review-webgpu-parity

name	review-webgpu-parity
description	Review changes to WebGPU/WebGL dual-backend parity. Use when a PR touches feedback managers, renderer adapters, shader lowering, or any code that affects both WebGPU and WebGL rendering paths.

Review WebGPU/WebGL Parity

Use this skill when reviewing or authoring changes to src/js/milkdrop/feedback-manager-*, src/js/milkdrop/renderer-adapter*, src/js/milkdrop/backend-behavior.ts, src/js/milkdrop/compiler/gpu-descriptor-plan.ts, or any shader-lowering code.

Why this exists

The most frequent source of fixes in this repo (~22% of fix commits) is parity drift between WebGPU and WebGL — still the #1 category: alpha blending order, feedback color math, wave interpolation, resolution scales, and shader lowering. This skill prevents those regressions at review time.

Pre-merge checklist

1. Both backends must be exercised

• bun run test:compat passes

• If the change touches shader generation or feedback sampling, run the parity reference suite:

  bun run test tests/milkdrop-renderer-adapter.test.ts
  bun run test tests/milkdrop-feedback-composite-profile.test.ts
  bun run test tests/milkdrop-shader-sampler-aliases.test.ts
  

• If the change touches compiler IR or GPU descriptor plans, run:

  bun run test tests/milkdrop-compiler.test.ts
  bun run test tests/milkdrop-compiler-seams.test.ts
  

2. No hardcoded backend-specific values without comment

• Every literal resolution scale, target size, sampler config, or blend factor is either:
  • a shared constant with a name explaining its parity role, or
  • accompanied by a comment explaining the WebGPU vs. WebGL semantic difference

3. Blend alpha order verified

• If changing wave, shape, or custom-wave blend behavior, confirm the additive/multiplicative/alpha order matches the projectM baseline.
• Prefer adding a regression test (see tests/milkdrop-renderer-adapter.test.ts for patterns).

4. Reference presets must not shift

• If the PR changes the render pipeline, verify against at least one of:

  • eos-phat-cubetrace-v2
  • krash-rovastar-cerebral-demons-stars
  • rovastar-parallel-universe

  bun run dev
  # Load preset, compare visual output to baseline or upstream projectM
  

5. Shader lowering comments

• Any change to compiler/gpu-descriptor-plan.ts, lowered field handling, or shader center normalization includes a comment explaining how the generated GPU code differs between WebGL and WebGPU.

What to reject in review

• Unconditional if (isWebGPU) branches that duplicate logic without a shared helper
• New innerHTML or string-built shader code without a corresponding test fixture
• Changes to feedback-manager-shared.ts that do not also update feedback-manager-webgpu.ts (or explain why not)
• Missing regression tests for fixed parity bugs

Related skills

• modify-preset-workflow — when the change is mainly preset content, not renderer parity
• modify-visualizer-runtime — when the change is broader runtime/shell work
• test-visualizer — for running the full validation suite