HartOMat/docs/plans/0001-step-to-usd-implementation.md

Implementation Plan: STEP → USD Canonical Scene Workflow

Execution checklist for ROADMAP.md Priorities 2, 4, and 5. RFC: docs/rfcs/0001-step-to-usd-workflow.md Date: 2026-03-11

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Prerequisites

• Priority 1 complete (step_tasks.py decomposed, blender_render.py decomposed)
• Decision: USD authoring library (see Open Questions below)
• Decision: seam/sharp payload encoding (primvars vs. JSON sidecar)

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Phase 1 — Dual-Write USD Beside GLB (Priority 2, M1–M2)

Goal: Export a valid usd_master alongside the existing GLB pipeline without changing any browser behavior.

Task 1.1 — export_step_to_usd.py scaffolding

File: render-worker/scripts/export_step_to_usd.py

What it must produce:

/Root                               — Stage root
/Root/Assembly                      — Top-level assembly prim (Xform)
/Root/Assembly/<AssemblyNode>       — Per-component prim (Xform)
/Root/Assembly/<AssemblyNode>/<PartKey>       — Leaf part prim (Xform)
/Root/Assembly/<AssemblyNode>/<PartKey>/Mesh  — UsdGeomMesh
/Root/Looks/<MaterialName>          — UsdShadeMaterial (placeholder binding)

Required per-mesh prim attributes:

Attribute	Value source
schaeffler:partKey	generate_part_key(xcaf_label_path)
schaeffler:sourceName	XCAF TDataStd_Name attribute
schaeffler:sourceColor	XCAF embedded color (hex string)
schaeffler:rawMaterialName	from CadFile.part_materials if available
schaeffler:tessellation:linearDeflectionMm	CLI arg value
schaeffler:tessellation:angularDeflectionRad	CLI arg value
primvars:schaeffler:seamEdgeVertexPairs	OCC B-rep seam edges (index pairs in mesh-local space)
primvars:schaeffler:sharpEdgeVertexPairs	sharp edges from _extract_sharp_edge_pairs()

CLI interface:

python3 export_step_to_usd.py \
  --step_path /path/to/file.stp \
  --output_path /path/to/output.usd \
  [--linear_deflection 0.03] \
  [--angular_deflection 0.05] \
  [--color_map '{"Ring": "#4C9BE8"}'] \
  [--tessellation_engine occ|gmsh]

Acceptance gate: python3 export_step_to_usd.py --step_path 81113-l_cut.stp --output_path /tmp/test.usd →

• File exists, parseable
• 25 part prims with schaeffler:partKey attribute
• Part count matches export_step_to_gltf.py output for same file

Task 1.2 — usd_master MediaAsset type

File: backend/app/domains/media/models.py

Add usd_master = "usd_master" to MediaAssetType enum.

Migration: backend/alembic/versions/060_usd_master_asset_type.py

ALTER TYPE mediaassettype ADD VALUE IF NOT EXISTS 'usd_master';

Acceptance gate: GET /api/media?asset_type=usd_master returns 200 (not 422 validation error).

Task 1.3 — generate_usd_master_task Celery task

File: backend/app/domains/pipeline/tasks/export_glb.py

New task that:

1. Resolves STEP file path from CadFile
2. Calls export_step_to_usd.py subprocess
3. Stores resulting .usd as a usd_master MediaAsset
4. Does NOT touch existing GLB tasks (dual-write, no removal yet)

Queue: step_processing (fast, < 30s — tessellation only, no Blender)

Called by: render_step_thumbnail task after render_step_thumbnail succeeds, OR triggered independently via admin action.

Acceptance gate: After triggering task for a CadFile, GET /api/cad/{id}/media includes an asset with asset_type: "usd_master".

---

Phase 2 — Canonical Part Identity and Assignment Layers (Priority 2, M3–M5)

Task 2.1 — part_key_service.py

File: backend/app/services/part_key_service.py

def generate_part_key(xcaf_label_path: str, source_name: str) -> str:
    """Deterministic slug from XCAF path + source name.

