OpenUSD layout and contract

How a piece is laid out in the file, the ground and physics scene, OpenUSD compliance, and what the Primvaria contract writes on every piece.

OpenUSD layout

Z up, metres, kilograms. /World is the default prim.

/World
  /Looks/<piece>      UsdPreviewSurface + UsdUVTexture (textures/<piece>.png)
  /Cloths/<piece>     UsdGeom.Mesh, flat on the ground, laid out in rows
  /PhysicsScene       gravity (shared, see below)
  /Ground             the shared ground, its top face at z = 0 (CollisionAPI)
  /GroundMaterial     its physics material

Ground and physics scene (the same in every Primvaria project)

Every file with physics carries the same two prims, so that assets from different generators can share one scene and every engine gets the same floor:

The code is one file, ground.py, identical in Granulaire, Maillage, Filaire and Quincaillerie; a test checks every generated file against it. The MuJoCo checks and the Blender rigid-body checks take the ground's friction from the file (Blender's cloth, in Maillage, has its own collision settings).

Each cloth mesh has subdivisionScheme = none, is double sided, and carries:

OpenUSD has no standard schema for cloth simulation (UsdPhysics covers rigid bodies and joints), so these properties are custom attributes. Blender's USD importer reads the geometry, UVs, material, texture and the hem and pin primvars (as mesh attributes); the drape test reads the maillage:* attributes with the OpenUSD bindings shipped with Blender.

OpenUSD compliance

python -m maillage.validate output/tea_towel

This runs every validator of OpenUSD's UsdValidation registry on each file, the checks usdchecker runs: composition, stage metadata (meters, Z up), geometry, material bindings, shaders, UsdPhysics rigid bodies and colliders. Each file is checked once per variant, since a variant that is not selected is not composed. The tests and CI require no error and no warning, on every preset.

Primvaria contract

python -m maillage.conformance output/tea_towel

Maillage is part of Primvaria and follows its mission and its OpenUSD contract, version 0, which the four generators share. Every piece is an asset any tool can find without knowing Maillage: a model of kind component with assetInfo (name, identifier, version) and the attributes primvaria:contract, primvaria:generator, primvaria:version, primvaria:preset and primvaria:seed; /World is an assembly and /World/Cloths a group. The command above checks the stage units, the model hierarchy and this identity; the tests and CI require it on every preset.

What was drawn for each piece is in the file too, its ground truth: primvaria:variation:aspects lists the aspects that were switched on (size, cut, corners, fabric, color) and primvaria:variation:<name> holds each drawn value (length_m, width_m, format_length_m, corner_radius_m, cut_amplitudes_m, cut_skew_deg, fabric, pattern, colors…). An aspect that is switched off is still recorded, at its nominal value, and the other aspects keep their draws.

What was measured on each piece once generated is there as well, as primvaria:measured:<name> (mass_kg, area_m2, perimeter_m, vertices, faces, edge_mean_mm, min_angle_deg…). The pack's instances.csv is read back from the OpenUSD files, one row per asset with the same names (variation:<name>, measured:<name>), and manifest.json says what the pack is; both have the same shape in the four generators, and the command above fails when the table and the files disagree.

Each piece also says what it is with OpenUSD's own semantic labels (UsdSemantics.LabelsAPI), which renderers such as Isaac Sim turn into segmentation masks: semantics:labels:class holds cloth and the preset (tea_towel), and semantics:labels:instance the asset's name. The classes are a column of instances.csv and are listed in manifest.json.