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Releases: newton-physics/urdf-usd-converter

URDF USD Converter v0.1.0a1

16 Jan 16:59
@andrewkaufman andrewkaufman
v0.1.0a1
db5c38f
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URDF USD Converter v0.1.0a1 Latest
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Features

  • USD Asset Structure
    • Output Assets are completely standalone with no dependencies on the source URDF, OBJ, DAE, or STL files
    • Atomic Component structure with Asset Interface layer and payloaded contents
    • Separate geometry, material, and physics content layers for easy asset-reuse across domains
    • Library-based asset references for meshes and materials to avoid heavy data duplication
    • Explicit USD stage metadata with units (meters, kilograms) and up-axis (Z)
  • Link Conversion
    • URDF links are converted as UsdGeom.Xform prims with UsdPhysics.RigidBodyAPI applied
    • The root link has UsdPhysics.ArticulationRootAPI applied to indicate the root of the kinematic tree
    • Links are nested in USD, reflecting the kinematic hierarchy of the source URDF rather than the XML file structure
    • Complete mass properties including explicit inertia & center of mass via UsdPhysics.MassAPI
  • Joint Conversion
    • Revolute joints as UsdPhysics.RevoluteJoint with angular limits
    • Continuous joints as UsdPhysics.RevoluteJoint without limits
    • Prismatic joints as UsdPhysics.PrismaticJoint with linear limits
    • Fixed joints as UsdPhysics.FixedJoint
    • Planar joints as UsdPhysics.Joint with the appropriate UsdPhysics.LimitAPI applied to constrain the locked DOFs
    • Floating joints (bodies are free by default in USD)
    • All joints have automatic joint frame alignment between Body0 and Body1, accounting for URDF joint axis, position, and orientation.
    • Joint limits for velocity & effort have no equivalent in UsdPhysics, but are authored as custom attributes urdf:limit:velocity and urdf:limit:effort respectively.
  • Geometry Conversion
    • All visual and collision geometry is converted to USD
      • Visuals are set with default UsdPurpose and colliders with guide UsdPurpose
    • UsdPhysics.CollisionAPI is applied to colliders
    • Meshes as UsdGeom.Mesh
      • Automatic mesh library generation with reference-based asset structure, to avoid duplicate topology
      • STL files converted to USD using numpy-stl and usd-exchange with normal processing
      • OBJ files converted using tinyobjloader and usd-exchange with UV coordinates and normal mapping
      • DAE files converted using pycollada and usd-exchange with UV coordinates, normal mapping, and UsdGeom.Subset support
      • OBJ and DAE files specifying multiple meshes convert as a list of meshes under a common parent prim
      • UsdPhysics.MeshCollisionAPI is applied to mesh colliders with convex hull specified as the approximation preference
    • Spheres as UsdGeom.Sphere
    • Boxes as UsdGeom.Cube with scale transforms
    • Cylinders as UsdGeom.Cylinder
  • Visual Material and Texture Conversion
    • All materials are converted to UsdShade.Material graphs using UsdPreviewSurface shaders, and encapsulated as instanceable material interfaces
    • PNG texture support with automatic texture copying and path resolution
    • URDF materials convert rgba as diffuse color and opacity, with support for diffuse textures
    • OBJ embedded materials (MTL files) convert diffuse color, specular color, dissolve (opacity), roughness (not shininess), metallic, and ior
      • diffuse, specular, normal/bump, roughness, metallic, and opacity textures are all supported
  • Prim Naming
    • If URDF/DAE/OBJ names are not valid USD specifiers they are automatically transcoded & made unique & valid
    • Display name metadata preserves the original source names on the USD Prims
  • Command Line Interface
    • Input is an URDF file and default output is a USD Layer as a structured Atomic Component with an Asset Interface USDA layer
      • All heavy data is compressed binary data (via USDC layers) while lightweight data is plain text for legibility
    • Optional comment string embedded into all authored USD Layers
    • Optional Stage flattening for single-file output
    • Optionally skip the UsdPhysics.Scene (this may be desirable for multi-asset setups)
    • Error handling with graceful failures
    • Enable verbose output for debugging (exposes any traceback info)
  • Python API
    • Full programmatic access via urdf_usd_converter.Converter class with configurable parameters for all CLI flags
    • Enables interactive editing of the USD Layers after conversion

Known Limitations

USD Data Conversion

  • Joint Conversion
    • Calibration has no equivalent in UsdPhysics and is omitted
    • Dynamics has no equivalent in UsdPhysics and is omitted
    • Safety Controller has no equivalent in UsdPhysics and is omitted
    • Mimic has no equivalent in UsdPhysics and is omitted
  • Geometry Conversion
    • No other file formats beyond OBJ/DAE/STL are supported
    • For DAE files, only "TriangleSet", "Triangles", "Polylist", and "Polygons" are supported
    • For OBJ files, only objects with faces are supported (i.e. no points, lines, or free-form curves/surfaces)
  • Visual Material and Texture Conversion
    • DAE embedded materials are not implemented
    • Projection shaders for basic geometry primitives (box, cylinder, sphere) are not implemented
    • More accurate PBR materials (e.g. OpenPBR via UsdMtlx) are not implemented
  • Other Elements
    • Transmission conversion is not implemented
    • Gazebo conversion is not implemented
    • Conversion of other out-of-spec URDF extensions are not implemented

Using the USD Asset in other USD Ecosystem applications

  • The USD Asset contains nested rigid bodies within articulations.
    • Support for nested bodies in UsdPhysics is fairly new (as of USD 25.11), and some existing applications may not support this style of nesting.
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