    Format: lowercase alphanumeric + underscores, max 64 chars.
    E.g.: 'ring_outer', 'ball_af0', 'cage_inner_ring'

    For duplicate names: append _2, _3, etc.
    For unnamed parts: 'part_{sha256(xcaf_path)[:8]}'
    """
    ...

def build_scene_manifest(cad_file: CadFile, usd_asset: MediaAsset) -> dict:
    """Read part metadata from USD or from CadFile.parsed_objects.

    Returns dict matching SceneManifest Pydantic schema:
    {
      "cad_file_id": "...",
      "parts": [
        {
          "part_key": "ring_outer",
          "source_name": "RingOuter_AF0",
          "prim_path": "/Root/Assembly/Bearing/RingOuter",
          "effective_material": "SCHAEFFLER_010102_...",
          "assignment_provenance": "manual|auto|default",
          "is_unassigned": false
        }
      ],
      "unmatched_source_rows": [...],
      "unassigned_parts": [...]
    }
    """
    ...

Task 2.2 — Three-layer material assignment model

File: backend/app/domains/products/models.py

Add three JSONB columns to CadFile:

source_material_assignments: dict | None    # from Excel / product import; keyed by source name
resolved_material_assignments: dict | None  # auto-matched; keyed by partKey
manual_material_overrides: dict | None      # browser-authored; keyed by partKey

Migration: backend/alembic/versions/061_material_assignment_layers.py

Keep existing part_materials column for backward compat during transition.

Service helper:

def get_effective_assignments(cad_file: CadFile) -> dict:
    """Priority: manual_overrides > resolved > source color > 'unassigned'."""
    ...

Task 2.3 — GET /cad/{id}/scene-manifest endpoint

File: backend/app/api/routers/cad.py

New endpoint returning SceneManifest. Calls build_scene_manifest() — reads from USD metadata if usd_master asset exists, otherwise reads from CadFile.parsed_objects.

Acceptance gate: Returns HTTP 200 with parts array; each part has part_key, effective_material, is_unassigned, assignment_provenance.

Task 2.4 — Update PUT /cad/{id}/part-materials

File: backend/app/api/routers/cad.py

Accept { "part_key": "ring_outer", "material": "SCHAEFFLER_010102_..." } body (or bulk map). Write to manual_material_overrides column (not the old part_materials column).

Acceptance gate: PUT with partKey → subsequent GET /scene-manifest shows that part's assignment_provenance: "manual".

---

Phase 3 — Seam/Sharp Payload to USD Mesh Prims (Priority 3 integration)

Task 3.1 — Port seam derivation into USD exporter

File: render-worker/scripts/export_step_to_usd.py

After tessellation (OCC or GMSH), for each mesh face:

1. Identify seam edges from face/batch boundaries (STEPper approach: adjacent faces with different batch IDs)
2. Identify sharp edges from _extract_sharp_edge_pairs() (already implemented)
3. Convert both to mesh-local vertex index pairs (not world-space coordinates)

Write to USD mesh prim:

mesh_prim.GetPrimvar("schaeffler:seamEdgeVertexPairs").Set(
    Vt.Vec2iArray(seam_pairs),  # [(vi0, vi1), ...]
)
mesh_prim.GetPrimvar("schaeffler:sharpEdgeVertexPairs").Set(
    Vt.Vec2iArray(sharp_pairs),
)

Why index-space: Survives transforms cleanly; avoids KD-tree matching workaround in _apply_sharp_edges_from_occ().

Task 3.2 — import_usd.py Blender helper

File: render-worker/scripts/import_usd.py

def import_usd_and_restore_topology(usd_path: str) -> list:
    """Import USD stage into Blender, restore seam/sharp from primvars.

    Returns list of imported mesh objects.
    """
    bpy.ops.wm.usd_import(filepath=usd_path)
    for obj in bpy.context.scene.objects:
        if obj.type != 'MESH':
            continue
        # Read custom attributes set by USD importer
        seam_pairs = obj.get("schaeffler_seamEdgeVertexPairs") or []
        sharp_pairs = obj.get("schaeffler_sharpEdgeVertexPairs") or []
        _mark_seams_from_index_pairs(obj, seam_pairs)
        _mark_sharp_from_index_pairs(obj, sharp_pairs)
    ...

