osmWorkflow/.trellis/tasks/08-03-extract-building-module/prd.md

Extract building module

Goal

Move building assembly out of blender/generate_scene.py into a dedicated bpy-layer module, keeping generated .blend / .glb / metadata structure unchanged. This continues the documented P2 feature-module direction after fountain.py.

Background

• .trellis/spec/guides/artifact-parity-guide.md records P2 as partial: water.py, grass.py, scrub.py, tree.py, and fountain.py are already split out, while building and roads still live in generate_scene.py.
• blender/generate_scene.py:72 defines _assemble_building().
• blender/generate_scene.py:179 defines add_building_details(), which creates office / industrial window bands with MeshBatch and add_wall_panel().
• blender/generate_scene.py:789 dispatches OSM ways with a building tag, increments counts["building_count"] and counts["industrial_count"], and extends focus_points with the returned ring.
• blender/generate_scene.py:710 creates building_mats from the existing catalog.MATERIALS["building_" + key] entries.
• blender/generate_scene.py:910 persists office_override_way_ids as a scene custom property from args["office_overrides"].

Requirements

1. Add a new bpy-layer module at blender/osmassets/building.py.
2. Move the current building assembly behavior into the new module:
   • _assemble_building()
   • add_building_details()
3. Expose an assemble(...) function from the new module that returns the same tuple shape currently returned by _assemble_building(): (added, industrial_added, ring_pts).
4. Keep generate_scene.py responsible for:
   • parsing --office-overrides;
   • creating building_mats;
   • filtering OSM ways with a building tag and len(ring) >= 3;
   • updating building_count, industrial_count, and focus_points;
   • writing scene["office_override_way_ids"];
   • writing the existing SCENE_DONE JSON shape.
5. Preserve existing building object names and mesh names:
   • Building_<way_id>
   • Building_<way_id>_Roof
   • Building_<way_id>_Windows
6. Preserve existing building custom properties:
   • osm_height
   • render_height
   • building_kind
   • osm_building_tag
   • office_override
7. Preserve current height rules:
   • industrial buildings use the parsed source height;
   • non-industrial buildings under 30m render at 11.4m;
   • source height has a 3.0m floor and parse_height(tag, 12.0) default.
8. Preserve current facade behavior:
   • industrial buildings use one high horizontal band;
   • office buildings use floor-based window bands;
   • bevel modifier name, width, and segment count stay unchanged.
9. Do not introduce a full features/ registry in this task.
10. Do not modify roads, material definitions, MATERIALS order, ROAD_LAYERS order, stdout markers, parity ignore lists, or known unrelated defects D1-D3.

Acceptance Criteria

• blender/osmassets/building.py contains building assembly behavior and imports only bpy-layer-safe dependencies.
• blender/generate_scene.py imports and calls the new building module; no inline _assemble_building() or add_building_details() remains there.
• Building counts, industrial counts, focus point behavior, scene custom properties, and SCENE_DONE JSON keys remain unchanged.
• python3 -m py_compile blender/osmassets/building.py passes.
• python3 -m py_compile blender/generate_scene.py passes.
• python3 -m unittest blender/tests/test_pure.py passes.
• A before/after parity comparison is run for the Blender/Cesium stages and any non-ignored diff is either absent or explicitly explained as expected. For this refactor, the expected result is no contract diff.

Out Of Scope

• Full feature registry implementation.
• Road extraction.
• Building visual redesign.
• Building material or Cesium export contract changes.
• Moving CLI parsing or office_override_way_ids scene metadata out of generate_scene.py.
• Changing geometry helper APIs in osmassets.mesh.

Open Questions

None.