixed GS smooth fvar interp for displacement

- Fixed GL version test to include minor version.
  (Issues: [#957](https://github.com/PixarAnimationStudios/USD/issues/957) and
  [#1001](https://github.com/PixarAnimationStudios/USD/issues/1001)

- Various improvements to Hgi:
  - Moved bindless texture management to HgiGL.
  - Exposed general and device specific capabilities.
  - Improved staging buffer for unified memory devices.
  - Use shaderc to compile GLSL to SPIR-V in HgiVulkan.
  - Check buffer storage mode before blit sync in HgiMetal.

### RenderMan Hydra Plugin
- Added support for UsdPreviewSurface's displacement in HdPrman.

- Added support for UsdUvTexture's "sourceColorSpace" input and fixed normal
  mapping.

- Added support for additional clip planes.

- Added flipped flag to RtxHioImage plugin.

- Adjusted HdPrman's fallback volume shader to match Storm's.

- Introduced codeless usdRiPxr schema domain for RenderMan lights and
  lightfilters. These are generated using usdgenschemfromsdr from rman light and
  lightfilter args files. Note that these schemas are compatible with RenderMan
  24.2.

- Removed use of "allowPresenceWithGlass" from UsdPreviewSurfaceParameters node.

- RtxHioImage can convert various texture formats to type float, so they can be
  passed to Renderman.
  (Issue: [#1313](https://github.com/PixarAnimationStudios/USD/issues/1313))

- Fixed HdPrman's fallback maxSamples to match the RenderMan fallback (64).

### MaterialX
- Updating the arguments passed into mx::loadLibraries to better accommodate
  relocating the MaterialX libraries folder.
  (Issue: [#1586](https://github.com/PixarAnimationStudios/USD/issues/1586))

## [21.08] - 2021-07-15

Ar 2.0 is feature-complete as of this release. We anticipate addressing
feedback and bug reports, deprecating Ar 1.0 and enabling Ar 2.0 by default
for the next release, then removing Ar 1.0 entirely in the release after
that. Developers should begin migrating to Ar 2.0 now.

RenderMan 24 is now required for the RenderMan plugin as of this release.
Support for RenderMan 23 was deprecated in 21.05 and has now been removed.

### Build
- Updated dependencies to align with VFX Reference Platform CY2019. In
  particular, USD now requires TBB 2018 or greater.
- Added support for Python 3.8+ on Windows. The usd-core PyPI package now
  supports Python 3.8 and 3.9 on all platforms.
  (PR: [#1511](https://www.github.com/PixarAnimationStudios/USD/pull/1511),
   [#1512](https://www.github.com/PixarAnimationStudios/USD/pull/1512),
   [#1513](https://www.github.com/PixarAnimationStudios/USD/pull/1513))
- Fixed build errors on ARM64 on Linux.
  (PR: [#1453](https://www.github.com/PixarAnimationStudios/USD/pull/1453))
- Fixed various build errors due to missing or bad includes on gcc 11,
  clang 11, clang 13, and for Android or emscripten.
  (PR: [#1497](https://www.github.com/PixarAnimationStudios/USD/pull/1497),
   [#1499](https://www.github.com/PixarAnimationStudios/USD/pull/1499),
   [#1530](https://www.github.com/PixarAnimationStudios/USD/pull/1530),
   Issue: [#1546](https://www.github.com/PixarAnimationStudios/USD/pull/1546))
- Fixed issue in build_usd.py with some versions of PySide2 on Windows.
  (PR: [#1454](https://www.github.com/PixarAnimationStudios/USD/pull/1454))

### USD
- Fixed bug with symbol demangling on Windows which broke some TfType alias
  registrations.
  (PR: [#1510](https://www.github.com/PixarAnimationStudios/USD/pull/1510))
- Replaced some uses of deprecated tbb::atomic with std::atomic.
  (PR: [#1486](https://www.github.com/PixarAnimationStudios/USD/pull/1486))

- Numerous changes to the Work library to adhere to TBB thread limits set by
  host applications.
  (Issue: [#1368](https://www.github.com/PixarAnimationStudios/USD/pull/1368)):
  - WorkGetConcurrencyLimit now returns tbb::this_task_arena::max_concurrency
  - Replaced WorkArenaDispatcher and other uses of task_arena in favor of new
    WorkWithScopedParallelism, which uses TBB's task isolation feature.

- Many changes for Ar 2.0:
  - Removed the majority of deprecated APIs, including AnchorRelativePath,
    FetchToLocalResolvedPath, ConfigureResolverForAsset, and others.
  - UsdStage::Refresh now calls ArResolver::RefreshContext to let resolvers
    update any internal state. Resolvers can send a ArNotice::ResolverChanged
    notice in that function to inform UsdStage and other listeners of any
    changes.
  - Added default implementations for context management and scoped
    caching to make it easier to implement resolvers. Note that resolvers
    that provide their own implementations must indicate this in the
    plugInfo.json file. See ArResolver documentation for more details.

- Added UTF-8 support for reading and writing asset paths.
  (Issue: [#1443](https://www.github.com/PixarAnimationStudios/USD/pull/1443))
- UTF-8 sequences are now written into .usda layers without hex encoding.
- Embedded NUL bytes are disallowed in string values in .usda layers.
- Dynamic file formats can now be set up to use anonymous layer identifiers to
  avoid the need for dummy resolvable assets.
  (Issue: [#1356](https://www.github.com/PixarAnimationStudios/USD/pull/1356))
- Made virtual SdfFileFormat functions protected instead of private.
  (PR: [#1519](https://www.github.com/PixarAnimationStudios/USD/pull/1519))
- Fixed bug where a layer would be dropped from the Sdf layer registry if its
  identifier were changed to an existing layer's via SdfLayer::SetIdentifier.
- Fixed SdfLayer::SetIdentifier to use same restrictions as SdfLayer::CreateNew.
- Fixed bug where entire layer would be parsed when reading layer metadata only
  from .usda layers via SdfLayer::OpenAsAnonymous if no such metadata existed.
- Fixed bug with vector usage during calls to SdfLayer::TransferContent and
  other methods that caused debug asserts on Windows.
  (Issue: [#1501](https://www.github.com/PixarAnimationStudios/USD/pull/1501))
- Fixed output of PcpPrimIndex::PrintStatistics on Windows.
  (PR: [#1480](https://www.github.com/PixarAnimationStudios/USD/pull/1480))

- Added support for "codeless" USD schemas that do not have any C++ or Python
  code. Changes to these schemas' definitions do not require any code to be
  recompiled; instead, the generatedSchema.usda just needs to be regenerated
  via usdGenSchema and reinstalled.

- Added usdgenschemafromsdr utility to generate schemas from Sdr nodes.
- Added ability to specify allowed prim types for API schemas in the customData
  dictionary for the schema in schema.usda.
- Added UsdPrim::CanApplyAPI and CanApply to all API schema classes to check if
  an API schema can be applied to a prim based on the restrictions above.
- Removed deprecated instancing "master" API from UsdStage and UsdPrim.
- Removed support for deprecated "UsdSchemaType" terminology in schemas.
- Fixed UsdStage::ResolveIdentifierToEditTarget to be consistent with asset
  path computations during composition.
- Fixed bug where UsdResolveInfo / UsdAttributeQuery would be invalidated by an
  insignificant layer stack change.
  (Issue: [#1500](https://www.github.com/PixarAnimationStudios/USD/pull/1500))
- Improved change processing performance in UsdStage. In an internal test that
  authored many attributes, runtime decreased by ~50% (from ~50s to ~25s).
- A UsdConnectableAPIBehavior can now be set to ignore shader encapsulation
  rules by overriding UsdConnectableAPIBehavior::RequiresEncapsulation.
- API schemas can now provide connectability behavior, which can override
  behavior based on prim type.
- Deprecated UsdShadeMaterial::ComputeSurfaceSource, ComputeVolumeSource, and
  ComputeDisplacementSource overloads taking a single token for renderContext.
- Renamed SdfTypeIndicator to NdrSdfTypeIndicator.
- Added initial version of UsdPhysics domain and schemas. For more details, see
  the [proposal](https://graphics.pixar.com/usd/docs/Rigid-Body-Physics-in-USD-Proposal.html).
  (PR: [#1424](https://www.github.com/PixarAnimationStudios/USD/pull/1424))
- Miscellaneous documentation fixes.
  (PR: [#1485](https://www.github.com/PixarAnimationStudios/USD/pull/1485),
   [#1553](https://www.github.com/PixarAnimationStudios/USD/pull/1553))

### Imaging
- Added basic support for Hydra Commands, a mechanism to invoke actions that
  renderers know how to perform.
- Added support for loading OpenVDB grids via ArAsset, allowing OpenVDB files
  to be read from within .usdz files.
- Added support for cameras in the testing Null render delegate.
- Added new rendertask param to disable clipping.
- Added UsdImagingGLEngine::SetPresentationOutput to explicitly set the
  framebuffer that AOVs are presented into.
- Fixed buffer allocation in vboSimpleMemory Manager.
- Fixed issue in UsdImaging that would occur when a CoordSys is bound to an
  Xform prim with a renderable descendants.
- Fixed resync of shaders in UsdImaging to also resync geometry bound to the
  shader's material.
  (Issue: [#1487](https://www.github.com/PixarAnimationStudios/USD/pull/1487))
- Fixed handling of edits to "purpose" in the native instance adapter in
  UsdImaging.
  (Issue: [#1515](https://www.github.com/PixarAnimationStudios/USD/pull/1515))
- Fixed unwanted export of stbImage symbols from Hio.
  (PR: [#1498](https://www.github.com/PixarAnimationStudios/USD/pull/1498))
- Allow point instancer prototypes from overs in UsdImaging.
  (PR: [#1436](https://www.github.com/PixarAnimationStudios/USD/pull/1436))
- Removed HdxMaterialTagTokens in favor of HdStMaterialTagTokens.

### usdview
- usdview and usdrecord now share the argument parsing facility for renderers.
- Fixed issue where usdview would sometimes open in the background on Windows.
  (PR: [#1537](https://www.github.com/PixarAnimationStudios/USD/pull/1537))
- Fixed to not error or warn in Rollover Prim Info if assetInfo has no
  identifier entry.
  (PR: [#1476](https://www.github.com/PixarAnimationStudios/USD/pull/1476))
- Fixed Qt window being displayed in usdrecord.
  (Issue: [#1521](https://www.github.com/PixarAnimationStudios/USD/pull/1521))

### Storm
- Added support for correct refinement of varying and face-varying primvars.
- Added support for compute program preprocessor defines.
- Added size validation for uniform primvars of basis curves.
- Improved performance of volume ray marching shader.
- Improved wireframe on surface display for adaptive tessellation.
- Improved wireframe on surface display for uniform refinement.
- Improved edge picking performance.
- Improved performance and appearance of patch edge drawing.
- Improved GPU memory usage by using a non-aggregated allocation for GPU stencil
  table buffers.
- Moved mesh edge handling out of the geometry shader.
- Changed default material tag in HdxTaskController to enable alpha-to-coverage.
- Changed behavior, when a dome light texture is invalid, issue a coding error
  and print the file path for debugging.
- Changed behavior, updated selection on tracker during Prepare instead of Sync.
- Use the new texture system to bind the texture in the render pass shader.
- Removed redundant ptexId field from patchParam.
- Removed HdStExtCompGpuComputationBufferSource.
- Removed remaining GS edge id shader code.
- Removed HdStMixinShader since its usage was very limited.
- Fixed multiple issues related to OSD drawing.
- Fixed Ptex layout texture sampler type.
- Fixed TCS/TES processing of primvar and drawingCoord.
- Fixed incorrect handling of left-handed meshes.
  (Issue: [#1075](https://www.github.com/PixarAnimationStudios/USD/pull/1075))
- Fixed resolution of symlinked UDIM textures.
  (Issue: [#1329](https://www.github.com/PixarAnimationStudios/USD/pull/1329))
- Fixed area light approximation when transforms are animated.
  (Issue: [#1524](https://www.github.com/PixarAnimationStudios/USD/pull/1524))

- Various improvements to Hgi:
  - Added 1D texture array upload support in HgiMetal.
  - Added 16-bit signed integer data formats.
  - Added buffer binding support for Hgi shader codegen.
  - Added codeGen for compute shader invocation ID.
  - Added HgiShaderFunctionDesc creation path to HdStGLSLProgram.
  - Moved Storm compute functions to Hgi shader generation.
  - Removed now unnecessary restoration of framebuffer in HgiGLScopedStateHolder
    and HgiInteropOpenGL.
  - Converted primvar refinement to Hgi compute shaders.

### RenderMan Hydra Plugin
- Updated RenderMan version required by HdPrman to 24.
- Unified HdPrman & HdxPrman into HdPrman.
- Added basic support to optionally use MaterialX networks. (see below)

### Alembic Plugin
- Changed default number of threads used for reading Ogawa-backed files from
  4 to 1. This may be changed with the `USD_ABC_NUM_OGAWA_STREAMS` env var.

### MaterialX Plugin
- Added support for reading MaterialX documents within .usdz files
  when Ar 2.0 is enabled.
- Imaging support:
    - Moved the code common to both Storm and HdPrman into imaging/hdMtlx.
    - Allow the output of a nodegraph to be used multiple times.
      (Issue: [#1489](https://www.github.com/PixarAnimationStudios/USD/pull/1489))
    - Update _GetMxNodeType to use MaterialX functions to support arbitrary node
      types not just UsdPreviewSurface and StandardSurface.
      (Issue [#1538](https://www.github.com/PixarAnimationStudios/USD/pull/1538))
    - Fixed issue with incorrect paths for finding nodes.
      (Issue: [#1488](https://www.github.com/PixarAnimationStudios/USD/pull/1488))
    - (Storm support) Added support for different primvar texture coordinate
      names to be passed down through the mtlx file.
    - (Storm support) Added support for textures to be used anywhere within the
      connected nodegraph instead of just as inputs for the material properties.
    - (Storm support) Added basic support for transparent MaterialX materials
      making sure the proper material tag is set.
    - (HdPrman support) Added basic support to use MaterialX networks.
    - (HdPrman support) Updated StandardSurface adapter node to pass the normal
      to PxrSurface for the bumpNormal.

### Security
- Fixed [CVE-2020-13520](https://talosintelligence.com/vulnerability_reports/TALOS-2020-1120)
- Fixed [CVE-2020-13531](https://talosintelligence.com/vulnerability_reports/TALOS-2020-1145)

## [21.05] - 2021-04-12

### Build
- Removed unnecessary check for Python executable in build_usd.py.
  (Issue: [#1414](https://www.github.com/PixarAnimationStudios/USD/issues/1414), 
  PR: [#1464](https://www.github.com/PixarAnimationStudios/USD/pull/1464))
- Updated build_usd.py to use MaterialX v1.38.0 and unified the MaterialX plugin 
  and imaging build options PXR_BUILD_MATERIALX_PLUGIN and 
  PXR_ENABLE_MATERIALX_IMAGING_SUPPORT into PXR_ENABLE_MATERIALX_SUPPORT.
- Exposed version numbers in pxrConfig.cmake for CMake client projects.
- Updated CHANGELOG.md to include links to pull requests and issues. 
  (Issue: [#1362](https://www.github.com/PixarAnimationStudios/USD/issues/1362))
- Fixed linking errors by removing extern C linkage from LZ4 symbols, to avoid 
  collisions with other libraries embedding LZ4. 
  (Issue: [#1447](https://www.github.com/PixarAnimationStudios/USD/issues/1447))
- Updated VERSIONS.md to include  Linux, Windows and MacOS versions used for 
  testing. Note that Windows and MacOS should still be considered experimental. 
  (PR: [#1393](https://www.github.com/PixarAnimationStudios/USD/pull/1393))

### USD
- Added UsdUtils.UpdateSchemaWithSdrNode functionality for creating or updating 
  a schema.usda from specially curated sdrShaderNode and sdrShaderProperty 
  metadata.
- Updated usdGenSchema to not error when generating a dynamic schema and 
  libraryPath is not specified.
- The default 'USD' SdrShaderNodes generated from UsdLuxLightFilter types now 
  have the context of "lightFilter" like other light filter shader nodes.
- Fixed bug where the registered URI resolver would not be invoked for a URI 
  asset path if it was shorter than the longest registered URI schema.
- The template asset path field for value clips now supports Sdf file format 
  arguments. 
  (PR: [#1450](https://www.github.com/PixarAnimationStudios/USD/pull/1450))
- SdfLayer::ListFields now includes "required fields" for each spec type.
- A collection of fixes and tests for Windows path handling, especially 
  involving symlinks. 
  (PR: [#1441](https://www.github.com/PixarAnimationStudios/USD/pull/1441),
  [#1465](https://www.github.com/PixarAnimationStudios/USD/pull/1465), 
  [#1468](https://www.github.com/PixarAnimationStudios/USD/pull/1468),
  Issues: [#1440](https://www.github.com/PixarAnimationStudios/USD/issues/1440))
- Fixed usdGenSchema to honor apiName metadata for auto generated tokens.h. This 
  results in correct doxygen documentation for the correct API names.
- UsdUtilsStitchClips now creates a separate manifest layer instead of using the 
  topology layer as the manifest layer. This is because the topology layer can 
  contain attributes that actually don't have any time samples in clips. 
  (Issue: [#1344](https://www.github.com/PixarAnimationStudios/USD/issues/1344))
- Added documentation for built-in and auto-applied API schemas.
- Added additional plugInfo mechanism (via AutoApplyAPISchemas metadata) to 
  define auto apply API schemas, to allow context/site specific auto application 
  of API schemas.
- Added FlattenTo function to UsdPrimDefinition to copy the contents of a prim 
  definition into a prim spec on a layer.
- Updated UsdShadeConnectableAPI::Get{Inputs,Outputs} to only return authored 
  inputs/outputs by default. Optional onlyAuthored=false argument can be passed 
  to explicitly include Builtins. Results in performance improvement at code 
  sites like UsdShadeShaderDefUtils.
- UsdLux Shadow and Shaping APIs are now connectable. All attributes on these 
  APIs have been given the "inputs:" prefix.
- Adding support for identifier aliases in Ndr/Sdr. This allows NdrRegistry to 
  use these aliases to find the node through the GetNodeByIdentifierAPIs when 
  there are no appropriate nodes that exactly match the identifier being 
  searched for.
- Performance improvement in terminal resolution in UsdShade. Introduced a 
  `shaderOutputOnly` flag on the GetValueProducingAttributes function, this 
  allows us to get only the type of connections we want and avoid the cost of 
  the extra checks.
- Updated NdrRegistry::GetNodeFromAsset() to use asset name and extension as the 
  node name instead of the identifier. Note that node name is not expected to be 
  unique for a NdrNode.
- Fixed a pcp change processing bug where we did not consider layers with 
  replaced content significant wrt layer stacks that use the layer. 
  (Issue: [#1407](https://www.github.com/PixarAnimationStudios/USD/issues/1407))
- Removed UsdShadeConnectableAPI implicit constructors from UsdShadeShader and 
  UsdShadeNodeGraph to enforce independence of connectability concept from 
  shader graphs. Callers are required to convert these explicitly now, which can 
  be done via the ConnectableAPI() method on Shader and NodeGraph.
- Updated connectability rules in UsdShadeConnectableAPIBehavior to improve 
  encapsulation. Refer to the UsdShade documentation for details on 
  connectability rules.
- usdGenSchema now adds a type alias for abstract typed schemas, which means 
  abstract typed schemas now have an official USD type name which is registered 
  with the UsdSchemaRegistry.
- Auto apply API schemas in USD can now specify abstract typed schemas to auto 
  apply to, indicating that the API should be applied to all schemas that derive 
  from the abstract schema type.
- Removed deprecated UsdConnectableAPI::IsShader() and IsNodeGraph() APIs.
- Fixed performance regression in UsdStage change processing when applying an 
  API schema.
- Make OSL parser for Sdr more robust to dynamic array type names.
- Backwards compatibility for the deprecated "UsdSchemaType" terminology will be 
  removed in the next release. Schemas that have not been updated since v21.02 
  must be regenerated to avoid breakage in the next release.


### Imaging
- Added SampleIndexedPrimvar() and GetIndexedPrimvar() scene delegate API to 
  allow users to access unflattened primvars and their indices.
- Added support for time-sampled computed primvars in the scene delegate API 
  (including an UsdImaging implementation for Skel). 
  (PR: [#1455](https://www.github.com/PixarAnimationStudios/USD/pull/1455))
- Added GetInstancerPrototypes() scene delegate API to return prim paths that 
  are prototypes of an instancer.
- Added support for GetMaterialResource() to the lightAdapter and 
  lightFilterAdapter classes so the usdImaging scene delegate can provide 
  HdMaterialNetworks for lights and filters.
- Added imaging adapters for UsdLuxPluginLight and UsdLuxPluginLightFilter.
- Added optimization when checking primvar existence, use Get() instead of 
  ComputeFlattened().
- Renamed render delegate API GetMaterialNetworkSelector() to 
  GetMaterialRenderContexts() to allow for multiple material render contexts.
- Moved version tracking of batches from HdChangeTracker to Storm's render 
  delegate.
- Moved garbage collection API to Storm's render delegate.
- Refactored the free camera code in HdxTaskController::_Delegate into its own 
  HdxFreeCameraSceneDelegate.
- Removed obsolete HdTexture bprim.
- Various improvements to Hio:
   - Refactored reading of HioImage and the conversion to an HgiFormat into 
     common functionality.
   - Moved volume field texture loading out of Glf. Volume data is loaded via a 
     plugin point (hioOpenVDB) in HioFieldTextureData.
   - Replaced GlfGetNumElements(HioFormat) and GlfGetElementSize(HioFormat) with 
     equivalent functions in Hio.
   - The HioOIIO OpenImageIO plugin supports format conversion when loading 
     image data, so we specify the target format in the destination image spec.
   - As the transition to Hio (and our new texture system) continues to move 
     forward, we are now removing many entry points in Glf.
- Fixed crash on Hydra update when a prototype primitive is removed (deleted or 
  deactivated) in the same update cycle where an inheritable primvar is added or 
  changed to an ancestor of the prototype that used to exist. 
  (PR: [#1418](https://www.github.com/PixarAnimationStudios/USD/pull/1418), 
  Issue: [#1417](https://www.github.com/PixarAnimationStudios/USD/issues/1417))
- Fixed skip creating unsupported AOVs in the task controller. 
  (Issue: [#1215](https://www.github.com/PixarAnimationStudios/USD/issues/1215))
- Fixed non-color AOV visualization. 
  (Issue: [#1437](https://www.github.com/PixarAnimationStudios/USD/issues/1437))
- Fixed material binding improvements for point instancer prototypes. 
  (Issue: [#1449](https://www.github.com/PixarAnimationStudios/USD/issues/1449))
- Fixed instance highlighting for native instances when the selected instance is 
  invisible. 
  (Issue: [#1461](https://www.github.com/PixarAnimationStudios/USD/issues/1461))
- Fixed PointInstancerAdapter Get() to properly call the fallback if it can't 
  find the requested key. 
  (Issue: [#1439](https://www.github.com/PixarAnimationStudios/USD/issues/1439))

### Storm
- Added filtering of instancer primvar bindings.
- Added shader configurations, mixins and a new material tag 
  'translucentToSelection' to enable occluded selection to show through.
- Added HdRenderPassState API to enable/disable the depth test.
- Added better support for a color mask to be specified per color attachment 
  rather than use the same mask for all attachments.
- Added SetPresentationOutput() API specifies the framebuffer the present task 
  will write the AOVs into.
- Added casts to avoid GLSL compile errors when enabling 
  HD_ENABLE_DOUBLE_MATRIX and using the volume raymarcher.
- Added GlfAnySharedGLContextScopeHolder that is a conditional 
  GlfSharedGLContextScopeHolder only switching the shared context if there is no 
  current valid context.
- Added number of texture objects and handles to 
  HdStResourceRegistry::GetResourceAllocation.
- Changed Ptex layout texture format in order to improve compatibility on 
  platforms which don't support 3-component textures.
- Disabled bindless textures while we investigate unexpected performance 
  regressions related to switching context and bindless.
- Optimization to bail out early for Storm tasks that have no work.
- Avoid invalidation in the renderpass shader when processing buffer binding 
  requests.
- Refactored basis curves points primvar processing such that it uses 
  HdSt_BasisCurvesPrimvarInterpolaterComputation.
- Fixed to trigger draw item gather when material tag changes
- Fixed HgiBufferDesc byteSize calculations to avoid reading beyond initialized 
  data when creating buffer objects.
- Renamed renderPassShader.glslfx to renderPassColorShader.glslfx to clarify its 
  intent.
- Renamed rprimUtils to primUtils to reduce code duplication.
- Removed unused mixin option to discard unselected fragments.
- Removed Bind/Unbind HdRenderPassState API
- Removed GlfHasLegacyGraphics - use GlfContextCaps instead.
- Removed render pass state override shader.
- Various improvements to Hgi:
   - Added support to pass an external framebuffer to HgiInterop.
   - Renamed HgiMipInfo::byteSize to HgiMipInfo::byteSizePerLayer for 
     clarification.
   - Moved HgiShaderSection::WriteAttributeToIndex to HgiMetalShaderSection.
   - Unified how attributes are stored across GL, Metal and Vulkan.
   - Fixed resolve op to transfer data from/to all attachments.
   - Fixed a potential bug where the wrong size was computed for a compressed 
     array texture.
   - Fixed code generation for unsigned and signed integer textures.
   - Fixed redundant depth buffer resolve and ensure that in the absence of 
     color attachments, it still gets resolved.
   - Fixed Metal error related to resource lifetime.

### RenderMan Hydra Plugin
- Updated RenderMan version required by hdPrman to 23.5.
- NOTE: The USD release following the RenderMan 24 release will increase the 
  required version once more to 24.0. In other words, this will likely be the 
  last USD release with hdPrman support for RenderMan 23.x.
- Made the rman args parser map `bxdf` types inputs to the Sdr `terminal` type.
- Finishing work to support fieldDataType and vectorDataRoleHint for UsdVol 
  prims in hdPrman.


