osmWorkflow/.trellis/tasks/archive/2026-08/08-03-use-low-poly-scrub-bush/prd.md

Use low-poly scrub bush asset

Goal

Prefer the new custom low-poly scrub bush asset for scrub edge instances and validate nantaizi-lake-innovation-valley Blender/Cesium/preview output.

The user supplied assets/models/custom/bush_low/bush_low.glb. The goal is to test whether using it reduces scrub bush mesh cost and improves Follow smoothness without losing scrub ground cover or breaking material tinting.

Requirements

1. Update blender/generate_scene.py to prefer assets/models/custom/bush_low/bush_low.glb for scrub edge bushes.
2. Keep assets/models/custom/bush/bush.glb as a fallback if the low asset is missing or fails to import.
3. Preserve deterministic scrub edge sampling and current scrub density budget: SCRUB_BUSH_SPACING = 1.8, SCRUB_BUSH_LIMIT_PER_PATCH = 60.
4. Preserve existing material tint behavior based on material names containing leaf, trunk, or twig.
5. Rebuild nantaizi-lake-innovation-valley stages blender,cesium,preview.
6. Measure and report:
   • SCENE_DONE.scrub_bushes
   • GLB total nodes
   • scrub bush node count
   • scrub bush mesh vertex/index bytes or primitive counts from GLB digest
   • output GLB size
7. Do not change road/building/tree/vehicle route behavior.

Acceptance Criteria

• generate_scene.py prefers bush_low.glb and falls back to bush.glb.
• python3 -m py_compile blender/generate_scene.py passes.
• python3 -m unittest blender/tests/test_pure.py passes.
• npm run build:area -- --config config/areas/nantaizi-lake-innovation-valley.json --stages blender,cesium,preview passes.
• GLB digest confirms scrub bush nodes remain about 266 and scrub ground metadata remains present.
• Task report/changelog records whether bush_low meaningfully reduced mesh cost and whether it is worth keeping.

Out Of Scope

• Compressing textures or changing baked material export.
• Replacing the preview car asset.
• Runtime LOD implementation.
• Further changing scrub density unless low asset validation shows it is needed.