# RTVolume

Ray tracing of **procedural (AABB) geometry** with an **any-hit** cut-out
on both backends — software WebGPU (issue #13) and native Vulkan hardware
RT (issue #33).

A 3×3×3 grid of unit boxes is registered as an AABB BLAS
(`Mesh::BuildProcedural` — `VK_GEOMETRY_TYPE_AABBS_KHR` on Vulkan, the
software AABB-leaf path on WebGPU). The hit group is procedural
(`RTShaderGroupType::ProceduralHitGroup` /
`VK_RAY_TRACING_SHADER_GROUP_TYPE_PROCEDURAL_HIT_GROUP_KHR`) carrying:

- `intersection.wgsl` / `intersection.glsl` — analytic ray–sphere test that
  turns each box into a radius-1 sphere (runs once per box the ray enters);
- `anyhit.wgsl` / `anyhit.glsl` — ignores the intersection 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` / `closesthit.glsl` — 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 (free camera, WASD + mouse):

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

Native Vulkan (fixed camera in raygen.glsl):

```
crafter-build -r
```