### MaterialX Support
- Minimum of MaterialX v1.38 is now required.
- MaterialX support in Storm:
   - Added basic support for textures.
   - Added basic support for direct lights.
   - Added a Mtlx render context, which gets consumed by Storm's render 
     delegate.
   - Register MaterialX nodes as having "mtlx" source type and allow Storm's 
     material network to accept both glslfx and mtlx sourcetypes.
   - Changed the MaterialX option from FIS to Specular Environment Prefilter.
   - Fixed u_envMatrix calculation to account for y-up/z-up/domelight 
     transform.
- Various improvements to the usdMtlx plugin:
   - Record "geomprop" references from materialx into Sdr's "primvars" 
     metadata. This lets clients query which primvars the material wants access 
     to (for purposes such as primvar filtering).
   - No longer copying default values on shader inputs to each USD shader 
     definition to improve performance and keep it more consistent with other 
     UsdShade constructs.
   - Removed deprecated references to shaderref, Color2.
   - Fixed a crash in UsdMtlx when reading mtlx nodes that use a MaterialX 
     standard shader nodedef.

### usdview
- Fixed a bug where we weren't updating for changes that affect the entire 
  stage. 
  (Issue: [#1407](https://www.github.com/PixarAnimationStudios/USD/issues/1407))
- Improved testusdview error reporting 
  (PR: [#1416](https://www.github.com/PixarAnimationStudios/USD/pull/1416))

## [21.02] - 2021-01-18

### Build
- Added build and packaging scripts for PyPI packages. Thanks to Nvidia for
  their work on this project!
- Updated documentation build to fix several issues and no longer require 
  Python. (Issue: [#718](https://www.github.com/PixarAnimationStudios/USD/issues/718))
- GLEW is no longer required for building imaging components.

- Various fixes and changes to build_usd.py:
  - Fixed command-line args not being respected for OpenVDB. 
    (PR: [#1406](https://www.github.com/PixarAnimationStudios/USD/pull/1406))
  - Updated boost to 1.70 on macOS for both Python 2 and 3. 
    (Issue: [#1369](https://www.github.com/PixarAnimationStudios/USD/issues/1369))
  - Updated MaterialX to 1.37.3 with shared libraries on Linux.

- Fixed various linking issues with OpenEXR. 
  (PR: [#1398](https://www.github.com/PixarAnimationStudios/USD/pull/1398))

### USD
- Allow setting malloc hook functions if they were previously set to malloc/etc.
- Fixed handling of symbolic links and mount points on Windows. 
  (PR: [#1378](https://www.github.com/PixarAnimationStudios/USD/pull/1378))
- Fixed incorrect handling of non-existent variables in TfEnvSetting in Python.
- Updated GfRect2i API to use function and argument names that are agnostic
  to the direction of the Y axis.
- Updated ilmbase half embedded in Gf to OpenEXR v2.5.3. 
  (Issue: [#1354](https://www.github.com/PixarAnimationStudios/USD/issues/1354))
- Added VtArray::AsConst, cfront, and cback methods to help avoid inadvertent
  copy-on-writes and thread-safety issues.

- Fixes for variety of issues with VtArray conversions in Python, including
  conversion from Python sequences if all elements are convertible and a
  bug with inadvertent copy-on-writes. 
  (Issue: [#1138](https://www.github.com/PixarAnimationStudios/USD/issues/1138))

- Initial implementation of Ar 2.0. This includes new features like support
  for URI resolvers and many changes to the ArResolver interface. For more
  details see https://graphics.pixar.com/usd/docs/668045551.html.

  Work on Ar 2.0 is not yet complete and will continue through the next few
  releases. Ar 2.0 is disabled by default but can be enabled for preview and
  initial testing by specifying `PXR_USE_AR_2=ON` when running CMake.

- Moved SdfFindOrOpenRelativeToLayer to SdfLayer::FindOrOpenRelativeToLayer.
- Fixed SdfLayer::FindRelativeToLayer to use the same anchoring logic as
  SdfLayer::FindOrOpenRelativeToLayer.
- Fixed string encoding issue in .usda file writer. 
  (Issue: [#1331](https://www.github.com/PixarAnimationStudios/USD/issues/1331))
- Improved behavior when hitting hard-coded composition graph limits in Pcp.
- Fixed incorrect native instancing behavior with sub-root references.

- Added support for auto-apply API schemas. This allows single-apply API
  schemas to be automatically applied to prims using one of a list of
  associated concrete schema types instead of requiring the user to manually
  apply the API schema.

- Renamed UsdSchemaType to UsdSchemaKind to disambiguate between the schema
  type (e.g. UsdGeomSphere) and kind (e.g. non-applied, single-apply, etc).
- Deprecated functions using the "schema type" terminology in favor of
  "schema kind".
- Added UsdVariantSet::BlockVariantSelection. 
  (Issue: [#1319](https://www.github.com/PixarAnimationStudios/USD/issues/1319), 
  PR: [#1340](https://www.github.com/PixarAnimationStudios/USD/pull/1340))
- Removed deprecated UsdAttribute::BlockConnections.
- Removed deprecated UsdRelationship::BlockTargets.
- Removed deprecated UsdCollectionAPI::ApplyCollection.
- Added "container" concept to UsdShadeConnectableAPI.
- Added support for connecting multiple sources to UsdShadeConnectableAPI.
- Deprecated API for connecting to single sources in favor of the more
  general multiple-source API on UsdShadeConnectableAPI.

- Deprecated UsdConnectableAPI::IsShader and IsNodeGraph in favor of
  IsContainer API. Warnings are emitted on first use unless the environment
  setting `USD_SHADE_EMIT_CONNECTABLE_API_DEPRECATION_WARNING` is set to 0.

- Updated various clients to apply the UsdShadeMaterialBindingAPI schema
  before binding materials.
- Fixed Python binding for UsdShade.CoordSysAPI.HasLocalBindings. 
  (PR: [#1360](https://www.github.com/PixarAnimationStudios/USD/pull/1360))
- Fixed Python binding for UsdSkel.SkinningQuery.ComputeExtentsPadding.
  (Issue: [#1375](https://www.github.com/PixarAnimationStudios/USD/issues/1375))
- Added UsdLuxPluginLight and UsdLuxPluginLightFilter schemas that allow for
  defining a light or light filter via an SdrShaderNode.
- UsdLuxLight and UsdLuxLightFilter schemas now publish an associated
  SdrShaderNode based on their built-in properties.
- Input attributes for UsdLuxLight and UsdLuxLightFilter schemas are now
  connectable and have been renamed to include the "inputs:" prefix.
- Deprecated UsdLuxLightPortal schema in favor of new UsdLuxPortalLight schema.

### Imaging
- Added new GL Loading Library (GLApi) to replace GLEW. GLEW is no longer 
  required.
- Added HioImage, removed GlfImage.
- Added new camera framing API. Introduces the display and data window and 
  storage size to separate these concepts. Updated Storm, HdPrman, and HdEmbree.
- Added support for normal buffers to HdxPickFromRenderBufferTask.
- Added standard prim API to HdInstancer and HdSceneDelegate::GetInstancerId.
- Added support for animated extents when using draw modes. 
  (PR: [#1365](https://www.github.com/PixarAnimationStudios/USD/pull/1365))
- Improved Hydra camera to better support physically based attributes.
- Extended HdDisplayStyle to house more advanced selection behaviors.
- Changed renderParams timeCode default values from Default to EarliestValue.
- Changed UsdImagingDelegate to map HdLightTokens to the new input attribute 
  names in queries through GetLightParamValue.
- Merged tokens textureResourceMemory into textureMemory for better performance 
  tracking.
- Renamed UsdImagingValueCache to UsdImagingPrimvarDescCache, it only stores 
  primvar descriptors.
- Removed implementations of deprecated HdSceneDelegate::GetTextureResource.
- Removed HwFieldReader volume material node in favor of typed nodes.
- Fixed bug in pick targets when resolving unique hits. 
  (Issue: [#1343](https://www.github.com/PixarAnimationStudios/USD/issues/1343))
- Fixed UsdSkel instance drawing at origin bug. 
  (Issue: [#1347](https://www.github.com/PixarAnimationStudios/USD/issues/1347))
- Fixed UsdImaging to discard coord sys bindings to non-existent xforms. 
  (Issue: [#1346](https://www.github.com/PixarAnimationStudios/USD/issues/1346))
- Fixed UsdSkelImagingSkeletonAdapter to forward GetMaterialId and 
  GetDoubleSided calls to skinned mesh primadapter. 
  (Issue: [#1384](https://www.github.com/PixarAnimationStudios/USD/issues/1384))
- Fixed an issue where UsdSkelImagingSkeletonAdapter::_RemovePrim() failed to 
  remove skeletons that did not have any bindings to skinned prims. 
  (Issue: [#1228](https://www.github.com/PixarAnimationStudios/USD/issues/1228), 
   Issue: [#1248](https://www.github.com/PixarAnimationStudios/USD/issues/1248))
- Fixed some display crashes after resyncing skeletons. 
  (PR: [#1397](https://www.github.com/PixarAnimationStudios/USD/pull/1397))
- Fixed dome light preview surface. 
  (PR: [#1392](https://www.github.com/PixarAnimationStudios/USD/pull/1392))
- Fixed links to OpenEXR for Alembic and OpenImageIO plugins. 
  (PR: [#1398](https://www.github.com/PixarAnimationStudios/USD/pull/1398))

### Storm
- Added support for varying interpolation of any basis curves primvar. 
  (Issue: [#1308](https://www.github.com/PixarAnimationStudios/USD/issues/1308))
- Added support for HdStResourceRegistry::ReloadResource to allow clients to 
  explicitly force textures to be reloaded by file path. 
  (Issue: [#1352](https://www.github.com/PixarAnimationStudios/USD/issues/1352))
- Added a new HdMaterialNetwork2 to combine (and then replace) 
  HdStMaterialNetwork and MatFiltNetwork.
- Added step to convert 3 channel textures to 4 channel to support Hgi backends 
  with 4 channel requirements.
- Added Resize() to barContainer.
- Added fieldTextureMemory render setting to specify the target memory of 
  volume textures.
- Added experimental MaterialXFilter to process a material network and convert 
  it into a MaterialX network. Also, added a first pass on a MaterialX 
  shadergen.
- Enabled bindless textures by default.
- Several improvements to volume rendering including a step size relative to the 
  sample distance so that the quality of the rendered volume does not depend on 
  the scale.
- Optimized HdStRenderBuffer performance by only allocating when the descriptor 
  changes.
- Optimized computations by avoiding copying the compute kernel for GPU 
  computations.
- Refactored face culling to remove fragment shader discards when possible.
- Refactored geometric shaders such that only prims with the "masked" material 
  tag can use alpha threshold based discards.
- Switched Storm over to using the "translucent" material tag rather than 
  "additive" as the default translucency state.
- Fixed handling of valid to invalid BAR transitions due to Scene graph 
  operations on primvars or primvar filtering. 
  (Issue: [#1182](https://www.github.com/PixarAnimationStudios/USD/issues/1182))
- Fixed GPU memory leak of certain buffers never getting garbage collected.
- Fixed fullscreen pass to preserve alpha.
- Removed instance primvar filtering.
- Various improvements to Hgi :
  - Added initial version of the Hgi codegen to be able to produce 
    GLSL/MetalSL/others from glslfx.
  - Added the first push of our experimental, incomplete, HgiVulkan backend, 
    which does not yet build by default.
  - Added memory barriers to Hgi.
  - Added CopyTextureToBuffer and CopyBufferToTexture to HgiBlitCmds.
  - Added 2D_ARRAY support to Hgi.
  - Moved Ptex and Udim loading from Glf to Hgi and Storm.
  - Added UINT16 format as prep work for Ptex support.
  - Fixed obj-c autorelease issue, and simplification of secondary command 
    buffer in HgiMetal compute encoder.
  - Fixed HgiTexture::GetByteSizeOfResource to take the mip levels into account.
  - Fixed for missing include. 
    (PR: [#1359](https://www.github.com/PixarAnimationStudios/USD/pull/1359))

### usdview
- Fixed StageView.pickObject when doubles are small or out of image-bounds. 
  (PR: [#1296](https://www.github.com/PixarAnimationStudios/USD/pull/1296))

### MaterialX Plugin
- Removed deprecated support for MaterialX 1.36.

### Embree Hydra Plugin
- Added support for new camera framing API and HdCamera API.

### RenderMan Hydra Plugin
- Bumped version requirement to RenderMan 23.5 or greater.
- Improved calculation of vertex, varying, and face-varying primvar counts in 
  BasisCurves to better match RenderMan.
- Added support for new camera framing API and HdCamera API.
- Added support for material node int array inputs. 
  (Issue: [#1294](https://www.github.com/PixarAnimationStudios/USD/issues/1294))
- Fixed issue that caused parameters from a shader to be incorrectly carried 
  over to the next shader that was parsed. 
  (Issue: [#1396](https://www.github.com/PixarAnimationStudios/USD/issues/1396))

## [20.11] - 2020-10-14

### Build
- Updated CMakeLists.txt to conform to recommended practice. 
  (Issue: [#1241](https://www.github.com/PixarAnimationStudios/USD/issues/1241))
- The build system now sets `Boost_NO_BOOST_CMAKE=ON` by default to
  work around issues with boost's cmake files. 
  (Issue: [#1255](https://www.github.com/PixarAnimationStudios/USD/issues/1255))

- Various fixes and changes to build_usd.py:
  - Added `--toolset` option for specifying CMake toolset. 
    (PR: [#1325](https://www.github.com/PixarAnimationStudios/USD/pull/1325))
  - Added `--prefer-safety-over-speed` and `--prefer-speed-over-safety`
    options. See below for more details.
  - Fixed issue with building libTIFF with Xcode 12. 
    (PR: [#1315](https://www.github.com/PixarAnimationStudios/USD/pull/1315))

### USD
- Fixed incorrect underlying container type for TfHashMultiMap. 
  (PR: [#1281](https://www.github.com/PixarAnimationStudios/USD/pull/1281))
- Fixed invalid conversions of Gf containers like GfVec3f in Python. Note this
  issue is still present in Python 3. 
  (Issue: [#1290](https://www.github.com/PixarAnimationStudios/USD/issues/1290))
- Fixed various tests to accommodate `PXR_OVERRIDE_PLUGINPATH_NAME`. 
(PR: [#1275](https://www.github.com/PixarAnimationStudios/USD/pull/1275))

- Added GetExternalAssetDependencies to SdfLayer and SdfFileFormat to allow
  file format plugins to declare additional dependencies that determine when
  a layer produced from that file format should be reloaded.

- Reduced memory usage from SdfPath::GetString and added SdfPath::GetAsString
  and SdfPath::GetAsToken to avoid populating internal caches. 
  (Issue: [#1287](https://www.github.com/PixarAnimationStudios/USD/issues/1287))

- Various fixes to composition behavior:
  - Fixed issues with sub-root references, payloads, and inherits, and
    ancestral variant selections.
  - Fixed issue with payloads on ancestors of sub-root references/payloads.
  - Fixed issue with combinations of root, sub-root, and ancestral inherits.

- Fixed bug in .usda file writer that could cause data corruption. 
  (Issue: [#1331](https://www.github.com/PixarAnimationStudios/USD/issues/1331))
- Fixed performance issue when writing many relationship targets or attribute
  connections to .usdc files. 
  (Issue: [#1345](https://www.github.com/PixarAnimationStudios/USD/issues/1345))

- Improved performance for certain queries with .usdc files. In one example,
  this decreased draw times in usdview after transform changes by ~90%.
  (Issue: [#1300](https://www.github.com/PixarAnimationStudios/USD/issues/1300))

- New .usdc files now default to version 0.8.0. These files cannot be read in
  USD releases prior to v19.03. Users can revert to writing older versions by
  setting the environment variable `USD_WRITE_NEW_USDC_FILES_AS_VERSION` to an
  older version.

- Added safety checks to guard against reading malformed .usdc files. These
  checks may negatively impact performance and may be disabled by specifying
  `PXR_PREFER_SAFETY_OVER_SPEED=OFF` when running CMake or
  `--prefer-speed-over-safety` with build_usd.py.

- Native instancing "master" prims are now referred to as "prototype" prims.
  The generated paths of prototype prims has been changed from /__Master_<X>
  to /__Prototype_<X>. Clients can use new UsdPrim::IsPrototypePath and
  UsdPrim::IsPathInPrototype API to determine if a given path is related
  to a prototype.

  UsdStage and UsdPrim API using "master" terminology like UsdPrim::GetMaster
  and UsdStage::GetMasters have been deprecated and will be removed in a future
  release.

- Added Get/Set/ClearChildrenReorder to UsdPrim for reordering prim children.
- UsdProperty::FlattenTo now allows flattening properties across stages.
- Deprecated UsdRelationship::BlockTargets and UsdAttribute::BlockConnections.
  Clients should use SetTargets({}) and SetConnections({}) instead.
- Changed UsdRelationship::GetTargets and UsdAttribute::GetConnections to return
  false when HasAuthored would return false.
- Fixed bug where UsdFlattenLayerStack was not applying the asset path
  resolution callback to asset paths in `clips` metadata. 
  (PR: [#1266](https://www.github.com/PixarAnimationStudios/USD/pull/1266))
- Added UsdNotice::LayerMutingChanged notice. 
  (Issue: [#676](https://www.github.com/PixarAnimationStudios/USD/issues/676))
- Fixed resource leak with Usd.PrimCompositionQuery in Python. 
  (PR: [#1297](https://www.github.com/PixarAnimationStudios/USD/pull/1297))
- Fixed crashes when printing expired Sdf.Layer and Usd.Stage objects in
  Python. (PR: [#1012](https://www.github.com/PixarAnimationStudios/USD/pull/1012))
- Removed `isPrivateApply` functionality from schema generation.
- Fixed error in UsdUtilsComputeAllDependencies when an invalid
  templateAssetPath value is specified. 
  (PR: [#1289](https://www.github.com/PixarAnimationStudios/USD/pull/1289))
- Improved UsdUtilsGetPrimaryUVSetName and UsdUtilsGetPrefName. 
  (PR: [#1283](https://www.github.com/PixarAnimationStudios/USD/pull/1283))
- Fixed bug in UsdUtilsStitchClips that led to inconsistent stitched results.

- Added CanContainPropertyName to UsdCollectionAPI, UsdGeomPrimvars,
  UsdShadeMaterialBindingAPI, and UsdShadeCoordSysAPI for checking if a
  property name is valid for these schemas.

- Deprecated UsdCollectionAPI::ApplyCollection in favor of Apply.
- Added fallback values for texture card attributes on UsdGeomModelAPI schema.
- Added exposure control to UsdGeomCamera.
- Added ease-of-use API to UsdGeom primitives for querying the size of primary
  geometric properties (e.g. UsdGeomMesh::GetFaceCount).
- Fixed plugin extent computations for primitives in UsdGeom to respect
  given timecode parameter. 
  (PR: [#1284](https://www.github.com/PixarAnimationStudios/USD/pull/1284))
- Fixed crash in UsdGeomSubset::GetUnassignedIndices when a given subset had
  negative indices. 
  (Issue: [#1227](https://www.github.com/PixarAnimationStudios/USD/issues/1227))
- Added UsdShadeNodeDefAPI schema to represent connectable nodes in a graph.
  UsdShadeShader has been modified to build off of this schema.
- Added UsdShadeConnectableAPIBehavior to allow plugins to customize
  connectability by prim type.
- Added API to retrieve material outputs for standard terminals in all
  render contexts on UsdShadeMaterial.
- Added more instancing support to UsdSkel. 
  (PR: [#1258](https://www.github.com/PixarAnimationStudios/USD/pull/1258))
- Updated UsdVol FieldAsset schemas.
- Various documentation fixes. 
  (Issue: [#1162](https://www.github.com/PixarAnimationStudios/USD/issues/1162), 
  PR: [#1335](https://www.github.com/PixarAnimationStudios/USD/pull/1335))

### Imaging
- Added new "sourceColorSpace" input to UsdUVTexture with values "raw", "sRGB",
  and "auto" for better control of with which color space a texture is read.
- Added ability to use mip maps authored in a file in the new texture system in
  Storm.
- Added UsdImagingPrimAdapter::ShouldIgnoreNativeInstanceSubtrees() to allow
  adapters to disable instancing of itself and its descendants.
- Added pre-multiply alpha functionality for UDIM textures. Also added sRGB
  internal formats for UDIMs.
- Added checks in HioGlslfxConfig to make sure the default value for an
  attribute and its type match.
- Added HdEngine::ClearTaskContextData() to avoid the task context to hold on to
  resources that are about to become invalid as the render delegate is destroyed.
- Added HDST_DUMP_FAILING_SHADER_SOURCEFILE debug flag to facilitate shader
  debugging.
- Added texture filepath resolution for symlinks.
- Added an "outHitNormal" parameter to UsdImagingGLEngine::TestIntersection().
- Added ReloadResource in Storm which reloads shader files when they change.

- Improvements to Hgi:
  - Added Draw, DrawIndirect and DrawIndexedIndirect to Hgi and backends.
  - Added tracking of HgiCmds submission to ensure they only get submitted once.
  - Added TextureCpuToGpu copy on BlitCmd, and CopyBufferGpuToCpu to read back
    the GPU frustum cull results.
  - Added guarantees that HgiGLInterop will restore GL state after interop.
  - Added guarantees that Hgi GPU-GPU sync between SubmitCmds calls.
  - Added TextureView to Hgi and convert domelight to use Hgi compute and
    texture views.
  - Added staging buffer in Hgi to reduce the amount of times small amounts of
    data are copied into the same GPU buffer.
  - Added HgiPrimitiveType, and added HgiSampleCount on HgiPipelineDesc.
  - Added support for BC texture compression.
  - Added component mapping to HgiTextureDesc and implementations for GL and
    Metal.
  - Added ability to share HgiCmds objects in Storm.
  - Added optional wait behavior for command submission in Hgi.

- Changed GLenum usage in GlfBaseTextureData and others to use new HioFormat.
- Changed Storm's frustum culling to be done via a vertex shader without a
  fragment shader.
- Changed HdxShadowTask to specify that it should only render opaque and masked
  materials.
- Changed cards UVs from face-varying to vertex interpolated.
- Changed to GL_SAMPLE_ALPHA_TO_ONE when using alpha to coverage, the alpha
  computed after the framebuffer resolve is more meaningful.
- Changed texture cards to require prims with UsdGeomModelAPI schema applied.
- Changed type of UsdPrimvarReader.varname input from token to string.
- Renamed various HdSt*GL classes to just HdSt*, this is part of the transition
  of Storm to support multiple rendering APIs.
- Enabled bindless shadow maps by default in Storm.
- On first material sync in Storm, now we try to batch prims with identical
  textures together.
- HdStDrawTarget's preferred mechanism are AOVs not GlfDrawTarget's. This change
  allows us to remove all GlfDrawTarget support from HdStDrawTarget.
- Since HdStDrawTarget's no longer rely on the resolve task, removing code to
  communicate the draw target task render passes to the resolve task.
- Deprecated HdSceneDelegate::GetTextureResource and will eventually be removed.
- An application can now disable the PresentTask in hdx task controller.
- When imaging basis curves, treat empty normals array the same as missing
  normals (i.e. draw as tube and not ribbon at higher complexities).
- Store HgiResourceBindings and HgiPipeline objects on HdSt resource registry.
- Switched (most of) the GPU frustum culling code from raw GL to Hgi.
- With the addition of HgiMetal, all platforms we currently support (for Storm)
  support color correction and 16F targets.
- Use BufferResource for GPU frustum cull result buffer so we can use the
  regular buffer binding APIs.
- Load mipmap data in HgiMetal textures.
- Changed signature of HgiDataSizeOfFormat so that it can return blockWidth and
  blockHeight. Returning block size in bytes for compressed formats.
- Describe shader constants (push constants) in pipeline.
- Using HgiCompute in smooth normals computation, switched flat normals and
  quadrangulate to HgiCompute, and converted HdStExtCompGpuComputation to Hgi.

- Removed UsdImaging value cache API for all data except primvar descriptors and
  instance indices.
- Removed unused hitMode field from HdxPickTaskContextParams.
- Removed UsdImagingGLEngine::InvalidateBuffers().
- Removed pre-multiply alpha behavior from volume.glslfx and changed
  renderPass.glslfx to allow fully transparent but still emissive volumes.
- Removed pre-multiply alpha behavior from stb_image and make it optional via
  SubtextureIdentifier within the HdSt texture system.
- Removed support of old-style draw targets using GlfDrawTarget instead of AOVs
  and render buffers.
- Removed UsdUvTexture's rgba output, and updated the cards draw mode to now use
  "rgb" and "a" outputs rather than rgba.
- Removed ReloadAllShaders from HdEngine.
- Removed HD_DRAWITEM_DRAWN as it uses raw gl calls that map/unmap a buffer.
- Removed HgiPipelineType. We split HgiPipeline into HgiComputePipeline and
  HgiGraphicsPipeline a little bit ago.
- Removed resource bindings handle from pipeline.
- Removed smooth normals CPU fallback path. Since previous releases required
  OpenGL 4.5 for Hgi to work, OpenGL compute must always be available.
- Removed HdBinding::TBO.
- Removed the implicit barriers from Hgi and let the external client manage
  synchronization via SubmitCmds.

- Fixed potential dead-lock caused by parallel loading plugins.
- Fixed lights to be turned off when invisible in UsdImaging.
- Fixed issue with UsdImagingGLEngine::TestIntersection such that it now
  correctly populates. Callers of this function are reminded to provide the
  "frame" field in UsdImagingGLRenderParams for correct and performant results.
- Fixed handling of computed primvar sources for meshes.
- Fixed normals after complexity change.
- Fixed HgiGL garbage collector leak when Hgi is recreated during process.
- Fixed HgiGL to better protect against long labels when using glObjectLabel.
- Fixed render settings behaviors so we test whether or not we're really
  changing anything before incrementing the settings version.
- Fixed for cycle detection in material network processing.
- Fixed Glf lighting resource binding.
- Fixed USD edit dependency tracking for cards prims.

