4e42d663a6
Replace the megakernel @compute entry with five wavefront kernels sharing one module, connected by GPU ray/hit/payload buffers and a GPU-driven indirect bounce loop: GENERATE -> (PREP -> TRACE -> SHADE) x maxDepth -> RESOLVE - TRACE contains zero user code (pure _rtwTraverseTlas/Blas, opaque-only). - PREP publishes dispatchWorkgroupsIndirect args from the live ray count; the indirect-args buffer lives in its own bind group so it is never bound read-write in the same dispatch that consumes it as INDIRECT. - New emit/accumulate API: rtEmitPrimaryRay / rtEmitRay / rtAccumulate, plus an optional user Resolve stage (tonemap hook; identity by default). - Per-pass WfParams via a dynamic-offset uniform ring (curIsA/bounce vary between passes within one submit). - Payload-typed wfPayload binding emitted in the codegen region after the user's struct Payload; payload travels with each ray (2*W*H slots). - Request maxBufferSize / maxStorageBufferBindingSize / maxComputeWorkgroups PerDimension so the W*H-sized work buffers fit past the 128MB baseline. VulkanTriangle ported to the new API and renders bit-identical to the megakernel baseline at maxDepth=1. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
69 lines
3.7 KiB
Markdown
69 lines
3.7 KiB
Markdown
# WebGPU wavefront RT rewrite — design & progress (issue #3)
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Replaces the single megakernel (`main`, 8×8 tile, per-pixel
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raygen→traceRay→CH/miss→store) with a streaming wavefront tracer:
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`GENERATE → PREP → (TRACE → SHADE → PREP)×maxDepth → RESOLVE`, each its own
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compute pass, dispatch sizes driven by `dispatchWorkgroupsIndirect`.
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## Kernels (all generated/assembled the same megakernel way, just split)
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- **GENERATE** (1 thread/pixel, 8×8): runs user `raygen_main(gid)` which calls
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`rtEmitPrimaryRay(...)`. Clears accum slot + payload slot for the pixel.
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- **PREP** (1 thread): reads emit counter for the just-filled ray buffer,
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writes indirect args `[ceil(n/64),1,1]`, publishes `traceCount=n`, swaps
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cur/next ray buffer, resets next emit counter. One PREP before first TRACE
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and one after each SHADE.
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- **TRACE** (1 thread/ray, 64-wide, indirect): ZERO user code. Reads ray i,
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runs `_rtTraverseTlas`, writes `HitResult` i (t/instanceId/primId/hg/attribs
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/objToWorld/customIndex/missFlag).
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- **SHADE** (1 thread/ray, 64-wide, indirect): reads ray i + hit i + payload
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slot p. miss→`runMiss`, hit→`runClosestHit` (unless SKIP_CLOSEST_HIT). User
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code calls `rtAccumulate(pixel,rgb)` and `rtEmitRay(...)`.
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- **RESOLVE** (1 thread/pixel, 8×8): reads accum slot, runs user `resolve_main`
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if present else passthrough; writes outImage.
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## Buffers (rtState, sized to 2*W*H rays)
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- `wfRaysA`,`wfRaysB`: array<WfRay>, ping/pong. WfRay = origin,tMin,dir,tMax,
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pixel,flags,cullMask,missIndex,sbtOffset,payloadSlot,kind,_pad.
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- `wfHits`: array<HitResult> (sized = ray capacity).
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- `wfPayload`: array<Payload> — declared in CODEGEN region after user Payload.
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- `wfAccum`: array<vec4<f32>> per pixel (W*H).
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- `wfCounters`: atomic counters: emitA, emitB, trace dispatch args, etc.
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- `wfIndirect`: INDIRECT dispatch-args buffer.
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## API (new, breaking)
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- raygen: `rtEmitPrimaryRay(origin,tMin,dir,tMax,flags,cullMask,sbtOff,missIdx)`
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→ allocates payloadSlot=pixel, writes ray to current buffer (atomic bump).
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- CH/miss: `rtEmitRay(origin,tMin,dir,tMax,flags,cullMask,sbtOff,missIdx,payload)`
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spawns into NEXT buffer carrying a payload slot; `rtAccumulate(pixel,rgb)`.
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- `rtGetPayload(slot)` / payload passed by value into CH/miss via slot.
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## Tonemap / resolve
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Accum buffer is linear. Optional user `WebGPURTStage::Resolve` entry
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`resolve_main(coord:vec2<u32>, hdr:vec4<f32>)->vec4<f32>`. None → passthrough.
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VulkanTriangle: no resolve (exact match). Sponza: resolve does Reinhard+gamma.
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## Indirect dispatch (Phase 2 de-risk)
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Prove `dispatchWorkgroupsIndirect` + cross-pass atomic visibility with a toy
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"emit N → dispatch N" before wiring real kernels. WebGPU inserts an implicit
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barrier between compute passes in one submit, so atomics written in PREP are
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visible to TRACE.
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## maxDepth
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Compile/runtime knob. JS unrolls the chain to maxDepth. VulkanTriangle
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maxDepth=1 (primary only). Sponza maxDepth=2 (primary + shadow).
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## Status / progress
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- [x] baseline VulkanTriangle renders (megakernel) — /tmp/baseline-triangle.png
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- [ ] wavefront prelude + codegen
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- [ ] VulkanTriangle on wavefront (maxDepth=1)
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- [ ] bounce loop + indirect + Sponza shadow port
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- [ ] RTStress example + timestamp queries
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- [ ] ordered traversal, dynamic TLAS depth, device limits
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- [ ] remove megakernel dual path; final validation; PR
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## Files
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- `additional/dom-webgpu.js` — prelude (`rtWgsl*`), `wgpuLoadRTPipeline`,
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`wgpuDispatchRT`, LBVH build, rtState/buffers, device-limit clamp (~L131).
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- `implementations/Crafter.Graphics-PipelineRTWebGPU.cpp` — assembles user
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WGSL + entry glue; must emit 5 entry points + payloadStore binding.
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- examples/{VulkanTriangle,Sponza,RTStress}/*.wgsl + main.cpp.
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