OpenUSD & UsdPhysics Digital Twins

ModelScript provides first-class support for OpenUSD (.usd, .usda, .usdz), the universal 3D scene description standard governed by the Alliance for OpenUSD (AOUSD). In ModelScript, OpenUSD serves as the high-fidelity 3D bridge connecting 1D/multibody physical simulation (Modelica, URDF, SIMPACK, Adams) and procedural 3D CAD (STEP, OpenSCAD) with photorealistic digital twins (NVIDIA Omniverse / Isaac Sim, Apple Vision Pro, Unreal Engine).

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Capabilities & Supported Formats

Format	Extension	Description
ASCII OpenUSD Stage	.usda	Human-readable UTF-8 scene description defining prim hierarchies, time-sampled attribute curves, and UsdPhysics schemas.
OpenUSD Zip Package	.usdz	Zero-dependency, uncompressed ZIP archive with strict 64-byte payload data alignment for direct mmap streaming on iOS / visionOS / AR.
Binary / Mixed OpenUSD	.usd	Generic USD container format routed automatically to ModelScript's streaming parser and 3D viewers.

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Key Architectural Features

1. UsdPhysics Simulation Ingestion (lowerUsdToDae)

ModelScript can parse OpenUSD stages authored in external digital twin tools (NVIDIA Isaac Sim, Blender, Omniverse) and extract physics properties:

• Rigid Bodies (PhysicsRigidBodyAPI, PhysicsMassAPI): Extracts scalar mass, center-of-mass offsets, and diagonal principal inertia tensors.
• Kinematic Constraints: Resolves parent/child body targets (physics:body0, physics:body1), axis tokens, and limits for PhysicsRevoluteJoint, PhysicsPrismaticJoint, and PhysicsFixedJoint.
• Linear DAE Lowering: Lowers generalized joint coordinates $(q,\dot{q},\tau )$ directly into ModelScript's WebAssembly Struct-of-Arrays DAEBuilder arena for simulation via CVODE/IDA or batched WebGPU integrators.

2. Time-Sampled Trajectory Animation

When forward dynamics are solved in ModelScript (from ROS URDF or Modelica MultiBody models), joint state trajectories are converted via forward kinematics into time-sampled USD animation curves:

double3 xformOp:translate.timeSamples = {
    0.0: (0.0, 0.0, 0.5),
    0.5: (0.1, 0.0, 0.7),
    1.0: (0.2, 0.0, 0.8),
}
quatf xformOp:orient.timeSamples = {
    0.0: (1.0, 0.0, 0.0, 0.0),
    0.5: (0.923, 0.382, 0.0, 0.0),
    1.0: (0.707, 0.707, 0.0, 0.0),
}
uniform token[] xformOpOrder = ["xformOp:translate", "xformOp:orient"]

These files can be immediately opened in Apple Vision Pro, Quick Look, or NVIDIA Omniverse for dynamic simulation playback.

3. Procedural CAD & CSG Export

Watertight 2-manifold triangular meshes synthesized by @modelscript/cad (ManifoldSurfaceMesh) and OpenSCAD (@modelscript/scad) map directly to UsdGeomMesh:

• Exact vertex points (point3f[] points)
• Outward unit surface normals (normal3f[] normals)
• Triangulated index buffers (int[] faceVertexIndices)
• Diffuse material colors (color3f[] primvars:displayColor)
• Part assembly transformations (matrix4d xformOp:transform)

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Command Line Interface (msx)

# Export URDF robot to human-readable ASCII OpenUSD with UsdPhysics schemas
msx urdf to-usd robot.urdf -o robot.usda

# Export to 64-byte aligned USDZ package for Apple Vision Pro & AR Quick Look
msx urdf to-usd robot.urdf --usdz -o robot.usdz

# Simulate robot forward dynamics and output trajectory
msx urdf simulate robot.urdf --stop-time=2.0 -o sim.json

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Semantic Linter Rules (USD_*)

Code	Rule	Description
USD1002	dangling-relationship	Joint or constraint targets a non-existent prim path in the stage (rel physics:body0).
USD1003	invalid-up-axis	Stage upAxis metadata is not 'Y' or 'Z'.
USD1004	non-positive-mass	Rigid body declares a mass $\le 0$ (physics:mass).
USD1005	invalid-inertia	Diagonal inertia tensor component is negative (physics:diagonalInertia).
USD1006	inverted-joint-limits	Joint lower limit exceeds upper limit (physics:lowerLimit > physics:upperLimit).
USD1007	invalid-axis-token	Joint axis token is not 'X', 'Y', or 'Z' (physics:axis).