- Updated UsdPreviewSurface clearcoat calculations to better match expected
  inputs for a specular lobe in Storm, and added clearcoat component to indirect
  lighting calculation in Storm. 
  (Issue: [#1307](https://www.github.com/PixarAnimationStudios/USD/issues/1307))
- Added support for UsdPreviewSurface's opacityThreshold parameter in
  Storm. (Issue: [#990](https://www.github.com/PixarAnimationStudios/USD/issues/990))
- Added support for normal mapping for UsdPreviewSurfaces in Storm. 
  (Issue: [#701](https://www.github.com/PixarAnimationStudios/USD/issues/701))
- Metallic materials should have an F0 equal to their base color in
  UsdPreviewSurface in Storm. 
  (Issue: [#1174](https://www.github.com/PixarAnimationStudios/USD/issues/1174))

- Removed pre-multiplication behavior from OIT resolve shader. The preview
  surface (and other shaders) pre-multiply within the fragment shader (unless
  the "diffuseColor" and "opacity" params are connected to the same
  texture). 
  (Issue: [#1269](https://www.github.com/PixarAnimationStudios/USD/issues/1269))

- Added support for UDIM texture scale and bias in Storm. 
  (Issue: [#1129](https://www.github.com/PixarAnimationStudios/USD/issues/1129))
- Converted render index to usdImaging cachePath before calling the adapter
  InvokeComputation method. 
  (Issue: [#1333](https://www.github.com/PixarAnimationStudios/USD/issues/1333))
- Fix draw mode adapter handling of transforms for instanced card prims. 
  (Issue: [#1251](https://www.github.com/PixarAnimationStudios/USD/issues/1251))
- _IsVarying no longer clears dirty bit and now ProcessPropertyChange is filled
  out to provide more efficiency. 
  (Issue: [#1250](https://www.github.com/PixarAnimationStudios/USD/issues/1250))

### usdview
- Added 'Enable Scene Lights' option.
- Changed UI to use bundled Roboto and Roboto Mono fonts.
- Fixed build issues due to stricter checks in PySide2 5.15.1. 
  (PR: [#1320](https://www.github.com/PixarAnimationStudios/USD/pull/1320))

### Alembic Plugin
- Alembic uv's are now converted to texCoord2f[] primvars:st. This can
  be disabled by setting the `USD_ABC_WRITE_UV_AS_ST_TEXCOORD2FARRAY`
  environment variable to 0.

### RenderMan Plugin
- Fixed infinite loop in material processing.
- Initial implementation of exposure.
- Volumes can react to changes to a field prim. 
- Fixed UsdUVTexture wrap mode support. 
- Add support for UsdPreviewSurface's opacityThreshold param in HdPrman. 
  (Issue: [#990](https://www.github.com/PixarAnimationStudios/USD/issues/990))
- Updated UsdPreviewSurface clearcoat calculations to better match expected
  inputs for a specular lobe in HdPrman. 
  (Issue: [#1307](https://www.github.com/PixarAnimationStudios/USD/issues/1307))
- Metallic materials should have an F0 equal to their base color in
  UsdPreviewSurface in HdPrman. 
  (Issue: [#1174](https://www.github.com/PixarAnimationStudios/USD/issues/1174))

### Security
- Fixed [CVE-2020-6147](https://nvd.nist.gov/vuln/detail/CVE-2020-6147)
- Fixed [CVE-2020-6148](https://nvd.nist.gov/vuln/detail/CVE-2020-6148)
- Fixed [CVE-2020-6149](https://nvd.nist.gov/vuln/detail/CVE-2020-6149)
- Fixed [CVE-2020-6150](https://nvd.nist.gov/vuln/detail/CVE-2020-6150)
- Fixed [CVE-2020-6156](https://nvd.nist.gov/vuln/detail/CVE-2020-6156)
- Fixed [CVE-2020-13493](https://nvd.nist.gov/vuln/detail/CVE-2020-13493)
- Fixed [CVE-2020-13524](https://nvd.nist.gov/vuln/detail/CVE-2020-13524)
- Fixed [CVE-2020-13498](https://nvd.nist.gov/vuln/detail/CVE-2020-13498)
- Fixed [CVE-2020-13497](https://nvd.nist.gov/vuln/detail/CVE-2020-13497)
- Fixed [CVE-2020-13496](https://nvd.nist.gov/vuln/detail/CVE-2020-13496)
- Fixed [CVE-2020-13494](https://nvd.nist.gov/vuln/detail/CVE-2020-13494)
- Fixed [CVE-2020-13495](https://nvd.nist.gov/vuln/detail/CVE-2020-13495)

## [20.08] - 2020-07-21

### Build
- The "master" branch on GitHub has been renamed "release".
- Improved error handling when building the RenderMan plugin. 
  (Issue: [#1054](https://www.github.com/PixarAnimationStudios/USD/issues/1054))

- Various fixes and changes to build_usd.py:
  - Added `--tools` and `--no-tools` options.
  - Updated OpenImageIO (2.1.16.0) and MaterialX (1.37.1) dependencies.
  - Specifying `--embree` will now build the Embree library. The
    `--embree-location` parameter has been removed.
  - CMake 3.14 is now required on Windows to support boost 1.70+.
  - Python 3.8 on Windows now causes an error, as USD does not support
    this version on Windows.
  - Improved handling of boost build failures.
  - Fixed locale decoding errors. 
    (Issue: [#1165](https://www.github.com/PixarAnimationStudios/USD/issues/1165))
  - Fixed incorrect detection of Draco library. 
    (PR: [#1239](https://www.github.com/PixarAnimationStudios/USD/pull/1239))

### USD
- Added support for "future division" in Python 2 to aid transition to Python 3.

- Added ArResolver::CreatePathForLayer to allow users to customize behavior
  when writing a layer. SdfLayer now uses this method instead of creating a
  directory, which was not appropriate for non-filesystem uses. 
  (Issue: [#1148](https://www.github.com/PixarAnimationStudios/USD/issues/1148))

- Added SdfReference::IsInternal. 
  (PR: [#1204](https://www.github.com/PixarAnimationStudios/USD/pull/1204))
- Added fallback prim types feature, allowing clients to specify alternative
  schemas to use if a schema can't be found. See documentation for more details.
- Added UsdPrim::ApplyAPI, RemoveAPI, AddAppliedSchemas,
  and RemoveAppliedSchemas. 
  (Issue: [#1218](https://www.github.com/PixarAnimationStudios/USD/issues/1218))
- Added Python __repr__ for generated schemas.
- Added methods to UsdGeomPrimvar for processing property names.
- Added methods to UsdGeomPrimvarsAPI for creating, removing, and blocking
  primvars. 
  (Issue: [#1100](https://www.github.com/PixarAnimationStudios/USD/issues/1100))
- Improved documentation for UsdVol schema domain. 
  (PR: [#1203](https://www.github.com/PixarAnimationStudios/USD/pull/1203))
- Improved documentation for UsdGeomMesh. 
  (PR: [#1254](https://www.github.com/PixarAnimationStudios/USD/pull/1254))
- Improved diagnostic messages for plugin registration and loading.

- Plugin search paths in PXR_PLUGINPATH_NAME can now be a symlink. Paths are
  now processed in order to ensure plugins in earlier entries take priority
  over those in later entries.

- Replaced SdrShaderNode::GetSourceURI and GetResolvedSourceURI with
  GetResolvedDefinitionURI and GetResolvedImplementationURI to properly
  represent RenderMan C++ shaders.

- Time sample times from layers whose timeCodesPerSecond value differ from
  the UsdStage are now automatically scaled to match. This can be disabled
  by setting the PCP_DISABLE_TIME_SCALING_BY_LAYER_TCPS environment variable,
  but we expect to remove this in a future release.

- Significant changes and additions to value clips functionality. See
  documentation for more details:
  - Added support for jump discontinuities in clip times metadata to encode
    looping behaviors.
  - The clip manifest is now used to determine which attributes have values
    in clips. The manifest will be generated in-memory at runtime if one
    hasn't been specified.
  - Value resolution will no longer fall through to weaker layers if a clip
    does not have values for an attribute declared in the manifest. In these
    cases, the value may come from the default value authored in the manifest
    or (optionally) be interpolated from surrounding clips.
  - Removed support for legacy value clips metadata.
  - Fixed numerous bugs, including incorrect time samples when reversing
    clips. (Issue: [#1116](https://www.github.com/PixarAnimationStudios/USD/issues/1116))

- Changed the strength ordering of entries in the apiSchemas metadata to be
  strongest-to-weakest, matching the ordering of references and other fields.
- Constructing a UsdGeom::XformOp with an invalid attribute is no longer a
  coding error for consistency with other schemas and objects.
- Deprecated many schemas in UsdRi in preparation for modernization efforts
  in a future release.
- Removed support for deprecated "hermite" and "power" basis from UsdGeomCurves.
- Fixed crash when removing many entries from an SdfPathTable. 
  (PR: [#1172](https://www.github.com/PixarAnimationStudios/USD/pull/1172))
- Fixed compile error due to missing virtual destructor for
  PcpDynamicFileFormatInterface. 
  (PR: [#1156](https://www.github.com/PixarAnimationStudios/USD/pull/1156))
- Fixed type conversions when setting metadata values using dictionaries
  in Python. (Issue: [#813](https://www.github.com/PixarAnimationStudios/USD/issues/813))
- Fixed handling of stage metadata and session layer muting in UsdStage.
- Fixed UsdFlattenLayerStack to handle mismatched attribute types and
  time-sampled asset paths. 
  (PR: [#1169](https://www.github.com/PixarAnimationStudios/USD/pull/1169))
- Fixed undefined behavior issue with iterator comparisons. 
  (Issue: [#1146](https://www.github.com/PixarAnimationStudios/USD/issues/1146))
- Fixed regression in handling of inherited bindings in UsdSkelCache.
- Fixed crash in UsdSkelBakeSkinning. 
  (PR: [#1213](https://www.github.com/PixarAnimationStudios/USD/pull/1213))

### Imaging
- Updated minimum required version of OpenGL to v4.5.

- Added new texture system to Storm. This enables multi-threaded texture
  loading, uses Hgi, and frees the scene delegate from having to load
  textures. In a performance test using Nvidia's Attic scene, time to first
  image dropped by ~90%, from 115s to 10s.

- Added first iteration of HgiMetal, a Metal-based Hgi implementation for
  Apple platforms. Currently, Hgi is used in several Hydra tasks to perform
  tasks like blitting, the goal is to slowly integrate Hgi in Storm.

- Numerous updates to Hgi:
  - Added HgiInterop to exchange rendered results between GL-GL or Metal-GL.
  - Added HgiSampler to represent texture samplers.
  - Added HgiComputeCmds to issue compute commands.
  - Added HgiBlitCmds for mipmap generation and GPU-GPU copy.
  - Added SRGB format and removed unsupported 24-bit formats.
  - Added GetRawResource function to various resource objects to expose
    low-level resource handles.

- Added garbage collection to HgiGL for proper handling of resource destruction.
- Added HdRendererPluginHandle, an RAII object for managing plugin lifetimes.
- Added HdxAovInputTask which takes HdRenderBuffer and (if needed) uploads
  it to the GPU as HgiTexture.
- Added support for down-sampling volumes in Storm.
- Improved performance of volumes in Storm by not re-creating the entire
  volume shader when fields are animated.
- Added support for UsdTransform2d node in Storm. 
  (Issue: [#1207](https://www.github.com/PixarAnimationStudios/USD/issues/1207))
- Added support for UsdUVTexture scale and bias in Storm. 
  (Issue: [#1129](https://www.github.com/PixarAnimationStudios/USD/issues/1129))
- Added USDIMAGINGGL_ENGINE_DEBUG_SCENE_DELEGATE_ID environment variable to
  specify scene delegate ID for debugging. 
  (Issue: [#1093](https://www.github.com/PixarAnimationStudios/USD/issues/1093))
- Changed default version of HdSceneDelegate::GetScenePrimPath to strip
  delegate ID.
- Replaced obsolete GL_GENERATE_MIPMAP with glGenerateMipmap. 
  (Issue: [#1171](https://www.github.com/PixarAnimationStudios/USD/issues/1171))
- Added support for uint16 type for GL textures and OpenImageIO. 
  (PR: [#1212](https://www.github.com/PixarAnimationStudios/USD/pull/1212))
- Restructured OpenImageIO support as a plugin to Glf. 
  (PR: [#1214](https://www.github.com/PixarAnimationStudios/USD/pull/1214))
- Converted common Hydra tasks (e.g. HdxColorCorrectionTask, HdxPresentTask,
  HdxColorizeSelectionTask) to use Hgi.
- Renamed HdxProgressiveTask to HdxTask.
- Storm now uses Hgi for buffers, shaders, and programs instead of OpenGL.
- HdStResourceRegistry is instantiated once per Hgi instance in Storm.
- Unshared computation BAR are reused when possible. 
  (Issue: [#1083](https://www.github.com/PixarAnimationStudios/USD/issues/1083))
- Improved handling of invalid cases in several areas. 
  (PR: [#1232](https://www.github.com/PixarAnimationStudios/USD/pull/1232), 
  PR: [#1201](https://www.github.com/PixarAnimationStudios/USD/pull/1201))
- Improved performance of UsdImagingDelegate::SetTime. 
  (Issue: [#1166](https://www.github.com/PixarAnimationStudios/USD/issues/1166))
- Improved performance for displaying meshes with large numbers of geometry
  subsets. (PR: [#1170](https://www.github.com/PixarAnimationStudios/USD/pull/1170))
- Normals are ignored on skinned meshes so they are computed post-skinning.
- Inherited UsdSkel bindings are now respected.
- Allow HdStRenderBuffer::Resolve() to change the size of the image. 
  (PR: [#1236](https://www.github.com/PixarAnimationStudios/USD/pull/1236))
- Removed "catmark" token in pxOsd in favor of "catmullClark".
- Removed UsdImagingGLMaterialTextureAdapter, UsdImagingGLDomeLightAdapter,
  GlfVdbTexture[Container], and GlfTextureContainer, which are unneeded with
  the new texture system.
- Removed unused HdxColorizeTask and HdxFullscreenShaderGL.
- Removed HdWrapLegacyClamp, which corresponded to the deprecated GL_CLAMP.
- Removed uses of HdStGLSLProgram as part of a new strategy to create these
  via HdStResourceRegistry (since it has access to Hgi).
- Removed transform feedback from non-instanced GPU culling in Storm.
- Fixed incorrect handling of buffer array resizing to 0 elements. 
  (Issue: [#1230](https://www.github.com/PixarAnimationStudios/USD/issues/1230))
- Fixed memory regression when calling UsdImagingDelegate::SampleTransform
  and SamplePrimvar.
- Fixed error in UsdImaging when changing from a time where a mesh provided
  points or normals to a time that does not.
- Fixed potential deadlocks if plugins were loaded at various points in
  UsdImagingGLEngine.
- Fixed error when processing skinned prim with no joint influences.
- Fixed change processing issue with native instanced prims. 
  (Issue: [#1163](https://www.github.com/PixarAnimationStudios/USD/issues/1163))
- Fixed handling of widths primvar with no value.
- Fixed "IsFlipped" computation for native instanced prims. 
  (Issue: [#1190](https://www.github.com/PixarAnimationStudios/USD/issues/1190))
- Fixed crash when updating a removed primvar. 
  (PR: [#1223](https://www.github.com/PixarAnimationStudios/USD/pull/1223))
- Fixed crash when reparenting an instance root. 
  (Issue: [#1245](https://www.github.com/PixarAnimationStudios/USD/issues/1245))
- Fixed various issues with cards prims, including a crash issue. 
  (Issue: [#1210](https://www.github.com/PixarAnimationStudios/USD/issues/1210))
- Fixed selection encoding/decoding of invisible native instances.
- Fixed invalidation when changing the "purpose" attribute. 
  (Issue: [#1243](https://www.github.com/PixarAnimationStudios/USD/issues/1243))

### usdview
- Added "Prototypes" pick mode that selects the prototype for a picked gprim
  in a PointInstancer and highlights the picked instance.
- Reverted "Prims" and "Instances" pick modes to original behavior of selecting
  the root boundable of the picked object. 
  (Issue: [#1196](https://www.github.com/PixarAnimationStudios/USD/issues/1196))
- Fixed "Reopen Stage" when usdview was launched with `--norender`. 
  (PR: [#1192](https://www.github.com/PixarAnimationStudios/USD/pull/1192))
- Fixed camera guide and reticles rendering on high DPI displays.

### Alembic Plugin
- Fixed conversion of mesh subdivision interpolation options between
  UsdGeomMesh and AbcGeom's ISubD and OSubD schemas. 
  (PR: [#1246](https://www.github.com/PixarAnimationStudios/USD/pull/1246))

### Embree Plugin
- Removed support for Embree 2.0. The HdEmbree plugin now requires Embree 3.

### MaterialX Plugin
- Added support for MaterialX 1.37 and deprecated support for 1.36. We
  anticipate requiring 1.37 in a future release.
- MaterialX "vectorN" datatypes are now translated to USD "floatN" datatypes.
- Changed default output name from "result" to "out" to match conventions
  in MaterialX specification.
- Fixed test issue found during Python 3 work. 
  (PR: [#1161](https://www.github.com/PixarAnimationStudios/USD/pull/1161))

### RenderMan Plugin
- Added support for arbitrary numbers of AOVs in HdPrman.
- Removed deprecated RenderMan 22 Hydra plugin.

- Material processing now uses SdrShaderNode::GetResolvedImplementationURI to
  locate shaders, which allows it consume shaders identified via 
  UsdShadeShader's info:sourceAsset property.

- Modified UsdVol support to allow easier extensions with custom volume plugin
  for RenderMan.
- Fixed calculation of specularFaceColor in UsdPreviewSurfaceParameters.osl
  to better match the spec.

## [20.05] - 2020-04-13

This release includes initial support for Python 3. Many thanks to our
collaborators at Nvidia and SideFX for all of their work to make this
happen!

The RenderMan 22 Hydra plugin is deprecated, and the deprecated Houdini
and Katana plugins have been removed from the repository. See details below.

### Build
- Added support for Python 3. This may be enabled by specifying
  `PXR_USE_PYTHON_3=TRUE` when running cmake or by running build_usd.py
  with Python 3.
- Updated C++ standard version to C++14 per VFX Reference Platform CY2018.
- Updated CMake minimum version to 3.12. 2.x support was deprecated in 19.11.
- Moved to Azure Pipelines for CI.
- PySide2 is now preferred over PySide if both are installed. Users can specify
  `PYSIDE_USE_PYSIDE=TRUE` when running cmake to force the use of PySide.
- Building examples and tutorials is now optional. 
  (PR: [#594](https://www.github.com/PixarAnimationStudios/USD/pull/594))

- Various fixes and changes to build_usd.py
  - Updated TBB (2017.6), Alembic (1.7.10), OpenImageIO (1.8.9), and
    OpenColorIO (1.0.9) dependencies for VFX Reference Platform CY2018 and/or
    to reflect versions used for testing internally.
  - Updated OpenSubdiv to 3.4.3. This removes the dependency on GLFW,
    which itself required X libraries on Linux. 
    (Issue: [#261](https://www.github.com/PixarAnimationStudios/USD/issues/261))
  - Miscellaneous fixes. 
    (Issue: [#1110](https://www.github.com/PixarAnimationStudios/USD/issues/1110), 
    Issue: [#1126](https://www.github.com/PixarAnimationStudios/USD/issues/1126))

- Made OpenEXR an optional dependency. It is only required for enabling
  OpenImageIO, OpenVDB, or OSL support. 
  (PR: [#1082](https://www.github.com/PixarAnimationStudios/USD/pull/1082))
- Fixed strict builds on Linux when building with boost version 1.61 or
  earlier. (PR: [#1081](https://www.github.com/PixarAnimationStudios/USD/pull/1081))
- Fixed issue where warnings from 3rd-party headers would cause strict builds
  to fail.
- Fixed bug where the boost_python library would not be found when building
  against boost 1.67+.
- Fixed bug where specifying `PXR_BUILD_DOCUMENTATION=TRUE` but not
  having required programs would result in an incomplete build. 
  (Issue: [#1127](https://www.github.com/PixarAnimationStudios/USD/issues/1127))
- Fixed incorrect instructions in README.md for running build_usd.py under
  Visual Studio 2017. 
  (PR: [#1120](https://www.github.com/PixarAnimationStudios/USD/pull/1120))
- Fixed bug where build_usd.py would pick up external builds of boost 1.70
  or later. 
  (Issue: [#1071](https://www.github.com/PixarAnimationStudios/USD/issues/1071))
- Fixed bug in build_usd.py when building boost with Visual Studio 2019.
  (Issue: [#1041](https://www.github.com/PixarAnimationStudios/USD/issues/1041))
- Fixed issue where non-debug builds with Visual Studio 2019 would not work
  properly or crash immediately. 
  (Issue: [#1095](https://www.github.com/PixarAnimationStudios/USD/issues/1095))
- Fixed several issues that caused errors when building a program against
  USD with clang on Windows. 
  (Issue: [#1030](https://www.github.com/PixarAnimationStudios/USD/issues/1030), 
  PR: [#1079](https://www.github.com/PixarAnimationStudios/USD/pull/1079))
- Fixed issue where debug mode builds on Windows with Python bindings enabled
  would fail without a special debug build of Python. 
  (PR: [#785](https://www.github.com/PixarAnimationStudios/USD/pull/785), 
  Issue: [#1006](https://www.github.com/PixarAnimationStudios/USD/issues/1006))

### USD
- Added typed value proxies to VtValue.
- Added SdfPathAncestorsRange for iterating over an SdfPath's ancestors.
- Added convenience scenegraph object accessor APIs to UsdPrim and UsdStage.
- Added UsdPrim::GetInstances to get all instances of a master. 
  (Issue: [#962](https://www.github.com/PixarAnimationStudios/USD/issues/962))
- Added UsdPrimDefinition to represent the built-in properties of a given prim.
  Users can access a prim's definition via UsdPrim::GetPrimDefinition.
- Added UsdUtilsConditionalAbortDiagnosticDelegate, a diagnostic delegate that
  aborts when an error or warning is encountered based on text-matching rules.
- Added UsdUtilsSparseValueWriter::GetSparseAttrValueWriters to help with
  debugging. (PR: [#1038](https://www.github.com/PixarAnimationStudios/USD/pull/1038))
- Added UsdGeomHermiteCurves schema.
- Deprecated 'hermite' and 'power' basis for UsdGeomBasisCurves.
- Removed remnants of relationship-based shader encoding from UsdShade schemas.
- Removed "shadow:include" and "shadow:exclude" from UsdLuxShadowAPI schema.
- Removed raw spec access API from UsdSchemaRegistry in favor of the new
  UsdPrimDefinition objects.
- For static USD builds, the Plug library now resolves plugin paths relative
  to the path of the executable that links to USD.
- SdfLayer and UsdStage API for timeCodesPerSecond now fall back to using
  framesPerSecond if no timeCodePerSecond is specified.
- Numerous optimizations for UsdStage load time and authoring speed.
- UsdStage now sends notification when population mask, load rules, and
  interpolation type are changed. 
  (Issue: [#1139](https://www.github.com/PixarAnimationStudios/USD/issues/1139))
- Applied API schemas now impart their properties and fallback values as
  builtins on the applied prim.
- Concrete typed schemas can now provide fallback metadata values.
- C++ typenames for schemas are no longer supported as prim typenames
  in scene description.

- UsdGeomImageable now computes 'purpose' using non-pruning semantics.
  See documentation for UsdGeomImageable::ComputePurpose for a description
  of the new behavior.

- Various fixes and changes for usdGenSchema
  - Added --headerTerminatorString option. 
    (Issue: [#1092](https://www.github.com/PixarAnimationStudios/USD/issues/1092), 
    PR: [#1097](https://www.github.com/PixarAnimationStudios/USD/pull/1097))
  - Output directory automatically created if it doesn't already exist.
  - Changed output in generatedSchema.usda for multiple-apply API schemas.
  - Changed output ordering for tokens in tokens.h.

