URDF USD Converter v0.1.0a1

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.