# 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

---

## Prerequisites

- [ ] Priority 1 complete (step_tasks.py decomposed, blender_render.py decomposed)
- [x] Decision: USD authoring library → **`usd-core` (pip)** — provides `pxr` module, no GPU tools needed, pip-installable in render-worker
- [x] Decision: seam/sharp payload encoding → **index-space primvars** (`primvars:schaeffler:seamEdgeVertexPairs`, `primvars:schaeffler:sharpEdgeVertexPairs`) — survives transforms, no KD-tree needed
- [x] Decision: preview GLB derivation → **co-author from same tessellation pass** during migration (avoid round-trip loss from USD→GLB export)
- [x] Decision: single-file vs override layers → **Option B: canonical geometry layer + material override layer, flattened via `UsdUtils.FlattenLayerStack()` for delivery** — preserves hierarchy AND allows instancing later (`FlattenLayerStack` keeps `instanceable` prims; `UsdStage.Flatten` would expand them). Note: Phase 1 uses no instancing (matching current GLB pipeline), but the delivery path is already instancing-safe.

---

## 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 1.0 — Install `usd-core` in render-worker

**File:** `render-worker/Dockerfile`

```dockerfile
# OpenUSD Python bindings — provides pxr module for USD authoring
RUN pip3 install --no-cache-dir "usd-core>=24.11"
```

Add after the `gmsh` line. `usd-core` is the Pixar-maintained pip distribution of OpenUSD — no GPU, no USD imaging, just the core authoring/scene APIs (`pxr.Usd`, `pxr.UsdGeom`, `pxr.Sdf`, `pxr.Vt`).

**Acceptance gate:** `docker compose exec render-worker python3 -c "from pxr import Usd; print(Usd.GetVersion())"` → `(24, 11, 0)` or later.

---

### 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:**

```bash
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`

```sql
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`

```python
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`:

```python
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:**

```python
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:

```python
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`

```python
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`

```typescript
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.

---

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

| # | Question | Options | Default recommendation |
|---|---|---|---|
| 1 | USD authoring library | ~~`pxr` full / `usda` text / `usd-core` pip~~ | ✅ **`usd-core` pip** — `pip install usd-core` in render-worker Dockerfile |
| 2 | Seam/sharp payload encoding | ~~primvars / JSON sidecar / GLB extras~~ | ✅ **index-space primvars** on mesh prim |
| 3 | Preview GLB derivation | ~~USD→GLB export / co-author from tessellation pass~~ | ✅ **co-author from same tessellation pass** during migration |
| 4 | Single-file USD or override layers | ~~flat single file / canonical + overlay layers~~ | ✅ **Option B: override layers + `UsdUtils.FlattenLayerStack()` for delivery** — hierarchy preserved, instancing-safe |

---

## 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)