- Fixed race condition in UsdGeomBoundable::ComputeExtentFromPlugins that
  could cause incorrect results for bounds computations.
- Fixed bug with value block handling in UsdFlattenLayerStack.
- Fixed incorrect documentation on crate file structure. 
  (Issue: [#1072](https://www.github.com/PixarAnimationStudios/USD/issues/1072))
- Fixed change processing bug for nested instancing that led to crashes.
- Fixed bug reading .usdc files in .usdz files if USDC_USE_PREAD=1 was set.
- Fixed usdedit failure on Windows due to file access conflicts. 
  (PR: [#1094](https://www.github.com/PixarAnimationStudios/USD/pull/1094))
- Fixed line ending issue causing some tests to fail on Windows. 
  (PR: [#848](https://www.github.com/PixarAnimationStudios/USD/pull/848))

### Imaging
- Added swizzle metadata to the output of UsdUVTexture. 
  (Issue: [#657](https://www.github.com/PixarAnimationStudios/USD/issues/657))
- Added ability to display unloaded prims as bounding boxes. 
  (PR: [#1145](https://www.github.com/PixarAnimationStudios/USD/pull/1145))
- Added selection highlight outline capabilities (not exposed in usdview).
- Added many more improvements to volume visualizations in Storm.
- Added basic support for transporting pinned curves to UsdImaging.
- Added support for computed vertex primvars for points prim in Storm.
- Added support for element invising and point colors in basis curves.
- Added hdTiny, a very simple example render delegate.
- Added support for cascaded shadow maps (not exposed in usdview).
- Added IsStopSupported/Stop/Restart APIs to render delegates.
- Added transport of light filters through usdImaging.
- Added HdResourceRegistry::ReloadResource API to allow for texture reloading.
- Added USDVIEW_OCIO_LUT3D_EDGE_SIZE environment variable for setting 3D LUT
  size for OpenColorIO.
- Removed IsFlipped() from certain Storm shaders for clean up.
- Many changes to remove or replace uses of boost with C++ features.
- Changed many APIs in Hgi as that subsystem continues to be built up.
- Changed HdResourceRegistry APIs for several cleanups and fixes.
- Replaced HdSceneDelegate::GetPrimPathForInstanceIndex with GetScenePrimPath.
- Moved HdMaterialParam to HdSt and made it private.
- Moved HdxSimpleLightingShader to HdSt.
- Fixed issue with Storm shutdown on certain platforms.
- Fixed several issues with selection highlighting (including issues relating
  to instancing) in usdImaging.
- Fixed issue with Storm not drawing geometry when a PxrSurface is attached.
  (Issue: [#1128](https://www.github.com/PixarAnimationStudios/USD/issues/1128))
- Fixed issue where UsdUVTexture alpha output was driven by the wrong channel.
  (Issue: [#657](https://www.github.com/PixarAnimationStudios/USD/issues/657))
- Fixed drawing of sphere implicit to be centered. 
  (Issue: [#1086](https://www.github.com/PixarAnimationStudios/USD/issues/1086))
- Fixed handling of primvar addition and removal in Hydra. 
  (Issue: [#1078](https://www.github.com/PixarAnimationStudios/USD/issues/1078))
- Fixed .glslfx processing to use Ar for resolving paths for includes.
- Fixed parsing of texture default values for .glslfx files in Sdr.
- Fixed issue with texture binding with certain configurations. 
  (Issue: [#1063](https://www.github.com/PixarAnimationStudios/USD/issues/1063))
- Fixed several cleanup issues. 
  (Issue: [#1103](https://www.github.com/PixarAnimationStudios/USD/issues/1103), 
  PR: [#1104](https://www.github.com/PixarAnimationStudios/USD/pull/1104))
- Fixed order of draw targets drawing in HdxDrawTargetTask.
- Fixed default values for displayColor, displayOpacity and widths. 
  (PR: [#1098](https://www.github.com/PixarAnimationStudios/USD/pull/1098))
- Fixed point instancer resync of prototypes. For example, we now resync
  prototypes properly if their material bindings change.
- Fixed point instancer refresh in response to structural primvar and prototype
  updates. (PR: [#1077](https://www.github.com/PixarAnimationStudios/USD/pull/1077))
- Fixed HdRenderIndex::Clear to always call Finalize() for cleared rprims.
  (Issue: [#1040](https://www.github.com/PixarAnimationStudios/USD/issues/1040))
- Fixed color banding issue when using OpenColorIO.
- Fixed reading stride with OpenImageIO. 
  (Issue: [#1080](https://www.github.com/PixarAnimationStudios/USD/issues/1080))
- Fixed drawMode updates with native instancing. 
  (Issue: [#1069](https://www.github.com/PixarAnimationStudios/USD/issues/1069))
- Fixed case sensitivity for image loading. 
  (PR: [#804](https://www.github.com/PixarAnimationStudios/USD/pull/804))
- Fixed issue with Storm shader generation when input and primvar names match.
- Fixed debug flag not properly displaying source when shader fails to link.
  (PR: [#1020](https://www.github.com/PixarAnimationStudios/USD/pull/1020))

### usdview
- Added ability to change font size.
- Added 'Apply' button to renderer settings dialog.
- Improved start up time by waiting until we've populated more of the UI.
- Made free-cam's FOV part of saved user settings.
- Fixed multi-line paste in the interpreter. 
  (Issue: [#1117](https://www.github.com/PixarAnimationStudios/USD/issues/1117))
- Fixed errors when using PySide2 5.14.0 or later. 
  (Issue: [#1111](https://www.github.com/PixarAnimationStudios/USD/issues/1111))
- Fixed errors when closing usdview with the native windowing system commands.

### Alembic Plugin
- Added file format arguments 'abcReRoot' for reparenting hierarchy beneath
  a new parent prim and 'abcLayers' for specifying a list of secondary files
  to load as layered Alembic. 
  (PR: [#1099](https://www.github.com/PixarAnimationStudios/USD/pull/1099))
- Only deliver interesting timeSamples for constant properties. 
  (PR: [#1114](https://www.github.com/PixarAnimationStudios/USD/pull/1114))
- Relaxed type-checking on position property and let any float[3] type through.
  (PR: [#1115](https://www.github.com/PixarAnimationStudios/USD/pull/1115))
- Removed need to link directly against Alembic backend libraries to open files.

### Houdini Plugin
- The Houdini plugin has been removed from the USD repository in favor of the
  native USD support in Houdini's Solaris suite.

### Katana Plugin
- The Katana plugin has been removed from the USD repository in favor of the
  Foundry-supported Katana plugins available at
  https://github.com/TheFoundryVisionmongers/KatanaUsdPlugins.

### MaterialX Plugin
- Fixed bug where implementation file relative paths were assumed to be
  relative to the .mtlx file with the implementation node instead of to
  the MaterialX library itself.

### RenderMan Plugin
- Deprecated support for RenderMan 22. We anticipate removing the renderman-22
  plugin in a future release.
- Added support for HdAov clearValue.
- Added support for Stop/Restart functionality.
- Added early initial support for light filters and trace groups.
- Fixed bug that caused empty renders when no scene camera was present.
  (Issue: [#1113](https://www.github.com/PixarAnimationStudios/USD/issues/1113))
- Fixed routing of ri:attributes properties when instancing is involved.
- Fixed restarting of RenderMan when application is told that resources
  (e.g., textures) need to be reloaded.

## [20.02] - 2020-01-24

We anticipate aligning with the version requirements from the VFX Reference
Platform CY2018 (https://vfxplatform.com) in the next release. In particular,
this includes requiring support for C++14.

This release adds a Hydra plugin for RenderMan 23 and removes the deprecated
Maya plugin from the repository. This release also deprecates support for the
Houdini and Katana plugins. See details below.

### Build
- The source tree has been restructured so that code no longer lives under a
  separate "lib" directory. This means that (with a few exceptions) include
  paths now match source tree paths.

- Added support for Visual Studio 2019 to build_usd.py. 
  (PR: [#1042](https://www.github.com/PixarAnimationStudios/USD/pull/1042))
- Added PXR_BUILD_USD_TOOLS option to disable building of command-line tools
  (e.g. usdcat, usdedit) 
  (PR: [#1048](https://www.github.com/PixarAnimationStudios/USD/pull/1048))
- The PXR_LIB_PREFIX option now defaults to "" and is respected for .lib files
  on Windows. 
  (Issue: [#1027](https://www.github.com/PixarAnimationStudios/USD/issues/1027))

### USD
- Added "subIdentifier" and "sourceType" parameters to GetNodeFromAsset API in
  Ndr and Sdr and "subIdentifier" attribute to UsdShadeShader schema. This
  allows clients to identify and retrieve definitions for a particular shader
  in source assets that have multiple shaders defined in them. 
  (Issue: [#919](https://www.github.com/PixarAnimationStudios/USD/issues/919))

- Removed unused legacy SdfVariabilityConfig variability value.
- Fixed bug where prims in masters would be populated incorrectly when a
  UsdStage's population mask contained paths descendant to masters.
- Fixed bug where layer stack flattening would lose some variant selections.
  (Issue: [#1060](https://www.github.com/PixarAnimationStudios/USD/issues/1060))
- UsdStage::CreateClassPrim is no longer restricted to root prims. 
  (Issue: [#994](https://www.github.com/PixarAnimationStudios/USD/issues/994))
- UsdVariantSets::GetNames() now returns the union of all variant set names
  across layer stacks.
- Improved performance of retrieving metadata with templated API in UsdObject.
- Numerous changes to improve performance when making edits to many properties.
- Removed support for inverse layer offsets which were deprecated in v0.8.2.
- Removed support for legacy UsdShade encodings, yielding cleaner code and
  improved performance.
- Improved diagnostics in usdGenSchema for non-identifier tokens in schema.usda.
- Improved documentation generated by usdGenSchema for schema attributes.
- usdGenSchema now treats all C++ and Python keywords as reserved and will
  prepend a "_" if they are encountered in any identifiers.
- Modified UsdGeomXformCommonAPI to be generated and behave like other API
  schemas (as the name implies) for consistency.
- Fixed bug in UsdGeomImageable::MakeVisible where the given prim would not be
  made visible if other "invis" opinions existed on ancestor prims. 
  (PR: [#1043](https://www.github.com/PixarAnimationStudios/USD/pull/1043))
- Fixed bug in UsdGeomSubset::GetUnassignedIndices where passing in an
  invalid element count could lead to a crash. 
  (Issue: [#989](https://www.github.com/PixarAnimationStudios/USD/issues/989))
- Removed deprecated shader binding API on UsdShadeMaterial in favor of
  UsdShadeMaterialBindingAPI schema.
- Improved fidelity of light-related UsdRi schemas.
- Added UsdLuxDomeLight::OrientToStageUpAxis utility for creating dome lights
  with an appropriate default orientation. 
  (Issue: [#938](https://www.github.com/PixarAnimationStudios/USD/issues/938))
- Added UsdMedia schema library and UsdMediaSpatialAudio schema.
- Fixed bugs in UsdSkelCache::ComputeSkelBinding where inherited skel:skeleton
  bindings were incorrectly evaluated and bindings would be returned for
  inactive prims.
- Python binding for UsdSkelSkinningQuery is now UsdSkel.SkinningQuery.

### Imaging
- Added RenderMan 23 Hydra plugin.
- Added support for specifying thread limits and convergence criteria in
  HdPrman via settings.
- Added initial support for light filters.
- Added an Sdr parser plugin for .glslfx files.
- Added support for material networks in Storm.
- Added initial support for volume rendering in Storm.
- Added basic support for bindless shadow maps in Storm.
- Added primvar/points editing support for points, basis curves, and meshes
  in UsdImaging. Previously everything was resync.
- Added Hydra changetracker dependencies between parent and child instancers.
- Added new task HdxColorChannelTask to output a specific display channel.
- Added command line option (--traceToFile) to usdview to start tracing as soon
  as the application is started.
- Changed HdPrman linking to be independent of RenderMan install.
- Changed "depth" AOV range from [-1, 1] to [0, 1].
- Changed name "linearDepth" AOV to "cameraDepth".
- Changed HdxPickHit "ndcDepth" field with a range of [-1,1] to
  "normalizedDepth" with a range of [0,1]
- Changed edge hashing to use triangular numbers in Hd. This is about 3x faster
  than the previous code.
- Changed UsdImaging's DrawModeAdapter to use HdMaterialNetwork.
- Changed default camera shutter in UsdImaging to match UsdGeomCamera [0,0].
- Improved Storm's instance registry and moved resource registries from Hd core
  to Storm.
- Improved aggregation of material shaders in Storm.
- Improved material primvar filtering in Storm.
- Improved property refresh support in UsdImaging for instance attributes.
- Removed HydraMaterialAdapter in UsdImaging since it is no longer needed.
- Removed old Hydra scene delegate API that was only used by the old material
  system in Storm. (GetSurfaceShaderSource, GetDisplacementShaderSource,
  GetMaterialParams and others..).
- Fixed instance adapter selection highlighting performance issues.
- Fixed bug where pick task was continuously running in usdview.
- Fixed Storm to skip string primvars.
- Fixed bug where rtx_glfImage was not honoring the channel offset and number
  of channels specified in the FillRequest parameter.
- Fixed nullptr exception in HdPrman when rendering without a camera.
- Fixed data ownership of instance indices. DirtyInstanceIndex is set on
  instancers and tells them they need to re-fetch their index buffer, relative
  to one of their prototypes.
- Fixed UsdImaging native instancing so instead of compensating for prototype
  transforms by overwriting prototype transforms, it compensates in the
  per-instance transform primvar.
- Fixed usdSkelImaging dirtyness propagation to the skinning aggregator
  computation.
- Fixed double color correction on MacOS.
- Implemented SamplePrimvar for native instancer adapter. 
  (Issue: [#996](https://www.github.com/PixarAnimationStudios/USD/issues/996))
- Fixed UsdPreviewSurface material displacement in Storm. 
  (Issue: [#922](https://www.github.com/PixarAnimationStudios/USD/issues/922), 
   Issue: [#1026](https://www.github.com/PixarAnimationStudios/USD/issues/1026))
- Fixed change processing when an inherited primvar is removed. 
  (Issue: [#1004](https://www.github.com/PixarAnimationStudios/USD/issues/1004))
- Fixed incorrect points buffer computation for implicit geometry. 
  (Issue: [#1024](https://www.github.com/PixarAnimationStudios/USD/issues/1024))
- Added GetHgiTextureHandle API on renderbuffer to avoid HdSt dynamic_casts
  and CPU -> GPU copies during post-process pipeline 
  (Issue: [#1008](https://www.github.com/PixarAnimationStudios/USD/issues/1008))
- Fixed TBB library linkage in hdEmbree. 
  (PR: [#991](https://www.github.com/PixarAnimationStudios/USD/pull/991))
- Added support for 'jpeg' extension for Glf_StbImage. 
  (PR: [#998](https://www.github.com/PixarAnimationStudios/USD/pull/998))
- Fixed bug where time-invariant geometry subsets would mask time-varying
  topology from Hydra. 
  (PR: [#1045](https://www.github.com/PixarAnimationStudios/USD/pull/1045))
- Fixed bug where UsdGeomSubset indices were always retrieved at the default
  time. (PR: [#1059](https://www.github.com/PixarAnimationStudios/USD/pull/1059))
- Fixed incomplete handling of UsdGeomMesh face-varying options. 
  (PR: [#1061](https://www.github.com/PixarAnimationStudios/USD/pull/1061))

### Houdini Plugin
- Deprecated Houdini plugins in favor of Houdini-native USD support.
  We anticipate removing the plugin from the USD repository in a future
  release.
- Additional changes for Houdini 17.5 compatibility.

### Katana Plugin
- Deprecated Katana plugin in favor of the Foundry-supported Katana plugins 
  available at https://github.com/TheFoundryVisionmongers/KatanaUsdPlugins.
  We anticipate removing the plugin from the USD repository in a future release.
- Changed how UsdGeomBasisCurve Widths and Normal attributes are read into
  Katana. Author either a geometry.point.* entry (old behavior) or a
  geometry.arbitrary.* entry (new behavior) based on the attribute's
  interpolation.
- Changes in preparation for reading UsdShadeMaterials which contain
  UsdShadeNodegraph subgraphs.
- Removed custom Viewer Modifier Plugins. These were used by Katana's legacy
  Viewer tab, which has been replaced by the Hydra Viewer tab.
- Removed support for /LooksDerivedStructure in USD shader libraries.
- Fixed incorrect geometry.instanceSource attribute for nested instances.
  (PR: [#1015](https://www.github.com/PixarAnimationStudios/USD/pull/1015))

### MaterialX Plugin
- Added support for versions 1.36.3 and later.

### Maya Plugin
- The Maya plugin has been removed from the USD repository. Development for
  the plugin has moved to the Autodesk maya-usd repository located at
  https://github.com/Autodesk/maya-usd.

## [19.11] - 2019-10-18

This release deprecates support for cmake 2.x and the Maya plugin.
See details below.

### Build
- Deprecated support for cmake 2.x. We anticipate requiring cmake 3.x in the
  next release.
- build_usd.py now fails if using 32-bit Python, which is a common gotcha on
  Windows. (Issue: [#921](https://www.github.com/PixarAnimationStudios/USD/issues/921))
- Fixed issue where build_usd.py would fail to build TBB on MacOS if CUDA was
  installed due to an issue in the TBB build system. 
  (Issue: [#767](https://www.github.com/PixarAnimationStudios/USD/issues/767))
- Made PXR_VERSION a valid and comparable integer in C++. 
  (PR: [#886](https://www.github.com/PixarAnimationStudios/USD/pull/886))
- Fixed issue where pxrTargets.cmake would include plugins with no exported
  symbols. This caused errors when building against USD on Windows. 
  (Issue: [#530](https://www.github.com/PixarAnimationStudios/USD/issues/530))
- Fixed issue where downstream cmake projects using pxrTargets.cmake could not
  find the USD headers.

### USD
- Added warning when reading a .usda layer with size in MB greater than value
  of SDF_TEXTFILE_SIZE_WARNING_MB environment variable. 
  (PR: [#980](https://www.github.com/PixarAnimationStudios/USD/pull/980))
- Added UsdStageLoadRules to support more robust and declarative intent about
  how payloads should be included in a UsdStage.
- Added UsdPrimCompositionQuery to facilitate discovery of composition arcs
  affecting a UsdPrim.

- Added "timecode" scene description value type. This type is represented by
  the SdfTimeCode class and can be used for attribute and metadata values.
  Layer offsets are applied to all timecode values during UsdStage
  value resolution, enabling "timing curve"-like attributes.

- Added support for Google's Draco compression library. This includes an
  SdfFileFormat plugin for reading .drc files and a standalone "usdcompress"
  tool that extracts and compress Mesh primitives from a USD file, creates
  .drc files for each one, and references them back into a result USD file.
  (PR: [#912](https://www.github.com/PixarAnimationStudios/USD/pull/912))

- Added --quiet flag to usdGenSchema for suppressing output. 
  (PR: [#480](https://www.github.com/PixarAnimationStudios/USD/pull/480))
- Added --brief flag to usddiff so differing lines are not printed.
- Added support for indexed uint primvars to UsdGeomPrimvar. 
  (PR: [#861](https://www.github.com/PixarAnimationStudios/USD/pull/861))

- Added "accelerations" attribute to UsdGeomPointInstancer schema and updated
  UsdGeomPointInstancer::ComputeInstanceTransformAtTime(s) to account for
  authored accelerations, if present.

- Added "accelerations" attribute to UsdGeomPointBased schema and added
  UsdGeomPointBased::ComputePointsAtTime(s) to compute point positions using
  authored velocities and accelerations, if present.

- Added initial version of UsdRender domain and schemas. See documentation and
  white paper on openusd.org for more details. 
  (Issue: [#910](https://www.github.com/PixarAnimationStudios/USD/issues/910))
- Added UsdShadeInput::GetValueProducingAttribute to facilitate more robust and
  correct processing of UsdShade shading networks.
- Added initial support for skinning mesh normals in UsdSkel.

- Removed SdfAbstractDataSpecId and updated the SdfAbstractData interface to
  use SdfPath in its place. Existing SdfAbstractData subclasses must be
  updated to match. See this usd-interest post for more details:
      https://groups.google.com/forum/#!topic/usd-interest/IVmd1t1GKBA

- Numerous fixes for C++14 / gcc6 warnings. 
  (PR: [#869](https://www.github.com/PixarAnimationStudios/USD/pull/869))
- Various optimizations and cleanups in trace library.

- Inherits and specializes arcs that target root prims ("global" arcs) are now
  combined with arcs that target sub-root prims ("local" arcs) for strength
  ordering in composition. Previously, local arcs would always be considered
  stronger than global arcs regardless of their authored order.

- Calling UsdReferences::SetReferences (and equivalent API on UsdPayloads,
  UsdInherits, and UsdSpecializes) with an empty vector now authors an
  explicit empty list op instead of being a no-op. 
  (Issue: [#749](https://www.github.com/PixarAnimationStudios/USD/issues/749))

- Numerous fixes and changes to Sdr and associated parser plugins. This
  includes the addition of a "terminal" property type and "role" and
  "renderType" metadata to better describe shader properties and their
  representation in scene description.

- Numerous optimizations for UsdStage load time and authoring speed.
- Improved performance of adding empty sublayers to an existing UsdStage.

- USD native instancing now includes both LoadRules and PopulationMasks in
  instancing keys, so different instances can have different load state.
  Instances that have equivalent load rules and population masks still share,
  but those that do not will use different masters.

- Load/Unload operations on a UsdStage now must operate on the "expanded" stage
  namespace. That is, the namespace as it would be if instancing was disabled.
- Updated UsdLuxDomeLight schema to clarify a dome light's orientation and
  its textures' orientation. 
  (Issue: [#938](https://www.github.com/PixarAnimationStudios/USD/issues/938))
- Changed 'pointIndices' attribute on UsdSkelBlendShape schema from uint[]
  to int[] for consistency with other core schemas. 
  (Issue: [#858](https://www.github.com/PixarAnimationStudios/USD/issues/858))
- Rewrote UsdSkelBakeSkinning to provide a more efficient baking solution.
- Fixed bug where converting .usdc files from version 0.7 to 0.8 when saving
  an existing file would produce incorrect layer offsets in payloads.
- Fixed crashes when calling methods on invalid Usd.SchemaBase objects in
  Python. 
  (Issue: [#872](https://www.github.com/PixarAnimationStudios/USD/issues/872), 
  PR: [#876](https://www.github.com/PixarAnimationStudios/USD/pull/876))
- Fixed crash when muting and unmuting layers on a UsdStage. 
  (Issue: [#883](https://www.github.com/PixarAnimationStudios/USD/issues/883))

### Imaging
- Renamed the real-time engine/rasterizer to "Storm"
- Added support for dome lights in Storm.
- Added support for screen-space adaptive tessellation and drawing for Loop
  scheme subdivision surface meshes when using OpenSubdiv 3.4 in Storm.
- Added basic support for AOVs in Storm. In usdview, you can now switch between
  color and depth when using Hydra Storm.
- Added Hgi (Hydra Graphics Interface) library. Hgi abstracts away graphics
  API calls in Storm.
- Added HgiGL library, an OpenGL backend implementation for Hgi.
- Added support for vstructs in Hydra RenderMan plugin.
- Added support for light attributes in UsdImaging and Hydra RenderMan plugin.
- Added an adapter in UsdImaging to read camera information, including custom
  camera parameters.
- Added support for physical camera parameters in Hydra and Hydra RenderMan
  plugin.
- Added support for camera motion blur in Hydra RenderMan plugin.
- Added support to read any number of time samples authored in Usd. Improved
  Hydra API to allow render delegates to extract any number of samples from
  scene delegates.
- Added API to HdRenderDelegate and the render thread to pause and resume
  renderers if supported. Updated Hydra RenderMan plugin to support it. 
  (PR: [#888](https://www.github.com/PixarAnimationStudios/USD/pull/888))
- Added API to HdRenderDelegate to return render stats to the client. Updated
  Hydra Embree to expose the number of samples completed this way. 
  (PR: [#888](https://www.github.com/PixarAnimationStudios/USD/pull/888))
- Added element/scope selection highlighting for raytraced backends.
- Added basic support for NURBS curves to UsdImaging. For now, it will
  send them to the render delegate as basis curves.
- Added support for UsdShadeNodeGraph to UsdImaging.
- Added support for acceleration primvars in UsdImaging and Hydra.
- Added support for inherited primvars to native instances.
- Added instance-inheritance support for string-typed primvars.
- Added convergence criteria to render settings in Hydra RenderMan.
- Added support for basis curves instancing in Hydra RenderMan plugin.
- Added support for OpenImageIO's IOProxy feature to allow supported image
  files (such as .exr) to be read from .usdz files.
- Added "Set As Active Camera' on USD camera prims context menu in usdview.
- Added a "Pause" menu item to usdview, and display the render status message.
- Replaced the key/fill/back lights in usdview by a dome light, still only
  works in Storm for now.

- Deprecated old material accessors on HdSceneDelegate in favor of using
  material networks in Storm.
- Removed TrackVariabilityPrep/UpdateForTimePrep in UsdImaging.

- Moved usdShaders from usdImaging/lib to usdImaging/plugin.
- Moved rendererPlugin and rendererPluginRegistry from hdx to hd.
- Moved UsdPreviewSurface conversion to the MatfiltFilterChain in
  Hydra RenderMan plugin.
- Changed return type of HdRenderBuffer::Map from uint8_t to void*.
- Improved handling of invalidations and edits of data such as instancers,
  primvars and others, in UsdImaging.
- Improved 32 bit color support when using Hydra and the default task controller.
- Improved draw item batching in Hydra Storm. 
  (PR: [#528](https://www.github.com/PixarAnimationStudios/USD/pull/528))
- Improved GLSL error reporting in Storm.
- Improved usdrecord to allow more consistent results with usdview (e.g.,
  default materials, purpose).
- Prim browser in usdview now maintains its expansion-state and view when the
  stage recomposes.

- Prim browser in usdview now behaves more like Maya's outliner with respect to
  viewport selections: the PrimView will no long expand/scroll to display the
  selected prim until you use the 'f' hotkey over it to frame the selection.
  Instead, all of the ancestors of selected prims will be highlighted in a
  secondary color.

- Searches in the usdview prim browser begin at the currently selected prim
  rather than at the root of the scene.
- Made usdview panes easier to resize by doubling size of splitter lanes.
- Made usdview timeline playhead much larger to make it easier to grab.
- Fixed inheriting primvars through instance boundaries when the primvar is an
  array.
- Fixed the way UsdGeomPointInstancer calculates purpose and visibility across
  instance/master boundaries.
- Fixed memory leak for draw items in Storm.
- Fixed handling infinite bounds explicitly during GPU frustum culling in Storm.
- Fixed UsdShadeShader material invalidations in UsdImaging.
- Fixed handling of topology changes for basis curves in Storm.
- Fixed ortho camera handling in Hydra RenderMan plugin.

### Houdini Plugin
- Added general methods for converting between USD values and GA_Attribute data.
- Fixed missing usdSkel dependency. 
  (PR: [#827](https://www.github.com/PixarAnimationStudios/USD/pull/827))

- Detect skinning influences stored as detail attribs when converting to
  capture attributes. This allows rigid shapes to be coalesced into deforming
  agent shapes.

### Katana Plugin
- Point instancers with velocities now get motion blur on the first frame,
  even if their positions are not sampled at any previous frames.
- Katana paths are now better aligned with USD paths when possible for point
  instancer prototypes.
- Includes "prmanLightParams.iesProfileNormalize" when reading UsdLux lights
  in as RenderMan representations

- Fixed bug where per-instance primvar values on point instancers could be
  inherited by the prototype geometry, causing warnings about mismatched sizes.
  (Issue: [#893](https://www.github.com/PixarAnimationStudios/USD/issues/893))

### MaterialX Plugin
- Fixed bug when reading a .mtlx file with more than 1 bindinput.
  (Issue: [#950](https://www.github.com/PixarAnimationStudios/USD/issues/950), 
  PR: [#956](https://www.github.com/PixarAnimationStudios/USD/pull/956))

### Maya Plugin
- Deprecated Maya plugin in favor of the Autodesk maya-usd plugin available at
  https://github.com/Autodesk/maya-usd. We anticipate removing the plugin from
  the USD repository in the next release and recommend users begin transitioning
  to the Autodesk plugin -- it contains the same features as the Pixar plugin
  (in fact, it is the same code).

