WebGPU RT: dynamic TLAS sweep-tree depth (next_pow2 instances)

The LBVH bitonic sort still runs over the full 16384 (sentinels sink to
the tail), but the sweep tree is now built and traced at depth
log2(next_pow2(nReal)) instead of a fixed 14. Add nPadded to LbvhPC; leaf
init + bottom-up refit use it; the host passes the same next_pow2 to the
trace via WfParams.tlasNPadded. Renders correctly at 512 instances
(depth 9). The fragile sort phases are untouched.

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

additional/dom-webgpu.js

@@ -2281,7 +2281,11 @@ struct BvhNode {
 // runtime resize-on-grow caused subtle BVH corruption (driver-level
 // memory recycling, suspected) and was the root cause of mid-game
 // geometry flicker when projectiles entered the TLAS.
-struct LbvhPC { nReal: u32, _pad0: u32, _pad1: u32, _pad2: u32 };
+// nPadded = next_pow2(max(nReal,1)), supplied by the host. The bitonic
+// sort still runs over the full N_PADDED (sentinels sink to the tail), but
+// the sweep tree is built (and traced) at depth log2(nPadded) so descent
+// tracks the real instance count instead of a fixed 14.
+struct LbvhPC { nReal: u32, nPadded: u32, _pad1: u32, _pad2: u32 };
 @group(0) @binding(5) var<uniform> lbvhPc : LbvhPC;
 
 const N_PADDED:      u32 = 16384u;
@@ -2436,29 +2440,36 @@ fn lbvhBuildMain(@builtin(local_invocation_id) lid: vec3<u32>) {
     storageBarrier();
 
     // ── Phase 4: initialize BVH leaf AABBs ───────────────────────────────
-    for (var k: u32 = 0u; k < K_PER; k = k + 1u) {
+    // Only the first nPadded sorted slots become leaves of the (smaller)
+    // sweep tree; reals occupy [0,nReal), the rest sink as sentinels.
+    let nPadded = max(lbvhPc.nPadded, 1u);
+    let leafPerThread = (nPadded + THREADS - 1u) / THREADS;
+    for (var k: u32 = 0u; k < leafPerThread; k = k + 1u) {
         let i = k * THREADS + tid;
-        let leafIdx = N_PADDED - 1u + i;
-        let leafKey = sortA[i];
-        if (leafKey == 0xFFFFFFFFu) {
-            outBvh[leafIdx].aabbMin = vec3<f32>( 1e30);
-            outBvh[leafIdx].aabbMax = vec3<f32>(-1e30);
-        } else {
-            let e = entries[leafKey & 0xFFFFu];
-            outBvh[leafIdx].aabbMin = e.aabbMin;
-            outBvh[leafIdx].aabbMax = e.aabbMax;
+        if (i < nPadded) {
+            let leafIdx = nPadded - 1u + i;
+            let leafKey = sortA[i];
+            if (leafKey == 0xFFFFFFFFu) {
+                outBvh[leafIdx].aabbMin = vec3<f32>( 1e30);
+                outBvh[leafIdx].aabbMax = vec3<f32>(-1e30);
+            } else {
+                let e = entries[leafKey & 0xFFFFu];
+                outBvh[leafIdx].aabbMin = e.aabbMin;
+                outBvh[leafIdx].aabbMax = e.aabbMax;
+            }
         }
     }
     workgroupBarrier();
     storageBarrier();
 
-    // ── Phase 5: bottom-up sweep-tree refit, LEVELS iterations ──────────
-    // Deepest internal level has N_PADDED/2 nodes; perThread = ceil of
-    // levelCount / THREADS is uniform per step, so workgroupBarrier
-    // stays in uniform control flow.
-    var levelCount: u32 = N_PADDED / 2u;
-    var levelStart: u32 = N_PADDED / 2u - 1u;
-    for (var step: u32 = 0u; step < LEVELS; step = step + 1u) {
+    // ── Phase 5: bottom-up sweep-tree refit, log2(nPadded) levels ───────
+    // Deepest internal level has nPadded/2 nodes. The loop bound is uniform
+    // across the workgroup (depends only on nPadded), so the barriers stay
+    // in uniform control flow.
+    var levelCount: u32 = nPadded / 2u;
+    var levelStart: u32 = nPadded / 2u - 1u;
+    loop {
+        if (levelCount == 0u) { break; }
         let perThread = (levelCount + THREADS - 1u) / THREADS;
         for (var k: u32 = 0u; k < perThread; k = k + 1u) {
             let nodeOff = k * THREADS + tid;
@@ -2723,11 +2734,12 @@ env.wgpuBuildTLAS = (instanceBufHandle, instanceCount, tlasOutBufHandle,
             { binding: 4, resource: { buffer: morton } },
         ],
     });
-    // Write the real instance count to the LBVH count uniform so the
-    // shader can iterate exactly the right number of entries even
-    // though the storage buffers stay sized for N_PADDED.
+    // Write the real instance count + the dynamic padded leaf count
+    // (next_pow2) to the LBVH uniform. The sort still runs over the full
+    // N_PADDED, but the sweep tree is built at depth log2(nPadded).
     const countBuf = new Uint32Array(4);
     countBuf[0] = instanceCount;
+    countBuf[1] = wfNextPow2(instanceCount);
     queue.writeBuffer(rtState.lbvhCountBuf, 0, countBuf);
 
     const lbvhBg = device.createBindGroup({
@@ -2798,7 +2810,15 @@ const WF_PAYLOAD_BYTES = 64;
 // Dynamic-offset uniform ring: one WfParams slot per wavefront pass. 128
 // slots covers maxDepth up to ~42 (1 + 3·maxDepth + 1 passes).
 const WF_PARAM_SLOTS   = 128;
-const WF_FIXED_TLAS_NPADDED = 16384;   // matches lbvhBuildWgsl N_PADDED
+const WF_TLAS_MAX_NPADDED = 16384;   // LBVH sort capacity (N_PADDED)
+// Smallest power of two >= max(n,1), clamped to the LBVH capacity. The
+// TLAS sweep tree is built and traced at this depth so descent tracks the
+// real instance count instead of a fixed 16384-leaf (depth-14) tree.
+function wfNextPow2(n) {
+    let p = 1;
+    while (p < n && p < WF_TLAS_MAX_NPADDED) p <<= 1;
+    return p;
+}
 
 function ensureWavefrontBuffers(W, H) {
     const cap = W * H;
@@ -3041,11 +3061,14 @@ env.wgpuDispatchRT = (pipelineHandle, pushPtr, pushBytes,
     //   1+3*d .. +2  PREP / TRACE / SHADE for bounce d
     //   1+3*depth    RESOLVE
     const passCount = 2 + 3 * depth;
+    // TLAS descent depth = log2(tlasNPadded); must match the value the
+    // build used (both derive next_pow2 from the same instance count).
+    const tlasNPadded = wfNextPow2(instanceCount);
     const ring = new Uint32Array(WF_PARAM_SLOTS * 64);  // 256 B = 64 u32 per slot
     const writeSlot = (slot, curIsA, bounce) => {
         const o = slot * 64;
         ring[o + 0] = W; ring[o + 1] = H; ring[o + 2] = cap; ring[o + 3] = curIsA;
-        ring[o + 4] = bounce; ring[o + 5] = depth; ring[o + 6] = WF_FIXED_TLAS_NPADDED; ring[o + 7] = 0;
+        ring[o + 4] = bounce; ring[o + 5] = depth; ring[o + 6] = tlasNPadded; ring[o + 7] = 0;
     };
     writeSlot(0, 1, 0);                              // GENERATE
     for (let d = 0; d < depth; d++) {