BVH Face Picking

Status: placeholder — content pending. Fill freely in English or 中文.

Scope

How a mouse click becomes a face index on a 1.5M-face mesh:

sequenceDiagram
    autonumber
    participant U as User click
    participant R as Raycaster
    participant B as BVH (three-mesh-bvh)
    participant F as Backend face id
    U->>R: pointer event (clientX, clientY)
    R->>B: intersectObject(mesh)
    B-->>R: closest hit (indirect mode)
    R-->>F: face.index == backend face id

Topics to cover: three-mesh-bvh build (computeBoundsTree({ indirect: true }), deferred to a macrotask so the UI doesn't stall) and accelerated raycast; why CPU raycast and not GPU render-target readback (FR-VIEW-07 forbids GPU readback — render-pipeline sync stalls; the old colour-ID RenderTarget design is retired); face-index stability with shared position/index buffers; hover + paint sharing ONE raycast per event (no double O(F) work).

Code pointers

• src/components/Viewport/Viewport.tsx — BVH setup, picker hook, shared raycast
• src/hooks/useMesh.ts — geometry build & index ordering

TODO

• Perf numbers (BVH build time / hit time per mesh size)
• DPR / coordinate correction notes