- FindMaya.cmake module on MacOS (once again) supports paths that include the
  "Maya.app/Contents" subdirectory when specifying MAYA_LOCATION. 
  (PR: [#878](https://www.github.com/PixarAnimationStudios/USD/pull/878))
- Visibility for Maya point instancers is now exported to UsdGeomPointInstancer.
- Maya "stroke" nodes are now exported as UsdGeomBasisCurves. 
  (Issue: [#867](https://www.github.com/PixarAnimationStudios/USD/issues/867))
- Various bug fixes and cleanup.

## [19.07] - 2019-06-17

### Build
- Improved detection of GLEW and Ptex libraries during build. 
  (PR: [#808](https://www.github.com/PixarAnimationStudios/USD/pull/808))
- Fixed bug in build_usd.py that caused errors when specifying the
  "Xcode" CMake generator.

### USD:
- Various optimizations and cleanups in trace library.
- Added iterator-based construction for VtArray. 
  (PR: [#644](https://www.github.com/PixarAnimationStudios/USD/pull/644))
- Added Python bindings for ArResolver::RefreshContext. 
  (PR: [#820](https://www.github.com/PixarAnimationStudios/USD/pull/820))
- Fixed incorrect results from NdrRegistry::GetAllNodeSourceTypes.
- Removed unused spec types from Sdf.
- Reimplemented SdfPath. The new implementation dramatically improves the
  performance of creating property paths, which is a key part of value
  resolution.
- SdfFileFormat::IsStreamingLayer was moved to SdfAbstractData::StreamsData.
  SdfAbstractData implementations are now responsible for expressing
  whether they stream data from their back-end instead of SdfFileFormat.
- Added file format arguments to SdfLayer::CreateAnonymous to match CreateNew.
- Fixed bug where strings in scene description that look like format
  specifiers would be interpreted as such during .usda output.
- Added dynamic payloads feature. Dynamic payloads allow file formats to
  procedurally generate a layer using metadata authored on the prim where the
  layer is introduced via a payload arc. See examples and "Advanced Features"
  documentation for more details.
- Fixed erroneous composition cycle detection for subroot references to
  sibling prims across reference boundaries.
- Added support for "schemaTokens" customData entry in schema.usda. This
  allows developers to associate tokens with a particular schema for clarity
  and organization. These tokens will continue to be emitted into the
  standard tokens.h and tokens.cpp files in each schema library.
- Removed deprecated UsdListPositionTempDefault and USD_AUTHOR_OLD_STYLE_ADD
  environment variable.
- Improved error detection for truncated .usdc files.
- Improved error messages and debug output for USD schema generation.
  (PR: [#427](https://www.github.com/PixarAnimationStudios/USD/pull/427), 
  PR: [#478](https://www.github.com/PixarAnimationStudios/USD/pull/478))
- Fixed various issues that caused build errors on Windows. 
  (Issue: [#812](https://www.github.com/PixarAnimationStudios/USD/issues/812))
- Fixed bug that caused corrupt .usdc files to be created in certain rare
  cases. (Issue: [#811](https://www.github.com/PixarAnimationStudios/USD/issues/811))
- Fixed bug where asset-valued stage metadata would not be resolved.
- Calling Usd.Prim.IsDefined on a null prim in Python now raises a
  Python exception. 
  (Issue: [#753](https://www.github.com/PixarAnimationStudios/USD/issues/753))
- Fixed bug in UsdUtils dependency extraction and .usdz creation where
  asset dependencies in layer metadata were ignored.
- Improved integer encoding and fixed a source of non-deterministic output
  in .usdc file format. 
  (Issue: [#830](https://www.github.com/PixarAnimationStudios/USD/issues/830))
- Added ability to usdcat to only show layer metadata via --layerMetadata
  command-line flag.
- UsdGeomBasisCurves schema now accepts "pinned" as a valid value for
  wrapping to represent pinned curves.
- UsdGeomPointInstancer will now use the "append" list op when authoring
  inactiveIds metadata instead of the deprecated "added" list op. This
  behavior can be reverted by setting the environment variable
  `USDGEOM_POINTINSTANCER_NEW_APPLYOPS` to 0, but this will be removed
  in a future release.
- Added UsdShadeCoordSysAPI schema for representing coordinate systems.
- Added shaping:ies:normalize attribute to UsdLuxShapingAPI.
- Numerous fixes and improvements to UsdSkel schemas.

### Imaging:
- Added early version of RenderMan Hydra render delegate with support for
  geometry,material networks, lights, volumes, coordinate systems and more!
- Added support for coordinate systems in Hydra and UsdImaging.
- Added support for categories per instance in Hydra to support light linking
  to individual instances.
- Added order independent transparency (OIT) to Hydra GL.
- Added support for backend-independent picking and highlighting.
- Improved Hydra support for scenegraph cameras.
- Added support for instance-rate primvar queries in Hydra GL shading code.
- Added support for generating normals for picking hits in Hydra GL.
- Added "DontConform" camera window policy.
- Added support for picking points in Hydra GL.
- Added depth compositing to HdxColorizeTask.
- Render delegates can now access "velocities" attribute on point-based schemas.
  (PR: [#824](https://www.github.com/PixarAnimationStudios/USD/pull/824))
- Refactored and clarified use of "path" and "prim" terminology in UsdImaging.
- Removed HdChangeTracker::MarkAllCollectionsDirty
- Removed render tags API from HdRprimCollection. They have become a Hydra
  task concept.
- Fixed instancing-related crashes in UsdImaging. 
  (Issue: [#838](https://www.github.com/PixarAnimationStudios/USD/issues/838), 
  Issue: [#847](https://www.github.com/PixarAnimationStudios/USD/issues/847))
- Added usdAppUtils library containing common functionality and utilities
  for applications that view USD stages.
- Added initial version of usdrecord command line tool for generating images
  from a USD file.
- Added "debug flags" configuration dialog in usdview.
- Added ability to specify first/last/current frame when launching usdview
  via "--ff", "--lf", and "--cf" command line parameters. 
  (PR: [#832](https://www.github.com/PixarAnimationStudios/USD/pull/832))
- Numerous correctness, interaction, and performance improvements to usdview's
  transport control (frame slider). 
  (PR: [#770](https://www.github.com/PixarAnimationStudios/USD/pull/770))

### Alembic Plugin:
- Added support for Alembic 1.7.9. 
  (PR: [#825](https://www.github.com/PixarAnimationStudios/USD/pull/825))

### Houdini Plugin:
- Support for Houdini 16.0 is deprecated. We anticipate bumping the minimum
  version requirement to 16.5 and dropping 16.0 support in the next release.
- Fixed bug which could cause visibility to be incorrect.
- Added support for export of additional primvar types.
- Fixed bug which could cause incorrect attribute typeinfo when importing
  array attributes.
- Added support for additional OSD subdiv interpolation tags.
- Added support for targeting the default prim on import via a reserved
  "defaultPrim" prim path.
- Miscellaneous improvements for converting between UsdSkel and Agent
  primitives.

### Katana Plugin:
- Added support for reading and resolving collection-based material bindings to
  PxrUsdIn. This is done as a two stage process in which bindings (for the
  purposes specified in "additionalBindingPurposeNames") are added within a
  "usd.materialBindings" group attribute. The PxrUsdInResolveMaterialBindings
  node (and its corresponding Op) transfers a purpose's binding to the
  system-level "materialAssign" attribute and optionally filters inherited
  redundancy.
- The "info.usd" attributes which advertise which API schemas are applied can
  be created in either array or group attribute form. The latter is convenient
  to match against via CEL.

### Maya Plugin:
- Add support for exporting assembly edits on pxrUsdReferenceAssembly nodes.
- Expose controls for tweaking selection parameters in pxrHdImagingShape.
- Importing shaders and lights will use mel commands to property add shaders
  to hypershade and lights to the defaultLightSet.

## [19.05] - 2019-04-10

### Build
- Added support for Ninja build system in build_usd.py. 
  (PR: [#590](https://www.github.com/PixarAnimationStudios/USD/pull/590))
- Added --build-debug option to build_usd.py. 
  (PR: [#502](https://www.github.com/PixarAnimationStudios/USD/pull/502))
- Better support for static builds in exported pxrConfig.cmake. 
  (PR: [#787](https://www.github.com/PixarAnimationStudios/USD/pull/787))
- .pdb files are now installed with the libraries on Windows. 
  (PR: [#502](https://www.github.com/PixarAnimationStudios/USD/pull/502))
- MacOS users can now run build_usd.py with Maya's Python interpreter (mayapy)
  to ensure that USD and the Maya plugin will be built against Maya's Python.
  This requires the --no-usdview option, as Maya does not provide the OpenGL
  module in Python. 
  (Issue: [#10](https://www.github.com/PixarAnimationStudios/USD/issues/10))
- Numerous fixes for FindMaya.cmake module. In particular, users on MacOS
  who specify MAYA_LOCATION should now use the root of the installation,
  without the "Maya.app/Contents" suffix.

### USD:
- Added usdtree utility for viewing the scenegraph at the command line. 
  (PR: [#619](https://www.github.com/PixarAnimationStudios/USD/pull/619))
- Added ability to configure default camera prim name via plugin and query
  the default name via UsdUtilsGetPrimaryCameraName.
- Added UsdUtilsTimeCodeRange for representing an iterable range of time codes.
- Added API to UsdGeom for setting and querying Stage-level linear units via
  the "metersPerUnit" metadata field.
- Changed SdfFileFormat interface to operate in terms of SdfLayer instead of
  SdfLayerBase.
- Improved performance of UsdGeomPointBased::ComputeExtent overload with an
  additional transform. 
  (PR: [#640](https://www.github.com/PixarAnimationStudios/USD/pull/640))
- Various cleanup changes to fix documentation, compiler warnings, and remove
  unused or legacy code.
- Numerous changes to UsdSkel schemas for resolving and imaging blend shapes.
- Removed UsdGeomFaceSetAPI schema.
- Removed SdfLayerBase. Its functionality was folded into SdfLayer.
- Fixed bug where parts of the codegen template for multiple-apply API schemas
  were specific to UsdCollectionAPI. 
  (Issue: [#799](https://www.github.com/PixarAnimationStudios/USD/issues/799))
- Fixed bug preventing the use of symlinks for generatedSchema.usda. 
  (Issue: [#763](https://www.github.com/PixarAnimationStudios/USD/issues/763))
- Fixed several bugs in .usdz creation related to nesting of .usdz files and
  file path syntax on Windows.

### Imaging:
- Added "MaterialTags" mechanism which allows the GL backend to render opaque
  primitives separated from translucent primitives.
- Added early implementation of CPU/GPU UsdSkel skinning support, working
  in both GL and Embree backends.
- Added early implementation of blend shapes support to UsdSkelImaging adapter.
- Refactored renderer-independent input/output code into new "hio" library.
- Added new phase for Hydra tasks called "Prepare", which runs after Sync
  phase and before Commit.
- Added "instanceId" and "elementId" AOVs in HdEmbree.
- Added new purpose cache in UsdImaging to improve performance.
- Added support for animated textures in drawModeAdapter. 
  (PR: [#735](https://www.github.com/PixarAnimationStudios/USD/pull/735))
- TaskController::TestIntersections now allows selection by nearestToCamera.
  (PR: [#760](https://www.github.com/PixarAnimationStudios/USD/pull/760))
- Split Hydra's color primvar into a float3 displayColor and a float
  displayOpacity.
- Removed hidden GL dependencies in UsdImaging tests.
- Removed unnecessary parameters from GetInstancerTransform and
  SampleInstancerTransforms in HdSceneDelegate API.
- Fixed several issues with curve drawing in GL backend. 
  (Issue: [#690](https://www.github.com/PixarAnimationStudios/USD/issues/690))
- Fixed bug where UsdGeomSubset called Populate instead of Resync during
  resyncs.
- Fixed leak of empty GlfSimpleShadowArray instance. 
  (PR: [#786](https://www.github.com/PixarAnimationStudios/USD/pull/786))

### Alembic plugin:
- Fixed double time scaling when converting Alembic sample times from seconds
  to USD time codes. 
  (Issue: [#662](https://www.github.com/PixarAnimationStudios/USD/issues/662))
- USD's "timeCodesPerSecond" metadata is now used to scale when reading or
  writing an Alembic file.
- Added support for facesets, which are represented using the UsdGeomSubset
  schema. (PR: [#758](https://www.github.com/PixarAnimationStudios/USD/pull/758))

### Houdini plugin:
- Added support for namespaced primvars in Houdini 17.5+. 
  (PR: [#747](https://www.github.com/PixarAnimationStudios/USD/pull/747))
- Added support for caching in-memory USD stages in GusdStageCache. 
  (PR: [#775](https://www.github.com/PixarAnimationStudios/USD/pull/775))
- Added support for import and export of holeIndices attribute on UsdGeomMesh.
- Added support for all registered USD file extensions.
- Changes in support of upcoming Houdini versions.
- Fixed spurious warnings when evaluating transforms on pseudo-root prims.

### Katana plugin:
- The Katana plugin no longer supports Katana 2.x. Support for 2.x was
  deprecated in release 19.03.
- Expanded support for and use of zero-copy construction of FnAttribute from
  VtArray.
- Ensure that a NullAttribute is used to block an inherited shading connection
  when the USD equivalent is present.
- PxrUsdIn will observe a "katana:useDefaultMotionSamples" USD attribute to
  hard-wire the equivalent behavior of a PxrUsdInDefaultMotionSamples node
  targeting that scope.
- Fixed bug where calling UsdKatanaCache::FindSessionLayer with a cache key
  derived from an empty GroupAttribute would not return a layer whose key
  was generated for an invalid/nonexistent GroupAttribute.
- Fixed crash in PxrUsdIn when an invalid regex value was given for
  "ignoreLayerRegex".

### Maya plugin:
- Added support for exporting units via new "metersPerUnit" metadata.
- Imported shaders now appear in the Hypergraph UI.
- Imported lights now appear in defaultLightSet.

## [19.03] - 2019-02-13

### Added
Build:
- Ability to download and build OpenColorIO dependency via `--opencolorio`
  argument to support new color management functionality in Hydra.

USD:
- Enabled usddumpcrate utility for viewing .usdc file information in the build.
  (PR: [#470](https://www.github.com/PixarAnimationStudios/USD/pull/470))
- Internal payloads, list editing and layer offsets for payload arcs. Payloads
  now provide the same set of features as references.

  This may affect backwards compatibility of layers written with this version
  of USD:
  - Layers containing payloads using these new features will not be readable
    in previous USD versions. .usdc files containing these payloads will be
    marked as version 0.8.0.
  - Layers containing payloads that only use the previously-available features
    will continue to be readable in previous USD versions.
- API for querying authored and inherited primvars in UsdGeomPrimvarsAPI schema.
- Optional 'jointNames' attribute to UsdSkelSkeleton to help DCC import/export.

Imaging:
- HdxColorCorrectionTask to do linear to sRGB conversions or color management
  via OpenColorIO.
- HdxProgressiveTask type tag for tasks that support progressive rendering.
- Initial support for computations in the UsdImaging scene delegate.
- Support for points rendering for basis curves in Hydra GL.
- Publish the normals attribute or primvar from points primitives through 
  Hydra. (PR: [#742](https://www.github.com/PixarAnimationStudios/USD/pull/742))
- usdview now responds to all scene edits from the interpreter. Plugins can
  connect to the usdviewApi.dataModel.signalPrimsChanged signal to be notified
  for these changes.

Houdini plugin:
- Support for more primvar types.

Katana plugin:
- Support for reading materials with multiple incoming shader connections.

Maya plugin:
- Initial work for a UsdPreviewSurface shader node and USD export support.

### Changed
Build:
- build_usd.py now explicitly detects Python version to link against on macOS
  to avoid conflicts between multiple installations. 
  (Issue: [#19](https://www.github.com/PixarAnimationStudios/USD/issues/19), 
   Issue: [#699](https://www.github.com/PixarAnimationStudios/USD/issues/699))
- build_usd.py only builds boost libraries like boost::filesystem when needed.

USD:
- UsdStage::MuteAndUnmuteLayers and LoadAndUnload now send a
  UsdNotice::StageContentsChanged notification. 
  (PR: [#710](https://www.github.com/PixarAnimationStudios/USD/pull/710))

Imaging:
- UsdImaging scene delegate now stops population traversal at non-imageable
  prims.
- Materials are no longer resynced on visibility changes.
- Hydra will now only reset varying state for clean prims.
- HdxCompositor can now copy depth without remapping its range and can also
  copy color and depth to a user-provided viewport size.
- Interacting with the 'Vis' or 'Draw Mode' columns in usdview no longer
  changes selection.
- When multiple prims are selected in usdview, changes in the 'Vis' column
  to one of those prims will affect all of them.

Katana plugin:
- Optimizations for reading gprims and point instancers via a new library,
  vtKatana. This library requires Katana 3.0 or later.

Maya plugin:
- Configuration of the default material scope name has been moved to usdUtils
  to facilitate sharing across plugins. See UsdUtilsGetMaterialsScopeName.

### Deprecated
USD:
- UsdAttribute::HasAuthoredValueOpinion in favor of HasAuthoredValue.
- UsdPrim::ClearPayload, HasPayload, and SetPayload in favor of API on
  UsdPayloads object returned by GetPayloads.
- Primvar API on UsdGeomImageable in favor of UsdGeomPrimvarsAPI schema.

Katana:
- Support for Katana 2.x in favor of Katana 3.x. We anticipate removing support
  for Katana 2.x in the next release.

### Removed
- Dependency on boost::filesystem 
  (Issue: [#679](https://www.github.com/PixarAnimationStudios/USD/issues/679))
- Ability to read pre-xformOp transform attributes on UsdGeomXformable prims.
- UsdShadeMaterial::CreateMaterialFaceSet, GetMaterialFaceSet,
  HasMaterialFaceSet functions.
- UsdGeomCollectionAPI schema.

### Fixed
Build:
- Detection of Visual Studio on non-English platforms in build_usd.py.
  (Issue: [#613](https://www.github.com/PixarAnimationStudios/USD/issues/613), 
   Issue: [#697](https://www.github.com/PixarAnimationStudios/USD/issues/697))

USD:
- Error when composing prims with both direct and ancestral specializes arcs.
- Incorrect strength ordering when composing specializes arcs declared across
  an internal reference arc.
- UsdAttribute::GetConnections and UsdRelationship::GetTargets could return
  incorrect paths in cases involving nested instancing and instance proxies
  inside of masters.
- Corrupted values when parsing OSL string arrays in Sdr via sdrOsl plugin.
- Various fixes for UsdSkelSkinningQuery and UsdSkelAnimMapper.
- Incorrect results from UsdGeomBBoxCache::ComputeRelativeBound.
- UsdGeomPointInstancer enums in Python are now properly scoped under
  UsdGeom.PointInstancer instead of UsdGeom.

Imaging:
- GlfDrawTarget::_BindAttachment could leave a different framebuffer bound
  than what was bound before calling the function.
- Multiple GLSL shader fixes to allow Hydra GL to render correctly with Intel
  GPUs.
- Errors when trying to use usdview on macOS due to incorrect handling of
  unsupported Hydra renderers. When no supported renderers are found, usdview
  will fall back to the legacy renderer.
- Missing symbol exports in UsdVolImaging that could cause build errors on
  Windows. (PR: [#742](https://www.github.com/PixarAnimationStudios/USD/pull/742))
- Regression in usdview that caused the "Redraw On Frame Scrub" option to
  always be in effect. 
  (Issue: [#734](https://www.github.com/PixarAnimationStudios/USD/issues/734))

Alembic plugin:
- Alembic curve basis, type, and wrap were being converted to varying 
  attributes in USD instead of uniform.

Houdini plugin:
- Incorrect display in Tree View panel when unimporting the top-most prim.
- USD Reference ROP behavior when updating or removing an existing reference.
- Various compilation errors with different Houdini versions.
- Incorrect default value for horizontal aperture offset in USD camera node.
- Vertex attributes on meshes with rightHanded winding order. 
  (Issue: [#631](https://www.github.com/PixarAnimationStudios/USD/issues/631), 
  PR: [#632](https://www.github.com/PixarAnimationStudios/USD/pull/632))

Maya plugin:
- Incorrect name conflict error on export when stripping namespaces and merging
  transform and shape nodes. 
  (PR: [#683](https://www.github.com/PixarAnimationStudios/USD/pull/683))

## [19.01] - 2018-12-14

### Added
USD:
- usdresolve utility for checking the results of asset resolution at the 
  command line. (PR: [#609](https://www.github.com/PixarAnimationStudios/USD/pull/609))
- SdfFileFormat::FindAllFileFormats and FindAllFileFormatExtensions
  for querying available file formats. 
  (PR: [#532](https://www.github.com/PixarAnimationStudios/USD/pull/532))
- Option to UsdGeomBBoxCache to ignore visibility.

Imaging:
- Render settings API to HdRenderDelegate. ("Enable Tiny Prim Culling" is the
  first example for Hydra GL)
- Support for UDIM textures in Hydra GL. 
  (PR: [#597](https://www.github.com/PixarAnimationStudios/USD/pull/597))
- Blending controls to HdRenderPassState for initial transparency support in
  Hydra GL.
- Ability to specify backend when starting usdview via "--renderer" parameter.
- Optional "Draw Mode" column to prim browser in usdview that provides control
  of model draw modes. (texture cards, bbox, etc.)

Houdini plugin:
- Optional layer scale, layer offset, and target prim parameters to the
  USD Reference ROP.
- Option on USD Output ROP to use or ignore obj-level transforms.
- Support for exporting ramp parameters in OSL shader networks.

Katana plugin:
- "info.usd.outputSession" attribute authored by PxrUsdIn. This is a sibling
  to "info.usd.session" but is not affected by the translation of PxrUsdIn's
  old "variants" parameter into session data. It acts as a more stable session 
  cache key for external apps to use.
- "forceArrayForSingleValue" parameter that allows PxrUsdInAttributeSet to
  author 1-length arrays.

Maya plugin:
- Support for instancing USD reference assemblies in instancers.

### Changed
Build:
- Symbols for wrapping functions are no longer exported from modules
  to avoid issues with using RTLD_GLOBAL in Python. 
  (Issue: [#641](https://www.github.com/PixarAnimationStudios/USD/issues/641))
- Updated minimum required version of GLEW to 2.0.0.

USD:
- Numerous fixes and cleanup changes throughout to improve performance, 
  remove dead code, convert code to more modern patterns, and remove 
  unneeded uses of boost.
- Replaced safe-bool idiom with explicit bool operator in several places.
- Improved performance of setting and erasing time samples in .usda 
  layers from linear complexity to logarithmic.
- Improved error messages when attempting to read a .usdz files using
  compression or encryption.
- Various improvements to UsdSkel documentation and API.
- Changed scene description name for UsdSkelBindingAPI from "BindingAPI"
  to "SkelBindingAPI" to distinguish it from other binding API schemas.

Imaging:
- Several improvements to the handling of AOVs in Hydra.
- Simplified class hierarchy in usdImagingGL. The primary API entry point
  is now UsdImagingGLEngine.
- Better performance for animated textures and large numbers of invisible
  prims in Hydra.
- Optimized batch removal of prims in Hydra and UsdImaging.
- Better handling of edits made in the interpreter in usdview.
- Mesh normals are suppressed when the subdivision scheme is something
  other than "none".
- The default stb-based image reader now extracts gamma information from
  .png images.
- Functionality specific to the prim browser in usdview have been moved
  to a browser-specific menu bar with "Navigation" and "Show" menus.
- Property browser in usdview now shows resolved material bindings and
  inherited primvars.

 Maya plugin:
- The name of the scope for exported material prims can now be customized 
  in the UI or via the "materialsScopeName" parameter to usdExport. The 
  default scope name is "Looks", matching the previous behavior.
- Refactoring and cleanup in preparation for shading export support.

### Removed
Build:
- Build-time dependency on Python. 
  (Issue: [#605](https://www.github.com/PixarAnimationStudios/USD/issues/605), 
  PR: [#615](https://www.github.com/PixarAnimationStudios/USD/pull/615))

Imaging:
- HdSceneTask in favor of HdTask.
- UsdImagingGL and UsdImagingGLHdEngine.

### Fixed
USD:
- Incorrect composition error in cases involving internal sub-root references 
  and variants. 
  (Issue: [#677](https://www.github.com/PixarAnimationStudios/USD/issues/677))
- Regression that caused prims to sometimes be composed incorrectly.
- Missing symbol exports that could cause build errors on Windows. 
  (PR: [#623](https://www.github.com/PixarAnimationStudios/USD/pull/623), 
   Issue: [#703](https://www.github.com/PixarAnimationStudios/USD/issues/703), 
   Issue: [#704](https://www.github.com/PixarAnimationStudios/USD/issues/704), 
   Issue: [#705](https://www.github.com/PixarAnimationStudios/USD/issues/705))
- Incorrect type validation and conversion when authoring metadata via Python
  that could lead to invalid scene description. 
  (Issue: [#529](https://www.github.com/PixarAnimationStudios/USD/issues/529))
- Various issues in usdzip and related usdUtils API when processing references 
  for packaging into a .usdz file.

Imaging:
- Batch aggregation for prims with face-varying primvars.
- Drawing coord initialization of instance primvar slots.
- Deep batch validation only happens when needed, improving performance.
- Crash in cases where a prim is removed from a stage and a collection
  targeting that prim is updated in the same round of changes. 
  (PR: [#685](https://www.github.com/PixarAnimationStudios/USD/pull/685))
- Crash when reading half-float .exr images in the OpenImageIO plugin.
  (Issue: [#581](https://www.github.com/PixarAnimationStudios/USD/issues/581))
- Incorrect handling of edits to material bindings.

Houdini plugin:
- Issue where visibility and purpose attributes weren't properly inherited 
  on import. 
  (Issue: [#649](https://www.github.com/PixarAnimationStudios/USD/issues/649))
- Crash when running ginfo on a USD file. 
  (Issue: [#673](https://www.github.com/PixarAnimationStudios/USD/issues/673), 
  PR: [#674](https://www.github.com/PixarAnimationStudios/USD/pull/674))
- Issue where imported string attribute values could be incorrect. 
  (Issue: [#653](https://www.github.com/PixarAnimationStudios/USD/issues/653))

Katana plugin:
- Issues with reading UsdGeomPointInstancers
  - Regression in instance transform computation with masked instances.
  - Prototype transforms are cleared out on the Katana side since they are
    folded in when computing instance transforms.

