Shader Segment Highlight

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

Scope

The GPU highlight scheme (Plan B, iter. 2026-08-08) that replaced geometry-rebuilding overlays:

flowchart LR
    A["per-vertex attribute<br/>aSegLabel"] --> S["material patched via<br/>onBeforeCompile"]
    U["uniforms<br/>uHighlightLabel / uHighlightColor"] --> S
    S --> O["fragment: label match → tint<br/>O(1) hover switch · flat varying"]

Topics to cover: O(1) hover switching (was O(F) geometry rebuild — the giant-segment freeze root cause); preserveDrawingBuffer removed; conditional frameloop="demand" (IdleFrameloop: sleep after 1.5 s idle, wake on interaction); tool-gated highlight (only fill / picker / segment tools compute it).

Code pointers

• src/components/Viewport/Viewport.tsx — material patch, uniform wiring, gating
• src/segmentStages.ts

TODO

• Shader snippet walkthrough
• Why highlight is display-only (never a paint/fill input — see fill-routing.md)