Crafter.Graphics/examples/Sponza/closesthit.wgsl

// Payload declared here so the WGSL assembler sees it before raygen
// (the assembler concatenates closesthit/anyhit/miss BEFORE raygen).
//
// WGSL forbids cycles in the function call graph, so closesthit_main
// CAN'T call traceRay (that would create closesthit → traceRay →
// runClosestHit → closesthit). The lighting + shadow trace therefore
// happens in raygen; closesthit's job is just to gather surface data
// into the payload.
//
//   shadowRay = 0 (primary): closesthit fills albedo/worldPos/normal/hit.
//   shadowRay = 1 (shadow):  closesthit is skipped (RT_FLAG_SKIP_CLOSEST_HIT),
//                            miss flips color to white = "lit".
struct Payload {
    color:       vec3<f32>,
    shadowRay:   u32,
    worldPos:    vec3<f32>,
    hit:         u32,
    worldNormal: vec3<f32>,
    _pad:        f32,
};

// User-bound resources at group(2). Matches the UICustomBinding span the
// host hands to PipelineRTWebGPU::Init.
//   binding 0 — albedo texture_2d_array, one layer per Sponza material
//   binding 1 — sampler (linear clamp)
//   binding 2 — camera storage buffer (read by raygen only)
@group(2) @binding(0) var albedos : texture_2d_array<f32>;
@group(2) @binding(1) var samp    : sampler;

// VertexNormalTangentUVPacked is `packed` on the outer struct but each
// inner `Vector<float, N, 4>` is SIMD-aligned to a 16-byte stride. So
// each vertex is 12 u32 words: normal at 0..2, tangent at 4..6, uv at 8..9.
const ATTRIB_STRIDE_U32:    u32 = 12u;
const ATTRIB_NORMAL_OFFSET: u32 = 0u;
const ATTRIB_UV_OFFSET:     u32 = 8u;

fn fetchUV(meshRec: MeshRecord, vertexIdx: u32) -> vec2<f32> {
    let base = meshRec.attribsOffset + vertexIdx * ATTRIB_STRIDE_U32 + ATTRIB_UV_OFFSET;
    return vec2<f32>(
        bitcast<f32>(vertexAttribs[base + 0u]),
        bitcast<f32>(vertexAttribs[base + 1u]),
    );
}

fn fetchNormal(meshRec: MeshRecord, vertexIdx: u32) -> vec3<f32> {
    let base = meshRec.attribsOffset + vertexIdx * ATTRIB_STRIDE_U32 + ATTRIB_NORMAL_OFFSET;
    return vec3<f32>(
        bitcast<f32>(vertexAttribs[base + 0u]),
        bitcast<f32>(vertexAttribs[base + 1u]),
        bitcast<f32>(vertexAttribs[base + 2u]),
    );
}

fn closesthit_main(ray: RayDesc, hit: HitInfo, payload: ptr<function, Payload>) {
    // Resolve hit triangle → 3 vertex indices.
    let meshIdx = tlasEntries[hit.instanceId].blasMeshIdx;
    let meshRec = meshRecords[meshIdx];
    let baseIdx = meshRec.indexOffset + hit.primitiveId * 3u;
    let i0 = indices[baseIdx + 0u];
    let i1 = indices[baseIdx + 1u];
    let i2 = indices[baseIdx + 2u];
    let bary = vec3<f32>(1.0 - hit.attribs.x - hit.attribs.y, hit.attribs.x, hit.attribs.y);

    // Albedo via barycentric UV interpolation.
    let uv0 = fetchUV(meshRec, i0);
    let uv1 = fetchUV(meshRec, i1);
    let uv2 = fetchUV(meshRec, i2);
    let uv  = uv0 * bary.x + uv1 * bary.y + uv2 * bary.z;
    // OBJ V is bottom-up; sampler is top-down. fract for manual tiling.
    let uvTiled = vec2<f32>(fract(uv.x), fract(1.0 - uv.y));
    let layer   = i32(hit.customIndex);
    let albedo  = textureSampleLevel(albedos, samp, uvTiled, layer, 0.0).rgb;

    // World-space smooth shading normal. Multiply through the
    // object-to-world rotation so this stays correct if a future scene
    // rotates instances (Sponza itself is all identities).
    let n0 = fetchNormal(meshRec, i0);
    let n1 = fetchNormal(meshRec, i1);
    let n2 = fetchNormal(meshRec, i2);
    let nObj = normalize(n0 * bary.x + n1 * bary.y + n2 * bary.z);
    let nWorld = normalize(vec3<f32>(
        dot(hit.objectToWorldR0.xyz, nObj),
        dot(hit.objectToWorldR1.xyz, nObj),
        dot(hit.objectToWorldR2.xyz, nObj)));

    (*payload).color       = albedo;
    (*payload).worldPos    = ray.origin + ray.direction * hit.t;
    (*payload).worldNormal = nWorld;
    (*payload).hit         = 1u;
}