Maya plugin:
- Reference assemblies weren't being drawn in "playback" representation.
  (Issue: [#675](https://www.github.com/PixarAnimationStudios/USD/issues/675))
- Issue where HdImagingShape prevented nodes from being reordered.

## [18.11] - 2018-10-10

### Added
USD:
- [usdVol] UsdVol schema for representing volumes. 
  (PR: [#567](https://www.github.com/PixarAnimationStudios/USD/pull/567))
- [usdShade] Ability to define shaders in the shader definition registry 
   using UsdShade. 
- [usdShaders] Shader definition registry plugin for core nodes defined in 
   the UsdPreviewSurface specification.

Imaging:
- [hd, usdImaging] Hydra support for UsdVol schema. 
  (PR: [#567](https://www.github.com/PixarAnimationStudios/USD/pull/567))
- [hd, hdSt] Topology visibility operations for meshes in Hydra.
- [hd, hdSt] Support for inverting the depth test of a draw target.
- [hd, hdSt] "Flat normals" computed buffer source for meshes.
- [hdx] ResolveNearestToCenter pick hit resolution method for ray picking.

- [usdview] Menu for switching AOVs for supported backends.
- [usdview] Menu for backend-specific settings.

Alembic plugin:
- [usdAbc] Behavior where an Xform with a single geometry or camera in an 
  Alembic file would be collapsed into a single prim in USD can now be disabled 
  by setting the environment variable `USD_ABC_XFORM_PRIM_COLLAPSE` to 0. 
  (PR: [#527](https://www.github.com/PixarAnimationStudios/USD/pull/527))

Maya plugin:
- [usdMaya] Display filter for USD proxy shapes.
- [usdMaya, pxrUsdMayaGL] pxrHdImagingShape node that reduces overhead when 
  imaging scenes with many Hydra-backed shapes.
- [pxrUsdMayaGL] Support for Maya lights casting shadows between Hydra shapes, 
  and Hydra shapes to Maya shapes, but not Maya shapes to Hydra shapes.

Houdini plugin:
- [gusd] Experimental build support for Houdini plugin on Windows.
- [OP_gusd] Multi-seg export to USD output ROP.

### Changed
- Numerous fixes and cleanup changes throughout to improve performance, 
  convert code to more modern patterns, and remove unneeded uses of boost.
  (PR: [#373](https://www.github.com/PixarAnimationStudios/USD/pull/373), 
   PR: [#453](https://www.github.com/PixarAnimationStudios/USD/pull/453), 
   PR: [#481](https://www.github.com/PixarAnimationStudios/USD/pull/481), 
   PR: [#484](https://www.github.com/PixarAnimationStudios/USD/pull/484), 
   PR: [#488](https://www.github.com/PixarAnimationStudios/USD/pull/488))

USD:
- [usd] UsdAPISchemaBase is now an abstract base class as originally intended.
- [usdGeom] Improved performance of UsdGeomPointBased::ComputeExtent. In one 
  example, this provided a ~30% improvement. 
  (PR: [#588](https://www.github.com/PixarAnimationStudios/USD/pull/588))
- [usdGeom] Improved performance of computing extents for UsdGeomPointInstancer.
  In one example with >5 million instances, extent computation time decreased
  from ~25 seconds to ~3 seconds.
- [usdSkel] UsdSkelBakeSkinning bakes skinning at additional time samples
  based on stage's configured sampling rate.
- [usdedit] Opening a .usdz file now forces read-only mode.
- [usdzip] "-l" or "-d" arguments will now provide information about existing 
  .usdz files.
- [usdzip] ARKit compliance checks now require valid sources for shader inputs
  with connections and valid prims and/or collections for material bindings.

Imaging:
- [hd] Improved Hydra AOV API.
- [hd] Split reprs into the topology indices they affect.
- [hd] Properly set up both the shadow projection and view matrices. 
  (PR: [#583](https://www.github.com/PixarAnimationStudios/USD/pull/583))
- [hd] Optimization to not run garbage collection if a texture hasn't change.
- [hd, hdSt, usdImaging] Update wrap mode fallback behavior to useMetadata.
- [hdx] Improved shadow support in HdxTaskController. 
  (PR: [#541](https://www.github.com/PixarAnimationStudios/USD/pull/541))
- [hdx] "Enable Hardware Shading" entry point is now "Enable Scene Materials".
- [hdSt] Support to generate GLSL-compatible names for resources/primvars.
- [usdImagingGL] Better support for animated textures. 
  (PR: [#488](https://www.github.com/PixarAnimationStudios/USD/pull/488))
- [usdview] Better error reporting if an unsupported backend is selected. 
  (PR: [#635](https://www.github.com/PixarAnimationStudios/USD/pull/635))
- [hdEmbree] Improved AO sampling (cosine-weighted sampling, stratified AO 
  samples LHS).

Maya plugin:
- [pxrUsdMayaGL] Many performance improvements in the Hydra batch renderer.
- [usdMaya, pxrUsdMayaGL] Using USD in live surface uses the normal at the 
  snap point.
- [usdMaya, pxrUsdTranslators] Various code cleanup in USD exporter.

Katana plugin:
- [usdKatana] Use bracketing sample closest to shutter open/close if no frames
  found in shutter interval.
- [usdKatana] Honor UsdStage's authored timeCodesPerSecond and framesPerSecond.
  If they differ, Katana will operate in frames, with current time, motion
  samples, and shutter interval scaled appropriately.

Houdini plugin:
- [gusd] Rebind materials when creating sub-root references.
- [gusd] Optimizations in handling of stage mask operations.
- [OP_gusd] USD output ROP now allows binding to shaders referenced from other 
  models.

### Removed

Imaging:
- [hd] Removed Repr requirements from GetRenderTag API.

### Fixed
Build:
- [cmake] User-specified CMAKE_MODULE_PATH was being ignored during build. 
  (PR: [#614](https://www.github.com/PixarAnimationStudios/USD/pull/614))
- [cmake] Installed headers could take precedence over source headers, leading 
  to build failures. 
  (Issue: [#83](https://www.github.com/PixarAnimationStudios/USD/issues/83)) 

USD:
- [tf] TfEnvSetting crash on MSVC 2017.
- [ar] ArDefaultResolver::AnchorRelativePath did not call IsRelativePath on
  derived resolvers. 
  (PR: [#426](https://www.github.com/PixarAnimationStudios/USD/pull/426))
- [sdf] SdfCopySpec did not handle copying prim specs to variants and vice-versa.
- [sdf] Incorrect notification was sent when removing subtrees of inert specs.
- [usd] Prim payload was not automatically loaded when a deactivated ancestor 
  prim with a payload was activated. 
  (Issue: [#604](https://www.github.com/PixarAnimationStudios/USD/issues/604))
- [usd] Incorrect lists of time samples would be returned in certain cases
  with value clips.
- [usd, usdzip] .usdz files were not fully compliant with the zip file format 
  specification. 
- [usdUtils, usdzip] Creating an ARKit .usdz file with a .usda asset would 
  create an invalid file.
- [usdMtlx] MaterialX plugin build failures on Windows and macOS. 
  (Issue: [#611](https://www.github.com/PixarAnimationStudios/USD/issues/611))

Imaging:
- [hdx] Picking with disabled scene materials generating shader errors.
- [hdSt] Fix immediate draw batch invalidation on buffer migration.
- [hdSt] Fix garbage collection on buffer migration.
- [hdSt] Bindless texture loading not being deferred.
- [usdImaging] Wrong transforms were being used when using usdLux light 
  transforms 
  (Issue: [#612](https://www.github.com/PixarAnimationStudios/USD/issues/612)).
- [usdImaging] Fix for usdImaging's TextureId not accounting for origin.

Houdini plugin:
- [gusd] Several fixes for building with Houdini 17. 
  (PR: [#549](https://www.github.com/PixarAnimationStudios/USD/pull/549))
- [gusd] Several fixes for stale caches and crashes.
- [gusd] Potential PackedUSD transform issues in Houdini 16.5.

## [18.09] - 2018-08-07

This release includes several major new features and changes, including:
  - .usdz file format for packaging assets into a single file
  - Introduction of UsdPreviewSurface
  - Switch to calendar-based versioning scheme for USD libraries

### Added
- Build:
  - build_usd.py can be given custom arguments for building libraries via the
    `--build-args` option.
  - Option to disable building usdview by specifying `PXR_BUILD_USDVIEW=OFF` 
    when running cmake or `--no-usdview` when running build_usd.py.
  - Allow specifying `Boost_USE_STATIC_LIBS` to cmake on Windows. Note that 
    static boost libraries may lead to issues with Python bindings. 
    (Issue: [#407](https://www.github.com/PixarAnimationStudios/USD/issues/407))

- USD:
  - ArAsset and ArResolver::OpenAsset interfaces allowing resolvers to control
    how data for a given asset is accessed.
  - "Package" asset and layer concepts to Ar and Sdf, including 
    "package-relative" asset path syntax for addressing assets within packages.
  - sdffilter utility for inspecting and summarizing the contents of a layer.
  - .usdz file format. This format allows multiple assets (including layers, 
    textures, etc.) to be packaged into a single file that can be consumed by
    USD without being unpacked to disk. These files can be created via the 
    "usdzip" command-line utility or via APIs like UsdZipFileWriter and 
    UsdUtilsCreateNewUsdzPackage.
  - UsdStage::GetObjectAtPath for retrieving a generic UsdObject. 
    (PR: [#390](https://www.github.com/PixarAnimationStudios/USD/pull/390))
  - UsdCollectionAPI can represent collections that include all paths or exclude
    some paths but include all others via the new "includeRoot" attribute.
  - Support for "sourceAsset" and "sourceCode" implementation sources in 
    UsdShadeShader.
  - Ndr and Sdr libraries that provide a registry for shader definitions that 
    can be extended via plugins.
  - sdrOsl plugin for populating shader definition registry from OSL 1.8.12. 
    OSL support must be enabled by specifying `PXR_ENABLE_OSL_SUPPORT=TRUE` 
    when running cmake.
  - usdMtlx plugin containing USD file format and shader definition registry 
    plugins based on MaterialX 1.36.0. MaterialX support must be enabled by
    specifying `PXR_BUILD_MATERIALX_PLUGIN=TRUE` when running cmake.
  - Initial UsdSkel schema for blend shapes, which are not yet factored into
    computed deformation.

- Imaging:
  - Per-prim adaptive refinement enabled via materials in hdSt.
  - Preliminary AOV support in Hydra with sample implementation in Embree 
    plugin.
  - HdRenderThread utility class to enable render delegates to render in 
    background threads. The Embree plugin uses this class as an example.
  - Support for scene-authored normals on meshes.

- UsdImaging:
  - Light linking support.
  - Initial support for USD material preview nodes. For more details, see:
    http://graphics.pixar.com/usd/docs/UsdPreviewSurface-Proposal.html 

    Currently unimplemented features include:
    - Support for automatically computed tangents when normal mapping. Primvars
      must be supplied.
    - Handling for ior (index or refraction).

- Alembic plugin:
  - Support for TextureCoordinate role and value types.

- Maya plugin:
  - Initial support for exporting .usdz files.
  - Support for importing and exporting volume and displacement in "pxrRis"
    shading mode.
  - pxrUsdPointBasedDeformerNode - an animation cache deformer that allows users
    to pose a Maya mesh based on a UsdGeomPointBased prim.
  - Option to apply Euler filtering to transforms during export. 
    (PR: [#299](https://www.github.com/PixarAnimationStudios/USD/pull/299))
  - Option to filter out certain Maya node types during export. 
    (PR: [#475](https://www.github.com/PixarAnimationStudios/USD/pull/475))

- Katana plugin:
  - Support for TextureCoordinate role and value types.
  - Support for importing inherit paths metadata as a info.usd group attribute
  - "additionalLocations" parameter to PxrUsdInVariantSelect
  - "asArchive" option to PxrUsdIn. When enabled, a "usd archive" location will 
    be created rather than loading the USD data directly. This is intended for 
    passing along to renderers which support reading USD natively.
  - PxrUsdIn supports a "sharedLooksCacheKey" attribute to permit sharing cached
    USD shading information across multiple references/instances which are known
    to be identical from a pipeline standpoint.

- Houdini plugin:
  - Support import of Scope prims.

### Changed
- Build:
  - USD resource files are now installed into <prefix>/lib/usd instead of
    <prefix>/share/usd/plugins to bring them closer to the installed libraries.
  - OpenGL dependency in imaging is now optional and may be disabled by 
    specifying `PXR_ENABLE_GL_SUPPORT=FALSE` when running cmake. This will 
    disable all GL-based functionality, including usdview.
  - OpenImageIO dependency imaging is now optional and is disabled by default. 
    Support must be enabled by specifying `PXR_BUILD_OPENIMAGEIO_PLUGIN=TRUE`
    when running CMake or `--openimageio` when running build_usd.py.
  - build_usd.py on Windows will now use powershell to download dependencies
    if it's available to avoid TLS 1.2 issues. 
    (Issue: [#449](https://www.github.com/PixarAnimationStudios/USD/issues/449))
  - build_usd.py now directs users to install PySide2 via pip on MacOS.

- USD:
  - Numerous fixes and cleanup changes throughout to improve performance, 
    convert code to more modern patterns, and remove unneeded uses of boost.
  - ArGetResolver no longer returns an instance of the ArResolver subclass used
    for asset resolution. Clients can use ArGetUnderlyingResolver in the special
    cases where access to the exact subclass is necessary.
  - ArResolver context and scoped cache functions are now public and have been
    renamed to match convention. Resolvers that override these functions will 
    need to be updated.
  - ArDefaultResolver default context for an asset now includes the directory
    of the asset in its search path.
  - Optimizations to prim change processing in UsdStage. In one example, 
    processing time for adding new prims decreased ~50%.
  - New .usdc files now default to version 0.7.0, which includes compression
    features introduced in earlier releases. These files cannot be read in USD
    releases prior to v0.8.4. Users can revert to writing older versions by
    setting the environment variable `USD_WRITE_NEW_USDC_FILES_AS_VERSION`
    to an older version.
  - Several changes to provide cleaner and more detailed runtime error messages.
  - Better support for generating code for properties of multiple-apply API
    schemas in usdGenSchema.
  - Schema types are now specified using the UsdSchemaType enum instead of 
    individual flags.
  - "expansionRule" attribute for UsdCollectionAPI is now optional; collections
    are created simply by calling UsdCollectionAPI::ApplyCollection.
  - UsdGeomBBoxCache's bounds computation now includes all defined prims,
    including those with no type specified.
  - UsdRiMaterialAPI now writes standardized "ri:surface" output by default 
    instead of deprecated "ri:bxdf".
  - UsdRiStatementsAPI now writes ri attributes as primvars by default.
  - Renamed UsdSkelPackedJointAnimation schema to UsdSkelAnimation
  - Renamed UsdSkelBakingSkinningLBS function to UsdSkelBakeSkinning
  - Numerous additional fixes and changes to UsdSkel schemas and APIs.

- Imaging:
  - Several API cleanup changes in HdSprim/HdBprim/HdRprim.
  - HdRenderDelegate::CanComputeMaterialNetworks has been replaced by
    GetMaterialBindingPurpose, which has a default value of "preview".
  - Improved handling of fallback texturing behavior when texture inputs are
    missing from a material.
  - .png, .jpg, .bmp, .tga, and .hdr images are now handled by a built-in
    image reader based on stb_image.
  - Texture loading is deferred until needed.
  - Truncate mesh vertex primvar data to expected length if larger than expected.
  - Improved ptex rect packing performance. In one large test case, processing
    time decreased from ~600s to ~40s.

- UsdImaging:
  - Avoid loading custom shaders and textures when the "Enable Hardware Shading"
    option is disabled in usdview.
  - Default values for wrapS/wrapT match the proposed value in the preview
    material spec. Also adds support for mirrored wrapS/wrapT.
  - Scene lights are now enabled by default. This can be disabled by setting
    the environment variable `USDIMAGING_ENABLE_SCENE_LIGHTS` to 0.
  - UsdShadeMaterialAPI is now used instead of UsdRi to look up material 
    networks.

- Alembic plugin:
  - Alembic writer now prefers writing to the "st" attribute instead of "uv". 
    (PR: [#463](https://www.github.com/PixarAnimationStudios/USD/pull/463))

- Maya plugin:
  - MAYA_SCRIPT_PATH and XBMLANGPATH environment variable settings have changed
    due to new resource file install location documented above. Refer to Maya
    plugin documentation for new values.
  - Large refactoring and cleanup of usdMaya. USD importers and exporters for
    built-in Maya nodes have been moved to pxrUsdTranslators plugin.
  - Proxy shape now redraws in response to changes on UsdStage.
  - Diagnostic messages are now batched together for better reporting behavior
    and to prevent log spewage.
  - Improved primvar export. 
    (PR: [#330](https://www.github.com/PixarAnimationStudios/USD/pull/330))
  - Camera shake now applied to USD cameras on export. 
    (PR: [#366](https://www.github.com/PixarAnimationStudios/USD/pull/366))

### Deprecated
- USD:
  - UsdSkelPackedJointAnimation schema in favor of UsdSkelAnimation
  - Specialized RIS and RSL shader schemas in UsdRi (e.g. UsdRisPattern and 
    UsdRisBxdf) in favor of generic UsdShadeShader prims.

### Removed
- Build:
  - boost::regex dependency.

- USD:
  - ArResolver::CreateDefaultContextForDirectory to better accommodate asset
    systems that are not file-system based.
  - SdfExtractExternalReferences. UsdUtilsExtractExternalReferences can be used
    as an alternative.
  - UsdLuxLinkingAPI, in favor of UsdCollectionAPI to represent light linking.

### Fixed
- Build:
  - Fixed issues with using Visual Studio 2017 in build_usd.py.
  - Issue preventing use of NMake generator on Windows. 
    (PR: [#519](https://www.github.com/PixarAnimationStudios/USD/pull/519))
  - Several fixes for static library builds.

- USD:
  - Bug where SdfLayer::GetDisplayName would return an empty string for 
    anonymous layers if the tag contained a ":". 
    (PR: [#440](https://www.github.com/PixarAnimationStudios/USD/pull/440))
  - Bug where SdfLayer::UpdateExternalReferences would not handle variants and 
    payloads correctly.
  - Incorrect load state handling in UsdStage for payloads introduced across 
    sub-root references, inherits, and specializes arcs.
  - Incorrect value clip time mapping during value resolution on UsdStage.
  - Change processing bug when adding a new inert prim spec.
  - Load/unload change processing bug with nested instances on UsdStage.
  - Crash when opening a UsdStage with a UsdStagePopulationMask containing 
    instance prims. 
    (Issue: [#497](https://www.github.com/PixarAnimationStudios/USD/issues/497))
  - Test failures when USD_EDITOR environment variable is set. 
    (Issue: [#505](https://www.github.com/PixarAnimationStudios/USD/issues/505))
  - Bug where non-constant primvars were inherited down namespace in
    UsdGeomPrimvarsAPI.

- Imaging:
  - Triangulation/quadrangulation of face varying for refined meshes.
  - Several selection and picking issues.

- UsdImaging:
  - Material binding to instanced prims.
  - Change processing for UsdShadeShader prims beneath UsdShadeMaterial prims.

- Alembic plugin:
  - Texture coordinate indices are now preserved. 
  (PR: [#520](https://www.github.com/PixarAnimationStudios/USD/pull/520))

- Katana plugin:
  - Several asset resolution fixes. 
    (Issue: [#535](https://www.github.com/PixarAnimationStudios/USD/issues/535))
  - Bug preventing correct inheritance of motion sample times overrides as
    authored by PxrUsdInDefaultMotionSamples and PxrUsdInMotionOverrides in
    Katana plugin.

- Houdini plugin:
  - Fixed several display and update issues with treeview panel.
  - Fixed bug in batched loading of masked prims.

## [0.8.5a] - 2018-05-21

### Fixed
- Fixed broken URL in build_usd.py for downloading libtiff. 
  (Issue: [#498](https://www.github.com/PixarAnimationStudios/USD/issues/498))

## [0.8.5] - 2018-05-07

### Added
- New 'trace' library providing performance profiling functionality.
- API to Ar for resolver implementations that wrap around other resolvers.
- TextureCoordinate role and TexCoord{2,3}{h,f,d}{Array} value types to Sdf 
  to indicate attributes that represent UV(W) texture coordinates.
- UsdUtilsSparseValueWriter and UsdUtilsSparseAttrValueWriter utility classes
  for authoring attribute values sparsely.
- UsdGeomPrimvar::GetIndicesAttr API for indexed primvars.
- UsdGeomPrimvarsAPI schema for accessing primvars, including primvar
  values inherited down namespace. This is intended as an eventual replacement 
  for the primvar API on UsdGeomImageable.
- API to UsdShadeMaterial to support render context-specific terminal outputs 
  and core support for three universal render context outputs: surface, 
  displacement, and volume.
- Time-sampling support for UsdGeomPointInstancer via new methods for
  computing extents and instance transforms at multiple times.
- Finalize method for sprims and bprims in Hydra.
- Time-sampling support in Hydra for geometry instancing.
- Maya plugin users can specify if colors coming from Maya are linear via the
  `PIXMAYA_LINEAR_COLORS` environment variable.
- Support for import/export with new TextureCoordinate role in Maya plugin.
- Support for cards drawMode in USD reference assembly in Maya plugin.
- Support for LiveSurface with proxy shapes in Maya plugin.
- Support for importing UsdSkel schemas and exporting locators and
  particles in Maya plugin.
- Option for placing exported data in parent scope in Maya plugin.
- Initial support for session layer metadata in PxrUsdIn in Katana plugin.
- Support for reading in UsdLuxCylinderLight schemas in Katana plugin.
- Gusd Python bindings for Houdini plugin.

### Changed
- USD now requires TBB version 4.4 Update 6 or later.
- Removed GLUT dependency for libtiff in build_usd.py. 
  (Issue: [#402](https://www.github.com/PixarAnimationStudios/USD/issues/402))
- build_usd.py will now use cURL to download dependencies if it's
  installed in the user's PATH. This can help avoid TLS v1.2 errors
  when downloading from certain sites. 
  (Issue: [#449](https://www.github.com/PixarAnimationStudios/USD/issues/449))
- Numerous documentation additions and fixes throughout the codebase.
- Improved performance of SdfCopySpec and UsdUtils stitching API; one test
  case showed a 35% speed improvement, from 160s to 118s.
- SdfLayer now uses the resolved path provided by Ar for a given identifier
  to determine the layer file format. 
  (Issue: [#144](https://www.github.com/PixarAnimationStudios/USD/issues/144))
- Simplified API for setting layer data in SdfFileFormat subclasses.
- Adding or removing invalid sublayers now results in composition errors.
- UsdStage::CreateNew and CreateInMemory now accept an InitialLoadSet argument.
  This controls whether new payloads are loaded automatically. 
  (Issue: [#267](https://www.github.com/PixarAnimationStudios/USD/issues/267))
- UsdStage::ExpandPopulationMask now considers attribute connections.
- UsdStage::Flatten now authors anchored asset paths in flattened results.
- UsdTraverseInstanceProxies() now includes instance proxy prims that pass the
  default predicate instead of all prims.
- .usdc files now support zero-copy array access, which can significantly
  improve performance. This feature is supported by all .usdc file
  versions, but may not be activated for certain files depending on their
  data alignment. In these cases, users can simply re-export the file 
  to activate zero-copy access.

  Users can set the environment variable `USDC_ENABLE_ZERO_COPY_ARRAYS` to 0 
  to disable this feature. Users can also track cases where array data is 
  copied due to modifications by setting the environment variable
  `VT_LOG_STACK_ON_ARRAY_DETACH_COPY` to 1.
- Other performance improvements in UsdStage composition.
- API schemas are now classified as either "applied" or "non-applied". See
  "Generating New Schema Classes" tutorial for more information. 
- Behavior of UsdUtils stitching API can now be customized via callbacks.
- UsdShadeMaterialBindingAPI now issues a warning when computing resolved
  material bindings if prims with old "look:binding" relationships are found.
  This can be disabled by setting the environment variable 
  `USD_SHADE_WARN_ON_LOOK_BINDING` to 0.
- UsdRiMaterialAPI now supports writing ri:surface outputs. ri:bxdf outputs are
  still written by default, but this can be disabled by setting the environment
  variable `USD_RI_WRITE_BXDF_OUTPUT` to 0.
- UsdRiStatementsAPI now supports encoding ri attibutes as primvars. This is
  disabled by default but can be enabled by setting the environment variable 
  `USDRI_STATEMENTS_WRITE_NEW_ATTR_ENCODING` to 1.
- Additions and improvements to UsdLux and UsdSkel schemas.
- Hydra backends that consume full network materials now receive all primvars
  for all rprims, not just those with bound materials.
- Several improvements to primvar handling and picking infrastructure in Hydra.
- Several performance improvements in Hydra, especially for material bindings
  and scenes with heavy nesting of instances.
- Ongoing work on Hydra compute framework.
- Conformed Hydra API to use "Primvar" instead of "PrimVar".
- HdMaterialNetwork is now emitted in topological order.
- Redundant animation data is no longer written during Maya export.
- Katana plugin now allows parent scope names for materials other than "Looks".
  This can be enabled by setting the `USD_KATANA_ALLOW_CUSTOM_MATERIAL_SCOPES`
  environment variable to 1.
- Refactor material loading in Katana plugin so that material locations aren't 
  computed all at once and site ops can run on each location individually.
- Improved construction of material terminal outputs in Katana plugin.
- Caching improvements in Houdini plugin to share USD stages between loaded
  primitives when possible.
- Improved error handling and reporting in Houdini plugin.
- USD stage masking in Houdini plugin can now be disabled via 
  `GUSD_STAGEMASK_ENABLE` environment variable.
- Improved performance when opening USD stages which don't use "component" kind
  in their model hierarchy.
- Object-level transforms are now written at a higher-level scope than leaf
  primitives if possible in Houdini plugin.

