wip: uncommitted changes from claude run on issue #3

examples/RTStress/closesthit.wgsl

@@ -0,0 +1,54 @@
+// RTStress closest-hit (runs in SHADE). Computes flat-shaded Lambert from
+// the hit triangle's geometric normal, accumulates ambient, and — if the
+// surface faces the sun — emits a shadow ray toward the sun. The shadow
+// ray's miss (sun visible) adds the direct term; its hit (occluded) adds
+// nothing because RT_FLAG_SKIP_CLOSEST_HIT suppresses closesthit on hit.
+//
+// Payload declared here so the assembler sees it before wfPayload / SHADE.
+struct Payload {
+    color:     vec3<f32>,   // shadow ray: pending direct contribution
+    shadowRay: u32,         // 0 primary, 1 shadow
+};
+
+const SUN_DIR_TO_LIGHT: vec3<f32> = vec3<f32>(0.40, 0.85, 0.35);
+const SUN_COLOR:        vec3<f32> = vec3<f32>(1.15, 1.05, 0.90);
+const AMBIENT_COLOR:    vec3<f32> = vec3<f32>(0.12, 0.13, 0.18);
+
+// Cheap per-instance albedo so the grid reads as distinct cubes (and any
+// TLAS flicker as instance count scales is obvious).
+fn instanceAlbedo(i: u32) -> vec3<f32> {
+    let h = i * 2654435761u;
+    return vec3<f32>(
+        0.35 + 0.6 * f32((h >>  0u) & 255u) / 255.0,
+        0.35 + 0.6 * f32((h >>  8u) & 255u) / 255.0,
+        0.35 + 0.6 * f32((h >> 16u) & 255u) / 255.0);
+}
+
+fn closesthit_main(ray: RayDesc, hit: HitInfo, payload: ptr<function, Payload>) {
+    let meshRec = meshRecords[tlasEntries[hit.instanceId].blasMeshIdx];
+    let verts = _rtFetchTri(meshRec, hit.primitiveId);
+    let nObj = normalize(cross(verts[1] - verts[0], verts[2] - verts[0]));
+    let nWorld = normalize(vec3<f32>(
+        dot(hit.objectToWorldR0.xyz, nObj),
+        dot(hit.objectToWorldR1.xyz, nObj),
+        dot(hit.objectToWorldR2.xyz, nObj)));
+
+    let albedo   = instanceAlbedo(hit.customIndex);
+    let worldPos = ray.origin + ray.direction * hit.t;
+    let viewDir  = -ray.direction;
+    let nFacing  = select(-nWorld, nWorld, dot(nWorld, viewDir) > 0.0);
+    let sunDir   = normalize(SUN_DIR_TO_LIGHT);
+    let nDotL    = max(0.0, dot(nFacing, sunDir));
+
+    rtAccumulate(albedo * AMBIENT_COLOR);
+
+    if (nDotL > 0.0) {
+        var sp: Payload;
+        sp.color     = albedo * SUN_COLOR * nDotL;
+        sp.shadowRay = 1u;
+        let shadowOrigin = worldPos + nFacing * 0.05;
+        rtEmitRay(shadowOrigin, 0.01, sunDir, 100000.0,
+                  RT_FLAG_SKIP_CLOSEST_HIT | RT_FLAG_TERMINATE_ON_FIRST_HIT,
+                  0xFFu, 0u, 0u, sp);
+    }
+}