Acceptance gate: Blender log shows [USD_IMPORT] 25 parts, 5044 seam edges restored from primvars — no KD-tree needed.

---

Phase 4 — Blender Render from USD (Priority 5)

Task 4.1 — blender_render.py USD path

File: render-worker/scripts/blender_render.py

Add --usd_path argument. When provided:

1. Call import_usd.py instead of export_gltf.py GLB import
2. Read schaeffler:partKey and schaeffler:canonicalMaterialName per mesh object after import
3. Apply materials by partKey → material library name lookup instead of object-name heuristics

Migration: Keep --glb_path working in parallel; switch production task to prefer --usd_path when usd_master asset exists.

Task 4.2 — Backend render service update

File: backend/app/services/render_blender.py

When building blender_cmd, check if usd_master MediaAsset exists for the CadFile:

• If yes: pass --usd_path <usd_master_local_path>
• If no: fall back to --glb_path <gltf_production_path> (unchanged)

---

Phase 5 — Frontend Migration (Priority 4)

Task 5.1 — Scene manifest fetch on product load

File: frontend/src/api/sceneManifest.ts

export interface PartEntry {
  part_key: string;
  source_name: string;
  prim_path: string;
  effective_material: string | null;
  assignment_provenance: 'manual' | 'auto' | 'default';
  is_unassigned: boolean;
}

export interface SceneManifest {
  cad_file_id: string;
  parts: PartEntry[];
  unmatched_source_rows: string[];
  unassigned_parts: string[];
}

export async function fetchSceneManifest(cadFileId: string): Promise<SceneManifest> {
  const res = await api.get<SceneManifest>(`/cad/${cadFileId}/scene-manifest`);
  return res.data;
}

Task 5.2 — ThreeDViewer partKey integration

File: frontend/src/components/cad/ThreeDViewer.tsx

• On GLB load: iterate scene.children, read mesh.userData.partKey (set by preview GLB derivation)
• On click: identify partKey from userData, not from mesh.name
• Pass partKey to MaterialPanel for display and persistence

Task 5.3 — MaterialPanel reconciliation section

File: frontend/src/components/cad/MaterialPanel.tsx

Add reconciliation section showing:

• "N unmatched Excel rows" (source names with no partKey match)
• "M unassigned parts" (partKeys with no resolved material)

Clicking an unassigned part in the viewer auto-focuses it in the MaterialPanel.

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Open Questions (must be decided before Phase 1 coding starts)

#	Question	Options	Default recommendation
1	USD authoring library	pxr (full OpenUSD, heavy) / usda text templating (no deps) / usd-core pip	Start with pxr — pip-installable, same OCC kernel available
2	Seam/sharp payload encoding	Custom primvars on mesh prim / separate JSON sidecar / GLB extras (current)	Index-space primvars — cleaner, survives transforms
3	Preview GLB derivation	USD → GLB export pass / co-author from same tessellation pass	Co-author during migration (avoid round-trip loss)
4	Single-file USD or override layers	Flat single file / canonical + overlay layers (flattened for delivery)	RFC recommends Option B (overlay layers, flatten for delivery)
5	pxr install in render-worker	Add to render-worker/Dockerfile / use system package	pip install usd-core — no NVIDIA/Pixar GPU tools needed

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Non-Regression Checklist

Before merging any Priority 2–5 work:

• Click a part in ThreeDViewer → selection resolves to stable partKey
• Pin selection → isolate, hide, ghost all work as before
• Unassigned parts are visually highlighted
• Assign a Blender asset-library material name via browser → persisted by partKey
• Reload page → same part still assigned
• Subsequent Blender render uses the same assignment
• CAD file with mismatched Excel names → system produces canonical scene, preview asset, unmatched row count
• geometryGltfUrl / productionGltfUrl distinction no longer required by frontend (removed from API contract)