### Deprecated
- UsdGeomFaceSetAPI in favor of UsdGeomSubset.
- UsdGeomCollectionAPI in favor of UsdCollectionAPI.

### Removed
- tracelite library, which has been replaced with the trace library.
- Conversions for Python's datetime and dependency on boost::date_time.
- Several unused classes and functions.
- usdHydra schemas. This functionality is being replaced with by a registry
  of shaders in a future release.
- API for ri:bxdf output on UsdRiMaterialAPI schema. ri:bxdf sources will still
  be returned by UsdRiMaterialAPI::GetSurface for backwards compatibility.

### Fixed
- Build errors when using ninja.
- Error when extracting boost on Windows in build_usd.py. 
  (Issue: [#308](https://www.github.com/PixarAnimationStudios/USD/issues/308))
- The build now prefers Alembic library specified at cmake time over any
  Alembic library found in PATH. 
  (Issue: [#409](https://www.github.com/PixarAnimationStudios/USD/issues/409))
- Several compile-time warnings on clang and other compilers.
- Bug where list-op valued metadata was not emitted by UsdStage::Flatten.
- Missing notifications for master prims affected by metadata/property changes.
- Crash in UsdCollectionAPI::ApplyCollection when given an invalid collection 
  name. (Issue: [#425](https://www.github.com/PixarAnimationStudios/USD/issues/425))
- Change processing for changes to drawMode property in imaging.
- Numerous fixes to cards draw mode.
- Change processing bug when removing nested point instancers.
- Crashes in pxOsd due to incorrect authored crease data.
- Bug where usdview would not redraw after switching renderer.
- Crash in Alembic plugin when reading files with object names beginning 
  with numerals.
- Change processing bug when reloading an Alembic layer. 
  (Issue: [#429](https://www.github.com/PixarAnimationStudios/USD/issues/429))
- Several crash bugs in Maya plugin.
- Material export bug in Maya plugin where the surface terminal was not
  being exported under the "ri" render context. This fix requires re-exporting
  any material networks exported from Maya using version 0.8.4.
- Prevent overwriting instance sources that resolve to the same master name
  in Katana plugin.
- Path resolution issue in Houdini plugin.
- Miscellaneous bug fixes in Houdini plugin.

## [0.8.4] - 2018-03-05

### Added
- The PXR_PLUGINPATH_NAME environment variable name may be changed by
  specifying `PXR_OVERRIDE_PLUGINPATH_NAME=<name>` when running cmake.
- Example sendmail plugin for usdview, located at
  extras/usd/examples/usdviewPlugins/sendMail.py
- ArDefaultResolverContext, a context object for the ArDefaultResolver asset
  resolution implementation that allows additional search paths to be used
  during asset resolution.
- Users can now query the UsdNotice::ObjectsChanged notice for the changed
  scene description fields that affected the reported objects.
- UsdAPISchemaBase base class for all API schemas.
- All UsdGeomBoundable schemas in usdGeom now have functions for computing
  extents. These functions are also used when calling the general 
  UsdGeomBoundable::ComputeExtentFromPlugins method.
- UsdLuxCylinderLight schema.
- Significant additions to edge and point selection and highlighting 
  capabilities in Hydra.
- Initial support for UsdSkel bones in usdImaging.
- Initial support for exporting joints and skin clusters using the UsdSkel
  schema in the Maya plugin.
- Documentation for third party plugin code is now included in the
  doxygen build.

### Changed
- build_usd.py no longer checks for pyside-uic or boost::python if Python
  support is disabled, and no longer builds OpenImageIO's Python bindings.
- Updated moduleDeps.cpp files to only register direct library dependencies.
  This makes it easier for users to generate their own file for custom schemas.
- ArDefaultResolver now allows search paths like "Dir/File.usd" to be anchored 
  to other paths via AnchorRelativePath. During composition, these asset paths 
  will be resolved relative to the layer where they were authored before
  falling back to the previous search path behavior.
- Updates to VtArray and .usdc code in preparation for zero-copy functionality.
- Inherit and specializes arcs to non-existent prims are no longer considered
  composition errors.
- Apply method on API schemas have been moved to UsdAPISchemaBase and now
  require a UsdPrim. Also improved documentation.
- Property queries on UsdPrim now accept a predicate for filtering results.
- UsdPrim::HasAPI now accepts an instance name argument to query if a prim has
  a particular instance of a multiple-apply API schema has been applied.
- Adding or removing an inert prim spec no longer causes affected prims to
  be resynced. These prims are now reported as "changed info only" in the
  corresponding UsdNotice::ObjectsChanged notice.
- UsdNotice::ObjectsChanged::GetResyncedPaths and GetChangedInfoOnlyPaths now
  return a custom range object instead of a SdfPathVector.
- Performance optimizations for querying properties on UsdPrim.
- Replaced UsdCollectionAPI::AddPrim/RemovePrim with IncludePath/ExcludePath.
- UsdGeomBoundable::ComputeExtentFromPlugins now accepts an optional 
  transform matrix, which may be used to provide more accurate bounds.
- UsdGeomBBoxCache now computes extents for all UsdGeomBoundable schemas.
- Performance optimizations in UsdShadeMaterialBindingAPI.
- Numerous changes and fixes to UsdSkel schemas.
- Significantly improved curve rendering in Hydra.
- Many improvements towards the goal of getting modern UsdShade materials
  through Hydra to various kinds of backends.
- Performance improvements to hydra gather phase via multi-threading and other
  optimizations.
- Changed complexity options in usdview to prevent users from inadvertently
  bumping the complexity value too high and hanging the application.
- Several tweaks and improvements to usdview UI.
- Refactored Maya/Hydra batch renderer to improve performance for imaging USD
  proxy shape nodes.

### Removed
- UsdShadeLook schema. This has been replaced by UsdShadeMaterial.
  Material bindings authored using the "look:binding" relationship are no
  longer respected.

### Fixed
- Various typo and compiler warning fixes throughout the codebase.
- Fixed bug where build_usd.py would not use the CMake generator specified at
  the command line.
- Fixed crash in Apply method on API schemas. 
- Fixed several bugs in UsdShadeMaterialBindingAPI::ComputeBoundMaterial.
- Changing the population mask for a UsdStage now correctly releases resources
  used by objects that have been excluded from the stage.
- Fixed quadrangulation bug in Hydra with handling topology with degenerate or 
  hole faces.
- Fixed patch param refinement for Loop meshes.
- Several fixes to the nascent Hydra lights pipeline.
- Fixed bug in the usdExport AlembicChaser in the Maya plugin where primvars 
  that match the primvarprefix do not get exported. They are now exported with 
  constant interpolation, and using _AbcGeomScope is no longer required.
- Fixed bug in Katana plugin where infinite recursion would occur in pxrUsdIn
  when sources were outside the scope of the point instancer. 
  (Issue: [#286](https://www.github.com/PixarAnimationStudios/USD/issues/286))

## [0.8.3] - 2018-02-05

### Added
- Compression in .usdc files for integer arrays and scalar floating point 
  arrays. The latter are compressed if the values are all integers or there 
  are a small number of unique values. In the example Kitchen Set asset on 
  the USD website, the total size of the geometry layers decreased by ~46%, 
  from 25 MB to 14 MB.

  .usdc files with this new compression enabled are marked as version 0.6.0
  and are not readable by earlier releases. These files are not written by
  default; this may be enabled by setting the environment variable 
  `USD_WRITE_NEW_USDC_FILES_AS_VERSION` to "0.6.0".
- Ability to record and query API schemas that have been applied to a prim
  via new Apply method on API schema classes, UsdPrim::GetAppliedSchemas and
  UsdPrim::HasAPI. Custom API schemas should be updated with these new
  methods by re-running usdGenSchema.
- GetUnionedTimeSamples and GetUnionedTimeSamplesInInterval functions for
  UsdAttribute and UsdAttributeQuery.
- Ability to offset time for active value clips when using template clip 
  metadata via "templateActiveOffset" entry.
- UsdUtilsGetDirtyLayers for retrieving dirty layers used by a UsdStage.
- GetTimeSamplesInInterval functions for UsdGeomXformOp, UsdGeomXformable 
  and UsdGeomXformable::XformQuery.
- UsdShadeMaterialBindingAPI, which provides an interface for binding 
  materials to prims or collections of prims and computing the final bound
  material for a prim via "material resolution".
- Numerous features and documentation for UsdSkel schema.
- "Save Flattened As" functionality in usdview.
- Plugin mechanism in usdview that allows users to add custom commands 
  and menus. See new tutorial for more details.
- Partial support for RenderMan for Maya lights in the Maya plugin.
- PxrUsdIn.BootstrapMaterialGroup op in Katana plugin for more robustly reading 
  a Looks scope from a .usd file

### Changed
- Build now supports versioned OpenEXR and IlmBase shared libraries. 
  (Issue: [#71](https://www.github.com/PixarAnimationStudios/USD/issues/71))
- Layer identifiers may now include '?' characters. 
  (Issue: [#289](https://www.github.com/PixarAnimationStudios/USD/issues/289))
- UsdStage preserves payload load state when processing instancing changes.
- UsdListPosition enum values now specify the "append" or "prepend" list as 
  well as a position to provide users with finer-grained control.
- The various Add... methods in Usd that take a UsdListPosition argument
  now author entries to the back of the "prepend" list by default if no
  "explicit" list exists. The old behavior of authoring to the (now 
  deprecated) "added" list can be restored by setting the environment
  variable `USD_AUTHOR_OLD_STYLE_ADD` to 1.
- Standard schema conventions are more strictly-enforced in usdGenSchema.
- UsdCollectionAPI::AddCollection has been renamed ApplyCollection.
- Enabled authoring of new UsdShade encoding by default. Authoring the old
  (now deprecated) encoding can be restored by setting the environment variable 
  `USD_SHADE_WRITE_NEW_ENCODING` to 0.
- The "joints" relationship on the UsdSkelSkeleton, UsdSkelPackedJointAnimation,
  and UsdSkelBinding API schemas is now a token array-valued attribute.
- UsdRiStatements API schema has been renamed to UsdRiStatementsAPI.
- sdfdump utility now shows all specs in a layer, even if they have no fields.
- Various improvements to Hydra reprs and geometry processing.
- More improvements to Hydra's handling of invalid data.
- Ongoing work to prepare Hydra to fully consume UsdShade schemas.
- Refactored GL dependency out of Hd library.
- Built-in variables in usdview interpreter are now accessed through a separate
  usdviewApi object to avoid name collisions.
- Performance improvements in Maya plugin when in Viewport 2.0.
- Inclusion of info.usd.opArgs in Katana plugin is now parameterized; it will
  be authored to the location where a "setOpArgsToInfo" attribute exists and is
  set to 1.

### Deprecated
- The "added" list for list ops in scene description is deprecated in favor 
  of the "prepend" and "append" lists.

### Removed
- GfCamera::ZUp and GfCamera::YUp.
- UsdSkelJoint schema.

### Fixed
- build_usd.py ensures OpenImageIO build does not pick up OpenEXR from other 
  locations, which could have led to runtime errors. 
  (Issue: [#315](https://www.github.com/PixarAnimationStudios/USD/issues/315), 
  Issue: [#325](https://www.github.com/PixarAnimationStudios/USD/issues/325))
- Headers are now installed properly for monolithic builds. 
  (Issue: [#277](https://www.github.com/PixarAnimationStudios/USD/issues/277))
- Original install location will no longer be searched for plugins after
  relocating builds. 
  (Issue: [#363](https://www.github.com/PixarAnimationStudios/USD/issues/363))
- Fixed thread-safety issue where plugins with the same name but in different
  locations could be loaded twice. 
  (Issue: [#358](https://www.github.com/PixarAnimationStudios/USD/issues/358))
- Fixed bug where layers that were muted via SdfLayer::AddToMutedLayers before
  they were first opened could not be unmuted.
- Fixed bug in usdGenSchema where changing an existing property's type in a 
  schema would not be reflected in the generated code.
- Fixed bug where a large (> 1460) number of variants in a .usda file would
  cause a "memory exhausted" error when parsing that file.
- Fixed broken pread mode for .usdc files.
- Fixed bug that caused UsdStage::CreateNew to crash on Windows. 
  (Issue: [#364](https://www.github.com/PixarAnimationStudios/USD/issues/364))
- Fixed bug when using a UsdStagePopulationMask with prims beneath instances. 
  (Issue: [#312](https://www.github.com/PixarAnimationStudios/USD/issues/312))
- Fixed bug where setting float-valued attributes to +inf in Python would fail.
- UsdAttribute::GetTimeSamplesInInterval now properly accounts for layer 
  offsets. (Issue: [#352](https://www.github.com/PixarAnimationStudios/USD/issues/352))
- Internal references or empty asset paths no longer cause errors in 
  UsdUtilsFlattenLayerStack.
- Fixed bug where UsdGeomPrimvar::GetTimeSamples would miss time samples for
  indexed primvars.
- Disabled tiny prim culling in Hydra by default. It can be re-enabled by
  setting the environment variable `HD_ENABLE_TINY_PRIM_CULLING` to 1.
  (Issue: [#314](https://www.github.com/PixarAnimationStudios/USD/issues/314))
- Fixed issues with using non-file-backed asset paths in various utilities
  and Maya and Katana plugins.
- Houdini plugin explicitly links against required libraries to avoid runtime
  errors with Houdini 16.5.
- Fixed several bugs with point instancing support in Houdini plugin.

## [0.8.2] - 2017-12-01

Release 0.8.2 increments the file format version for .usdc files. New .usdc
files created in this release will not be readable by earlier releases. See
below for more details.

### Added
- SdfCopySpec for copying scene description specs in layers.
- Usd.GetVersion() for retrieving USD version in Python 
  (Issue: [#306](https://www.github.com/PixarAnimationStudios/USD/issues/306))
- UsdProperty::FlattenTo for copying a UsdProperty's resolved metadata/values.
- IsTyped and IsConcrete schema queries on UsdSchemaRegistry and UsdSchemaBase. 
- UsdCollectionAPI schema for representing collections of objects. This schema
  is more expressive than UsdGeomCollectionAPI and can represent large numbers
  of objects compactly.
- Utility functions in UsdUtils for determining compact representations of
  objects for UsdCollectionAPI.
- UsdUtilsCoalescingDiagnosticDelegate, a Tf diagnostic delegate that provides
  condensed output for diagnostic messages.
- UsdUtilsFlattenLayerStack for flattening a UsdStage's layer stack into a
  single layer, as well as a --flattenLayerStack option to usdcat.
- Support for named clip sets for clip stitching API in UsdUtils.
- Schema for cards rendering in UsdGeomModelAPI and imaging support. This
  provides a lightweight way to collapse model hierarchies into simple
  geometry for visualization of large scenes.
- Ability to encode familyType on owners of families of UsdGeomSubsets.
- Size properties for UsdLuxDiskLight, UsdLuxSphereLight, and UsdLuxRectLight. 
  (Issue: [#290](https://www.github.com/PixarAnimationStudios/USD/issues/290))
- Support for computing bounding boxes for UsdGeomPointInstancer instances 
  using UsdGeomBBoxCache.
- Support for residual GPU computations in Hydra.
- Sheer display mode in Hydra, a 'see-through' display mode akin to the
  mesh being covered in a sheer mesh fabric instead of its regular surface.
  Implemented as a regular grid stipple pattern of the surface at 20% opacity.
- Testing framework for usdview.
- PxrUsdMayaXformStack for representing transform stacks in the Maya plugin.
- Support for exporting Maya instancers to UsdGeomPointInstancers in the Maya
  plugin.
- Support for exporting groups of objects with the same material binding as
  collections encoded with UsdCollectionAPI in the Maya plugin.
- Support for vec4 primvars in the Katana plugin.
- Support for point instancers with varying topology that don't have authored 
  velocities in the Katana plugin.
- Support for importing collections encoded with new UsdCollectionAPI schema
  in the Katana plugin.
- Support for overriding data on a subset of point instancer instances in
  the Houdini plugin.

### Changed
- Build system now checks that the compiler being used supports 64-bit builds. 
- Tf diagnostic warnings and status messages issued in secondary threads are
  no longer printed to the terminal.
- Multiple delegates may now be registered with the Tf diagnostic system.
- SdfLayer::RemoveInertSceneDescription removes inert overs in variants.
- When saving a new .usdc file over an existing file, data is first saved to a 
  temporary file then renamed to the destination file. This provides some 
  protection for other processes that were reading the old file.
- Compression for structural sections in .usdc files is now enabled by default.
  New .usdc files will be marked as version 0.4.0 instead of 0.3.0 to 
  accommodate a bug fix described below. This means that any new .usdc files 
  created in this release will not be readable by previous USD releases.

  Users can disable the compression functionality by setting the environment
  variable `USD_WRITE_NEW_USDC_FILES_AS_VERSION` to "0.0.1". This will ensure 
  new .usdc files will be readable in older releases.

- UsdStage now uses the composed layer offset instead of its inverse when
  mapping times in layers to the stage. For example, to shift the time samples
  in a referenced layer stack forward by 10, users previously authored an
  offset of -10. Under the new behavior, users need to author an offset of 10
  instead. 

  Users can revert to the old behavior by setting the environment variable
  `USD_USE_INVERSE_LAYER_OFFSET` to 1. This will be deprecated in a future 
  release.

- UsdStage now allows a prim to be loaded without loading its descendants via
  a new UsdLoadPolicy argument supplied to UsdStage::Load and LoadAndUnload.
- UsdInherits, UsdSpecializes, and UsdReferences now map non-root prim paths
  to the namespace of the edit target.
- UsdAttribute::GetTimeSamplesInInterval now supports open/finite end points.
- UsdClipsAPI now authors the dictionary-style clips metadata introduced in
  release 0.7.6 by default.
- Schemas may now define fallback values for the majority of metadata fields,
  including custom metadata fields.
- Schemas may now have more than 256 tokens defined.
- usdedit now checks `USD_EDITOR` environment variable first when determining
  which editor to use.
- Ongoing refactoring to move code from hd to hdSt in preparation for material
  support in Hydra and use of Hydra in other renderer backends.
- Several performance optimizations in Hydra and UsdImaging.
- usdview now uses a new JSON-based format for its settings file. Settings
  files from older releases will be ignored.
- Several changes to provide better support for Viewport 2.0 in Maya plugin.
- Enum attributes in Maya now have their integer values exported to USD 
  instead of their descriptive names.
- Improved attribute transfer on USD Unpack node in Houdini plugin.
- usdvisible and usdactive attributes in Houdini plugin are now integers
  instead of strings.
- Several changes to Houdini plugin for compatibility with Houdini 16.5
  and improved HDK usage.

### Deprecated
- UsdGeomCollectionAPI, in favor of new UsdCollectionAPI.

### Removed
- TfDiagnosticNotice. The Tf diagnostic system no longer emits notices; 
  consumers relying on these notices should use diagnostic delegates instead.
- UsdGeomPointInstancer prototypeDrawMode attribute. Consumers should use
  the new drawMode attributes on UsdGeomModelAPI instead.

### Fixed
- Issue where users were forced to define certain preprocessor macros in 
  order to build code against USD. In particular, an issue introduced in 0.8.1
  caused users who did not #define PXR_ENABLE_PYTHON_SUPPORT to run into odd
  errors at runtime. 
  (Issue: [#304](https://www.github.com/PixarAnimationStudios/USD/issues/304))
- Bug that caused invalid .usdc files to be written. This affected all files
  with version 0.3.0; however, this file version was not enabled by default in
  release 0.8.1 and would only affect users who had explicitly enabled the
  new version via the `USD_WRITE_NEW_USDC_FILES_AS_VERSION` environment setting.
- Precision loss when writing time sample times to .usda files.
- Performance issue in UsdStage where memory would still be consumed by 
  child prims after their parent prim had been deactivated.
- Bug that caused UsdStage::ExpandPopulationMask to add invalid paths.
- Bug that caused UsdObject::GetMetadata to not fill in resolved paths for
  asset-valued metadata.
- Bug where layer offsets were incorrectly applied when authoring time samples.
- Bug in UsdUtilsStageCache that led to crashes during static destruction.
- Unpickable objects now act as occluders during picking in Hydra.
- Bug that caused usdview to stop drawing after "Reopen Stage" or "Open Stage".
- Bug that caused usdview to not play all frames if the step size was changed.
  (Issue: [#321](https://www.github.com/PixarAnimationStudios/USD/issues/321))
- Several performance and UI issues in usdview.
- Crash in Maya plugin with nested assemblies and variant set selections.
  (Issue: [#288](https://www.github.com/PixarAnimationStudios/USD/issues/288))
- Bug in Maya plugin where exporting over a previously-imported layer failed.
- Bug in Maya plugin that caused lights to flip between directional and 
  non-directional in legacy viewport.
- Various issues that prevented UsdKatana Python module from being imported.
  (Issue: [#323](https://www.github.com/PixarAnimationStudios/USD/issues/323))
- Material bindings above point instancer prototypes are now preserved in 
  the Katana plugin.
- Reworked how USD masters/sources are built in the Katana plugin so 
  that material bindings use the correct Katana paths.
- Hang in Houdini plugin when importing from variant paths. 
  (Issue: [#309](https://www.github.com/PixarAnimationStudios/USD/issues/309))

## [0.8.1] - 2017-10-02

### Added
- "append" and "prepend" operations for SdfListOp-valued fields.
  Scene description layers that contain these new operations will not be
  readable by earlier USD releases.
- `--validate` argument to sdfdump for checking file validity.
- Support for sub-root references and payloads.
- Ability to specify amount of data to prefetch when reading a .usdc
  file via the `USDC_MMAP_PREFETCH_KB` environment variable.
- Debugging output that shows memory mapping information for .usdc files.
  This can be enabled via the `USDC_DUMP_PAGE_MAPS` environment variable.
- UsdGeomMotionAPI schema.
- UsdGeomSubset schema for representing a subset of a geometric prim.
- Hydra GL support for displacement shaders
- Initial experimental support for PySide2 in usdview. By default, the 
  build will search for PySide, then PySide2. Alternatively, users may specify 
  `PYSIDE_USE_PYSIDE2=TRUE` to CMake to force the use of PySide2.
- Ability to view connections and relationship targets in usdview.
- Support for camera clipping range in Alembic plugin.
- Support for array attributes with "RfM Shaders" shading mode in Maya exporter.
- Support for angularVelocities attribute on point instancers in Katana plugin.
- Filtered view and improved selection to Tree View panel in Houdini plugin.
- Support for prototype offsets in point instancer exports in Houdini plugin.

### Changed
- Made Python dependency optional. Python support is enabled by default but
  may be disabled by specifying `PXR_ENABLE_PYTHON_SUPPORT=FALSE` to CMake
  or `--no-python` to build_usd.py.
- build_usd.py now requires that users install PyOpenGL manually. 
  (Issue: [#264](https://www.github.com/PixarAnimationStudios/USD/issues/264))
- Replaced addedOrExplicitItems attribute on Sdf.*ListOp in Python with
  GetAddedOrExplicitItems method.
- Renamed Append... methods on composition arc APIs (e.g. UsdReferences, 
  UsdInherits) to Add... and added a position argument to support new
  "append" and "prepend" operations. 

  Calling these functions with the default UsdListPositionTempDefault argument
  will author "add" operations, maintaining the previous behavior. We anticipate
  removing UsdListPositionTempDefault and replacing it with UsdListPositionFront
  so that "prepend" operations will be authored by default in 0.8.2. Users may 
  enable this behavior now by setting the `USD_AUTHOR_OLD_STYLE_ADD` environment
  variable to false.

- UsdStage now issues warnings for invalid layers it encounters instead of
  errors, which cause exceptions in Python.
- Code generation templates and schema.usda files used by usdGenSchema are 
  now installed as part of the build. Schemas can now use asset paths like
  "usdGeom/schema.usda" or "usdShade/schema.usda" instead of absolute paths
  to add installed schemas as sublayers. 
  (Issue: [#158](https://www.github.com/PixarAnimationStudios/USD/issues/158), 
   Issue: [#211](https://www.github.com/PixarAnimationStudios/USD/issues/211))
- Improved USD scenegraph instancing to allow more sharing of assets with
  inherits or specializes arcs. In one large example, this decreased stage load
  time by ~60%, stage memory usage by ~65%, and time to first image in usdview 
  by ~20%.
- Several optimizations to reduce I/O operations for .usda and .usdc files.
- Add support for compressed structural sections in .usdc files. This can
  significantly decrease file sizes for files that are dominated by prim and
  property hierarchy; one production shading file decreased in size by ~94%,
  from 21 MB to 1.3 MB.

  Compressed files are marked as version 0.3.0 and are not readable by earlier 
  USD releases. To help with transition, USD will not write these compressed 
  files by default until a future release. Users may enable this feature now 
  by setting the `USD_WRITE_NEW_USDC_FILES_AS_VERSION` environment variable 
  to "0.3.0". We plan to add compression for geometry data and time samples
  in a future release.

- UsdUtilsExtractExternalReferences now checks for references to external files
  in property values and metadata.
- Increased precision for rotations attribute in UsdSkelPackedJointAnimation 
  schema to 32-bit.
- UsdGeomPrimvar now allows primvar names with arbitrarily-nested namespaces.
- Sizeable refactor in Hydra to support sprims more like rprims in anticipation 
  of future lights and material support.
- Hydra no longer syncs rprims that are not visible.
- Several performance and stability fixes to point instancer handling in Hydra.
- "Reload All Layers" in usdview will try not to reset the camera or timeline.
- Numerous other improvements to usdview.
- The Alembic reader now converts single samples for properties in .abc files
  to single time samples in USD, instead of default values.
- Updated required Katana version to Katana 2.5v1.
- Improved bounds computations for point instancers in Katana plugin
- Account for motion sample time overrides when computing bounding boxes in
  Katana plugin.
- Improved stage cache in Houdini plugin, which can have significantly
  better performance in some circumstances.
- Improved handling of instanced USD primitives in Houdini plugin.

### Deprecated
- UsdGeomFaceSetAPI and related API in usdShade and usdLux in favor of new 
  UsdGeomSubset schema.

### Removed
- UsdStage::Close.
- Camera zUp-related functionality
- Support for faceVaryingInterpolateBoundary in UsdGeomMesh schema.
  See faceVaryingLinearInterpolation instead.

