catbot 5dd1086f08 docs(webgpu-rt): add RTVolume example (procedural spheres + any-hit cut-out)

A 3x3x3 grid of AABB-geometry spheres rendered through an analytic
ray-sphere intersection shader, with an any-hit spherical-checkerboard
cut-out so the background shows through. Exercises both features end to
end on the WebGPU wavefront tracer.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

2026-06-02 22:09:30 +00:00

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RTVolume

WebGPU software ray tracing of procedural (AABB) geometry with an any-hit cut-out — the two features added for issue #13.

A 3×3×3 grid of unit boxes is registered as an AABB BLAS (Mesh::BuildProcedural, the WebGPU analog of VK_GEOMETRY_TYPE_AABBS_KHR). The hit group is a RTShaderGroupType::ProceduralHitGroup carrying:

intersection.wgsl — analytic ray–sphere test that turns each box into a radius-1 sphere (runs in TRACE, once per box the ray enters);
anyhit.wgsl — returns RT_ANYHIT_IGNORE for half the cells of a spherical checkerboard, so the ray passes through and the background / spheres behind show through (the visible proof any-hit runs);
closesthit.wgsl — normal-based Lambert shading, tinted per instance.

The geometry is registered non-opaque and the instances clear their force-opaque flag, which is what lets the any-hit shader run. Flip the instance flag to kRTGeometryInstanceForceOpaque (or build the mesh with opaque = true) to skip any-hit and see solid spheres.

WebGPU/DOM only:

crafter-build --target=wasm32-wasip1 -r

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RTVolume

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