# Smooth Cesium preview vehicle cruise report

## Root Cause

The first preview-only optimization helped but did not address the main load.
User testing showed Follow still stuttered in scrub-heavy areas, while disabling
`Scene` improved smoothness substantially.

Current nantaizi GLB before the scrub-density change:

- GLB nodes: `1271`
- Scrub bush nodes: `707`
- Scrub polygons: `5`

The route data is small (`9` route segments, preview uses at most `5` vehicles),
so route interpolation is not the primary bottleneck.

## Changes

- `scripts/lib/cesium-preview.js`
  - Route helper polyline is now opaque instead of alpha blended.
  - Route helper polyline uses `arcType: Cesium.ArcType.NONE`.
  - Vehicle orientation and chase-follow camera callbacks reuse scratch
    `JulianDate`, `HeadingPitchRoll`, `Quaternion`, and `Matrix4` objects.
  - Default Cesium resolution and antialiasing are preserved; no blurry
    performance mode is enabled by default.
- `blender/generate_scene.py`
  - `SCRUB_BUSH_SPACING`: `0.82 -> 1.8`
  - `SCRUB_BUSH_LIMIT_PER_PATCH`: `180 -> 60`
  - Scrub ground cover remains unchanged.

## Validation

Commands run:

```bash
node --check scripts/lib/cesium-preview.js
python3 -m py_compile blender/generate_scene.py
python3 -m unittest blender/tests/test_pure.py
npm run build:area -- --config config/areas/nantaizi-lake-innovation-valley.json --stages blender,cesium,preview
node scripts/glb-digest.js outputs/nantaizi-lake-innovation-valley/nantaizi-lake-innovation-valley.glb --out /tmp/nantaizi-after-scrub-glb-digest.json
```

Results after rebuild:

- `SCENE_DONE` reports `scrub_bushes: 266`.
- GLB nodes: `830`
- Scrub bush nodes: `266`
- Scrub polygons in metadata: `5`
- Route segments in preview route JSON: `9`
- GLB size: about `31.96 MB`

## Deferred

If Follow still stutters after this density reduction, the next root-level work
should target the bush asset itself: lower-poly bush mesh, explicit LOD, or a
proper instancing path supported by Cesium/glTF.