### Fixed
- Bug in build_usd.py where specifying `--no-embree` did the opposite.
  (Issue: [#276](https://www.github.com/PixarAnimationStudios/USD/issues/276))
- Bug in build_usd.py where specifying `--force-all` or `--force` on a
  Python dependency would cause the script to error out early. 
  (Issue: [#263](https://www.github.com/PixarAnimationStudios/USD/issues/263))
- Race condition between TfType consumers and TfType registry initialization.
- Composition bug where references with the same prim path and relative asset
  path would be considered duplicates even when anchoring location differed.
- Composition bug where payloads on arcs requiring ancestral opinions could
  result in errors.
- Change processing bug when removing sublayers specified by search path.
- Stack overflow crash when reading .usdc files due to excessive recursion.
- Possible crash in Hydra when using basis curves.
- Memory leak in Hydra task controller.
- Build issue for Alembic plugin with HDF5 support disabled when
  precompiled headers are enabled (which is the default on Windows).
- Several other bugs in Alembic reader.
- Performance issue in Katana plugin due to inefficiency in UsdShadeNodeGraph.

## [0.8.0] - 2017-07-29

### Added
- Added option to build_usd.py for building monolithic shared library
  and for building new sample Embree plugin.
- Added --mask option to usdcat, matching the --mask option in usdview.
- Added CPU-side external computation support to Hydra.
- Added Embree backend to serve as an example of how to add renderer 
  plugins to Hydra.
- Added camera gates to usdview.
- Added ability to Houdini plugin to set active state via usdactive 
  attribute and to write static data to default time via usdwritestatictopology,
  usdwritestaticprimvars, and usdwritestaticgeo attributes.
- Added UsdExportAttributes hda to Houdini plugin for setting primitive and 
  detail attributes which control various USD attributes or metadata.

### Changed
- Restored export of CMake targets from shared library builds.
- Asset paths in .usda files may now contain any printable character.
  (Issue: [#73](https://www.github.com/PixarAnimationStudios/USD/issues/73))
- Legacy special behavior for variant sets named "standin" has been disabled
  by default.
- Interpolation is now applied to time samples from clips when no sample
  exists at a specified clip time.
- Removed deprecated UsdShadePShader schema.
- Enabled vertex primvar sharing in Hydra. Hydra can now use significantly 
  less GPU memory.
- Many internal performance improvements to Hydra.
- usdview HUD now shows the backend renderer plugin used by Hydra
- Wireframe drawing now ignores opacity.
- Katana plugin properly interprets schema-supported triangle subdivision rule 
  when meshes are read in and rendered.

### Fixed
- Fixed several issues in build_usd.py related to building dependencies.
  (Issue: [#225](https://www.github.com/PixarAnimationStudios/USD/issues/225), 
   Issue: [#230](https://www.github.com/PixarAnimationStudios/USD/issues/230), 
   Issue: [#234](https://www.github.com/PixarAnimationStudios/USD/issues/234))
- Fixed bug where UsdGeomPrimvar::IsIndexed() would not work for attributes
  with only authored time samples. 
  (Issue: [#238](https://www.github.com/PixarAnimationStudios/USD/issues/238))
- Fixed small platform inconsistencies for Windows in Hydra.
- Fixed crash in Katana plugin when using point instancers that did not have
  scales or orientations specified. 
  (Issue: [#239](https://www.github.com/PixarAnimationStudios/USD/issues/239))
- Fixed issue in Houdini plugin where the "w" attribute was not converted
  to the "angularVelocities" attribute for point instancer prims.
	
## [0.7.6] - 2017-06-30

### Added
- Added `build_scripts/build_usd.py` for building USD and its dependencies.
- Added support for building static libraries or a single monolithic
  shared library. See [BUILDING.md](BUILDING.md#linker-options) for more
  details.
- Added support for color spaces to Usd. Color configuration and management 
  system can be specified on a UsdStage (via the stage's root layer), and 
  colorSpace metadata is available on any UsdAttribute.
- Added clip set functionality to Usd. This provides the ability to specify 
  multiple sets of value clips on the same prim, which allows users to compose 
  different sets of clips together. See UsdClipsAPI for more details.
- Added initial UsdLux schemas for representing interchangeable lights and 
  related concepts, and UsdRi schemas for Renderman-specific extensions. 
  UsdImaging/Hydra support will be coming in a future release.
- Added ability to specify smooth triangle subdivision scheme for catmullClark
  surfaces in UsdGeomMesh and imaging support in Hydra.
- Added backdrops for node layout description in UsdUI.
- Added support for sharing immutable primvar buffers to Hydra.
  This can greatly reduce the memory required on the GPU when displaying
  typical scenes. It is currently experimental and disabled by default. It
  can be enabled for testing with the environment variable
  `HD_ENABLE_SHARED_VERTEX_PRIMVAR`.
- Added support for uniform primvars for basis curves in Hydra.
- Added ability to Alembic plugin to control number of Ogawa streams used for 
  each opened archive via the environment variable `USD_ABC_NUM_OGAWA_STREAMS`,
  which defaults to 4.
- Added Katana plugin support for reading UsdLux lights.
- Added `--camera` command line argument to usdview to specify the initial 
  camera to use for viewing.
- Added display of instance index and (if authored) instance ID to rollover
  prim info in usdview. 
- Added initial version of Houdini plugin.

### Changed

- Removed unnecessary dependency on OpenImageIO binaries.
- Removed unnecessary dependency on boost::iostreams.
- Made HDF5 backend for Alembic plugin optional. HDF5 support is enabled by 
  default but may be disabled by specifying `PXR_ENABLE_HDF5_SUPPORT=FALSE`
  to CMake.
- Metadata fields for value clips feature in UsdStage (e.g. clipAssetPaths,
  clipTimes, etc.) have been deprecated in favor of the new clips metadata
  dictionary. Authoring APIs on UsdClipsAPI still write out the deprecated
  metadata unless the environment variable `USD_AUTHOR_LEGACY_CLIPS` is set 
  to 0. This will be disabled in a future release.
- When using value clips, Usd will now report time samples at each time 
  authored in the clip times metadata.
- Completed support for encoding UsdShade data using UsdAttribute connections.
  Authoring APIs still write out the old encoding, unless the environment 
  variable `USD_SHADE_WRITE_NEW_ENCODING` is set to 1. This will be enabled
  in a future release.
- Removed support for deprecated UsdShade schemas UsdShadeParameter and
  UsdShadeInterfaceAttribute, which have been superseded by UsdShadeInput.
- Removed deprecated UsdRiLookAPI, which has been replaced by UsdRiMaterialAPI.
- Removed deprecated Hydra scene delegate APIs.
- Significant refactoring and refinements to allow multiple backends to be
  plugged in to Hydra.
- UsdImaging now pushes GL_DEPTH_BUFFER_BIT to avoid any potential state
  pollution issues when integrating it with other renderers. 
- Improved error handling in Hydra for:
  - Invalid point instancer input
  - Inconsistent displayColor and displayOpacity primvars
  - Direct instances of imageable prims
- usdview more gracefully handles the situation where it doesn't get a
  valid GL context (e.g., due to limited resources on the GPU).
- Improved how edits are handled on assembly nodes in the Maya plugin.
- Improved UsdGeomPointInstancer support in the Katana plugin:
  - Overhauled methods for computing instance transforms and aggregate bounds. 
  - Backward motion and multi-sampled point instancer positions, orientations, 
    and scales are now supported.
  - Removed opArgs attributes that were used for configuring the reader's 
    behavior.
  - More robust primvar transfer to point instancer's Katana locations.
  - Restructured prototype building logic to preserve material bindings
    when creating prototype prims.
- Numerous changes and fixes for Mac and Windows ports.

### Fixed

- Fixed issue that caused IDEs and other tools to find headers in the build
  directories rather than the source directories.
- Fixed race condition in TfType.
- Fixed long-standing memory corruption bug in Pcp that surfaced when 
  building on MacOS with XCode 8.3.2.
- Fixed potential deadlocks in Usd and Pcp due to issues with TBB task group 
  isolation.
- Fixed bug where UsdStage::Flatten was not preserving attribute connections.
- Fixed infinite loop on glGetError in the presence of an invalid GL context.
  (Issue: [#198](https://www.github.com/PixarAnimationStudios/USD/issues/198))
- Fixed build issues that caused the Maya plugin to be unusable after building
  on MacOS.

## [0.7.5] - 2017-05-01

C++ namespaces are now enabled by default. The `PXR_ENABLE_NAMESPACES` CMake
option may be used to disable namespaces if needed.

This release adds initial experimental support for building the USD core
libraries on Windows.

### Added
- Added ability to build doxygen documentation. See
  [BUILDING.md](BUILDING.md#documentation) for more details.
- Added support for pre-compiled headers. By default, this is disabled
  for Linux and Mac builds and enabled for Windows builds. This may be
  configured with the `PXR_ENABLE_PRECOMPILED_HEADERS` CMake option.  
- Added many unit tests for core libraries
- Added UsdStage::Save and UsdStage::SaveSessionLayers
- Added attribute connection feature that allows consumers to describe
  connections between an attribute and other prims and properties.
- Added instance proxy feature that allows consumers to interact with
  objects on a UsdStage as if scenegraph instancing were not in use while
  retaining the performance benefits of instancing.
- Plugins providing Boundable schema types can now register functions for
  computing extents. This allows code in the USD core to compute extents for
  prims with types provided by external schemas.
- Added support for specializes arcs to usdShade.
- Added UsdUISceneGraphPrimAPI for representing prim display properties.
- Added `HD_DISABLE_TINY_PRIM_CULLING` flag for `TF_DEBUG` to make it easier
  to turn off tiny prim culling for debugging.
- Added support for generic metadata to GLSLFX, allowing hardware shaders to
  pass information to downstream rendering code.
- Added ability to adjust basic material properties in usdview.
- Plugins can now register MayaPrimWriter subclasses for exporting Maya 
  nodes to USD prims.
- Added ability to override default motion data (sample times, current time, 
  shutter open/close) on a per-scope basis in Katana plugin. Users can author
  these overrides by modifying the global graph state, then decide to use
  the default motion values with or without these overrides applied.

### Changed
- Removed OpenEXR dependency from core libraries. Note that OpenEXR is still
  required for imaging.
- Made Ptex dependency optional. It is only required if Ptex support for
  imaging is enabled. This is on by default, but can be disabled by specifying
  `PXR_ENABLE_PTEX_SUPPORT=FALSE` to CMake.
- Cleaned up and expanded doxygen documentation:
  - Added documentation for value clips and scenegraph instancing USD features.
  - Enabled XML generation for external tools
  - Fixed invalid tags and formatting in several places
- UsdTreeIterator has been replaced by UsdPrimRange, which provides 
  container-like semantics in C++ (e.g., the ability to use them in range-based
  for loops).
- Removed UsdPrim::GetPayload API
- UsdRelationship and UsdAttribute will no longer forward target/connection
  paths that point to objects within instances to objects in master prims.
  Users can use the new instance proxy functionality to work with these paths
  instead.
- Relationships belonging to prims in instances that point to the root of the
  instance no longer cause errors.
- Value clip metadata can now be sparsely overridden across composition arcs.
- Deprecated shading property schemas UsdShadeParameter and 
  UsdShadeInterfaceAttribute. These are replaced by UsdShadeInput, which can
  represent both input parameters on shaders and interface inputs on node
  graphs.
- The `--compose` command line parameter for usddiff is now `--flatten`
- Numerous cleanup changes for Hydra API. In particular:
  - HdEngine::Draw has been retired in favor of HdEngine::Execute.
  - HdSceneDelegate::IsInCollection has been deprecated in favor of render-tag
    based API.
- Meshes with subdivisionScheme = none will no longer be bilinearly subdivided.
- usdview now displays prim hierarchies beneath instances inline in the 
  browser. The "Jump to Master" command has been removed in favor of this
  new functionality.
- The default specular response in usdview has been reduced by a factor of 5.
- The Maya plugin now excludes attributes during import if they are tagged
  with customData specifying them as generated.
- Numerous changes for ongoing Mac and Windows port efforts.
- Changed scoping of built-out point instancer prototypes in Katana plugin
  so that their hierarchy more closely matches the original USD hierarchy.

### Fixed
- Added workaround to avoid redefined "_XOPEN_SOURCE" and "_POSIX_C_SOURCE" 
  macro warnings due to Python includes on Linux. 
  (Issue: [#1](https://www.github.com/PixarAnimationStudios/USD/issues/1))
- Fixed issue where code would be generated directly into the install location 
  when running cmake, interfering with the use of make's DESTDIR functionality.
  (Issue: [#84](https://www.github.com/PixarAnimationStudios/USD/issues/84))
- Fixed memory leak when converting Python tuples to C++ GfVec objects.
- Fixed thread-safety issue with `PCP_PRIM_INDEX` and `PCP_PRIM_INDEX_GRAPHS`
  `TF_DEBUG` flags. 
  (Issue: [#157](https://www.github.com/PixarAnimationStudios/USD/issues/157))
- Fixed invalid memory access bug in Pcp
- Fixed memory leak when saving .usdc files.
- Fixed bug with removing an attribute's last time sample in a .usdc file.
- Fixed bug where instance prims with locally-defined variants would incorrectly
  share the same master prim.
- Reader-writer locks are now used for various registry to improve performance.
- Improved speed of adding an empty sublayer to an opened stage. In one large
  test case, time to add an empty sublayer went from ~30 seconds to ~5 seconds.
- Fixed issues with opening .usda files and files larger than 4GB on Windows.
  (Issue: [#189](https://www.github.com/PixarAnimationStudios/USD/issues/189))
- Fixed backwards compatibility issues with UsdShadeInput and 
  UsdShadeConnectableAPI.
- Memory footprint has been significantly reduced in Hydra's adjacency tables
  for subdivs that contain vertices of high valence.
- Fixed crash in imaging due to invalid textures.
- Fixed shadow banding artifacts when using vertex color with basis curves.
- Fixed buffer overrun when using GPU smooth normals.
- Fixed issue in Maya plugin where multiple assemblies that reference the
  same file might all be affected by the edits of one particular assembly 
  when viewing them via a proxy.
- Fixed issue in Maya plugin where path resolution was being done by the
  plugin itself rather than deferring to Usd and Sdf.

## [0.7.4] - 2017-03-03

USD now supports C++ namespaces. They are disabled by default in this release
to ease transition, but we anticipate enabling them by default in the next 
release. 

Specify the `PXR_ENABLE_NAMESPACES` CMake option to enable namespaces.
See documentation in [BUILDING.md](BUILDING.md#c-namespace-configuration) for more details.

### Added
- Added GfHalf type to represent half-precision numeric values. This type 
  is currently an alias to the "half" type supplied by the ilmbase library 
  USD is built against. This will be changed in an upcoming release to point 
  to a version of "half" that is included in the USD codebase, which will
  allow us to remove the USD core's dependency on ilmbase.
- Added UsdShadeInput class to represent shader or NodeGraph input.
  These are encoded as attributes in the "inputs:" namespace. By default, 
  this encoding will not be written out via UsdShadeConnectableAPI unless 
  the 'USD_SHADE_WRITE_NEW_ENCODING' environment variable is set.
- Added "Composition" tab to usdview that allows users to inspect the 
  composition structure of a selected prim.
- Added tests for Hdx. These tests are currently not run as part of the unit 
  test suite, but are being included for users to try on their platforms and 
  to serve as example code.
- Added icons for the USD assemblies in Maya.
- Added initial implementation of Katana plugin for reading 
  UsdGeomPointInstancer locations. 

### Changed
- CMake module now additionally looks for the pyside-uic binary under the name 
  pyside-uic-2.7 to accommodate package managers on OSX.
- Removed GLUT dependency.
- Removed double-conversion dependency.
- Modified all uses of "half" type from ilmbase to use new GfHalf type.
- Renamed UsdShadeSubgraph to UsdShadeNodeGraph to conform with final
  MaterialX terminology.
- Numerous changes for in-progress work on refactoring Hydra to support render 
  delegates.
- Updated UI in usdview to display legends for text coloring in collapsible 
  panes and to include more information.
- The import UI in the Maya plugin now enables reading of anim data by default.
- Updated API in Maya plugin that referred to "look" to "material".
- Numerous changes for ongoing Mac and Windows porting efforts.

### Fixed
- Fixed several issues when building with C++ namespaces enabled.
- Fixed bug with selection highlighting for instanced prims in usdview.
- Fixed crash that occurred when an invalid color primvar was encountered while
  using usdview's "simple" renderer.
- Fixed issue with TF_REGISTRY_DEFINE* macros that caused build failures
  for the Maya plugin on OSX. 
  (Issue: [#162](https://www.github.com/PixarAnimationStudios/USD/issues/162))

## [0.7.3] - 2017-02-05

### Added
- Added support for RelWithDebInfo and MinSizeRel release types.
- Added initial support for C++ namespaces. This feature is 
  currently experimental but can be enabled by specifying the 
  `PXR_ENABLE_NAMESPACES` option to CMake. See documentation in
  BUILDING.md for more details.
- Added population masking to UsdStage. 
  - Consumers can specify what parts of a stage to populate and use
    to reduce resources used by the stage.
  - usdview now has a --mask option allowing users to view just the
    specified portion of a stage.
- Added UsdPrim API for collecting relationship targets authored 
  in a given prim subtree.
- Added UsdShade API to allow creating and connecting outputs
  on subgraphs and shaders instead of terminals.
- Added support for bool shader parameters in Hydra. 
  (Issue: [#104](https://www.github.com/PixarAnimationStudios/USD/issues/104))
- Added ability to independently toggle display of geometry with
  purpose=proxy in usdview.
- Added initial set of unit tests for pxrUsd library in the Maya plugin.
- The Maya plugin now supports multi-sampling of frames during export.
- Attributes in Maya may now be tagged to be converted from double
  to single precision during export.
- Alembic tagging for attributes in Maya are now respected during 
  export. This includes support for primvars and custom prefixes.
- Added support for playing back animation in assemblies via Hydra
  when the Playback representation is activated in Maya.
- Added support for reading material references in Katana plugin.

### Changed
- CMake will no longer look for X11 on OSX. 
  (Issue: [#72](https://www.github.com/PixarAnimationStudios/USD/issues/72))
- The build system now uses relative paths for baking in plugin
  search paths, allowing for relocatable builds as long as all
  build products are moved together as one unit.
- Removed hard-coded /usr/local/share directory from plugin search path.
- Optimized creation of prim specs in Sdf. Creating 100k prim specs 
  previously took 190s, it now takes 0.9s.
- SdfPath::GetVariantSelection now returns a variant selection only
  for variant selection paths.
- UsdStage no longer issues an error if requested to unload a path that
  does not identify an unloadable object.
- Changed API on UsdInherits, UsdReferences, UsdRelationship,
  UsdSpecializes, UsdVariantSet, and UsdVariantSets for clarity.
- Hydra GPU compute settings have been consolidated under a single
  HD_ENABLE_GPU_COMPUTE environment setting which is enabled by
  default if the OpenSubdiv being used supports the GLSL compute kernel.
- Numerous changes for in-progress work on refactoring Hydra to
  support render delegates.
- Changed 'Display' menu in usdview to 'Display Purposes...' and menu
  items to match USD terminology.
- Updated required Maya version to Maya 2016 EXT2 SP2.
- Normals are now emitted by default when exporting polygonal meshes in Maya.
- Katana plugin now supports deactivating prims directly on
  UsdStage for improved efficiency.
- Continued work on deprecating UsdShadeLook in favor of UsdShadeMaterial.
- Numerous changes for ongoing Mac and Windows porting efforts.
- Changed coding style to use symbol-like logical operators
  instead of the keyword-like form (e.g., '&&' instead of 'and').
- Various cleanup changes to fix comments, compiler warnings, and
  to remove unused/old code.

### Fixed
- Fixed composition bug where nested variants with the same name
  were composed incorrectly.
- Fixed composition bug where variant selections from classes
  were not being applied in certain cases. 
  (Issue: [#156](https://www.github.com/PixarAnimationStudios/USD/issues/156))
- Fixed .usda file output to write out floating point values
  using the correct precision and to use exponential representation
  for values greater than 1e15 (instead of 1e21) to avoid parsing
  issues.
- Fixed bug where negative double values that could be represented as
  floats were not stored optimally in .usdc files.
- Fixed bug where UsdPrim::Has/GetProperty would return incorrect results.
- Fixed bug where UsdObject::GetMetadata would return incorrect results
  for dictionary-valued metadata in certain cases.
- Fixed bug where reading certain .usdc files while forcing single-threaded
  processing via PXR_WORK_THREAD_LIMIT would lead to a stack overflow.
- Fixed missing garbage collection for shader and texture resource
  registries in Hydra.
- Several fixes for nested instancing support in Hydra (native
  USD instances containing other instances or point instancers, etc.)
- Fixed "Jump to Bound Material" in "Pick Models" selection mode in usdview.
- Fixed bug where USD assemblies would not be drawn in Maya under VP2.0.
- Workaround for performance issue when drawing proxies in Katana plugin.
- Fixed issue causing USD Alembic plugin to fail to build with
  Alembic 1.7. 
  (Issue: [#106](https://www.github.com/PixarAnimationStudios/USD/issues/106))
- Various other bug fixes and performance improvements.

## [0.7.2] - 2016-12-02

### Added
- Added ability to apply a custom prefix to shared library names
  by specifying the `PXR_LIB_PREFIX` option to CMake.
- Added ability to disable tests by using the `PXR_BUILD_TESTS` option
  with CMake.
- usddiff can now diff two directories containing USD files.
- Value clips on a prim may now be specified using 'template' clip
  metadata. This is a more compact representation for simple cases
  where each value clip corresponds to a single time code.
- Better support for non-file-backed asset resolution
  - Made ArDefaultResolver public, so that subclasses can layer
    behavior on top of it.
  - Added ArResolver API for retrieving files from data store when
    needed.
- Initial version of UsdGeomPointInstancer schema and preliminary
  support in usdImaging.
- pxrUsdReferenceAssemblies in Maya can now import animation from a USD scene.
  See http://openusd.org/docs/Maya-USD-Plugins.html#MayaUSDPlugins-ImportingasAssemblies 
  for details. 
- Core USD metadata (e.g. hidden, instanceable, kind) now roundtrips through 
  Maya, stored as "USD_XXX" attributes on corresponding Maya nodes.

### Changed
- Removed dependency on Qt. Note that PySide is still a dependency for 
  usdview.
  - Removed imaging/glfq library as part of this work.
- Modified variant behavior in composition:
  - Variants may now be nested within other variants in scene description.
    Nested variants may be authored by using nested UsdEditContext objects
    returned by UsdVariantSet::GetVariantEditContext.
  - Weaker variants may now introduce variant selections that affect
    stronger variants. Previously, these selections would be ignored 
    and the variant fallbacks would be used instead.
- Optimized .usdc file format performance in certain cases:
  - Output is buffered in memory to improve write times for layers 
    containing many nested dictionaries. In one such example, wall-clock
    time improved by ~64% and system time improved by 89%.
  - Time samples are now written out grouped by time to improve read 
    performance for frame-by-frame access patterns.
- Several optimizations to improve USD scenegraph authoring and composition 
  speed and reduce memory consumption. In some use cases, we observed:
  - ~5-7x less memory consumed by Pcp dependency tracking structures
  - ~40% less time spent to author large stages
- Exporting USD from the Maya Export window now produces binary .usd
  files by default instead of .usda files.

### Fixed
- Fixed bug where edits to a layer opened in Maya would persist when
  reopening that layer in a new scene.
- Fixed issue where exporting USD from the Maya Export window would
  produce files ending with ".usda", even if a different format
  was specified. The exporter should now respect any extension that
  is entered, so long as the "Default file extensions" option is
  turned off.
- Fixed several issues with example plugins being improperly built, making
  them unusuable, and missing functionality.
- Various other bug fixes and performance improvements.

## [0.7.1] - 2016-09-21

### Added
- UsdMaterial schema for shading, intended to replace UsdLook. Also added
  support for this new schema to Hydra.
- Initial version of UsdUI schema library, intended for encoding GUI 
  information on USD prims.
- Parallel teardown of some data members in UsdStage, speeding up overall
  teardown by ~2x.
- Support for packed vertex normals to Hydra, reducing GPU memory consumption
  by 20-30% in some cases.
- Ability to compare two composed stages to usddiff by specifying the
  "--compose" or "-c" options.
- Support for soft-selection for collapsed USD assemblies in Maya.
- Support for exporting color sets and UV sets as indexed primvars from Maya.

### Changed
- Refactored Hydra libraries to move higher-level concepts out of the core
  hd library and into hdx.
- Removed use of opensubdiv3/ include path from imaging code in favor of
  standard opensubdiv/ include.
- Modified UsdStage to automatically load newly-discovered payloads if
  their nearest ancestor's payload was also loaded. For example, consumers
  will no longer have to explicitly call UsdStage::Load to ensure payloads
  are loaded when switching variants on a prim, so long as their nearest
  ancestor is also loaded.
- Refactoring and other changes to help with Mac and Windows ports.
- Updated doxygen and other documentation.

### Fixed
- Fixed issue that caused Alembic plugin (usdAbc) to be misconfigured at
  build time, which required users to manually update its plugInfo.json
  and set an environment variable at runtime to make it work. This plugin
  is now installed to $PREFIX/plugin/usd/ and requires no additional
  steps to use once it has been built.
- Fixed issue with .usdc files that resulted in corrupted files on Windows.
  The file structure was changed, so the .usdc version has been bumped
  from 0.0.1 to 0.1.0. USD will continue to write 0.0.1 files by default,
  but will begin writing 0.1.0 files in the near future.

  Users may choose to write files with the new version immediately
  by setting the environment variable `USD_WRITE_NEW_USDC_FILES_AS_VERSION`
  to "0.1.0".

  Note that 0.0.1 files are still readable using this release, 
  except for those that have been generated on Windows using 0.7.0. 
  Early testers on Windows will need to regenerate those files
  with this release.
- Fixed issue that caused usdGenSchema to generate files that could
  not be compiled.
- Added a workaround for TBB-related issue in Maya that caused hangs when 
  using the USD Maya plugin. This can be enabled at build time by 
  specifying the `PXR_MAYA_TBB_BUG_WORKAROUND` option to CMake.
- Various other bug fixes and performance improvements.

## [0.7.0] - 2016-08-01

Initial release