Phase 4.2: GPU toping rendering pipeline + winding/lighting fixes

Add instanced rendering for toping bevels: dedicated shaders
(voxelTopingVS/PS), PSO, GPU buffers (t4 vertices, t5 instances),
per-group DrawInstanced in a separate render pass with LoadOp::LOAD.
Fix inverted face winding (emitTri auto-winding condition flipped for
CW front-facing), slope normals (use inward direction not outward),
and PS lighting (negate sunDirection like voxelPS). Update CLAUDE.md
with Phase 4.1/4.2 documentation.

CLAUDE.md

@@ -18,7 +18,8 @@ bvle-voxels/
 │   │   ├── VoxelTypes.h        # Types fondamentaux (VoxelData, PackedQuad, MaterialDesc, ChunkPos)
 │   │   ├── VoxelWorld.h/.cpp   # Monde voxel (hashmap de chunks, génération procédurale)
 │   │   ├── VoxelMesher.h/.cpp  # Binary Greedy Mesher CPU
-│   │   └── VoxelRenderer.h/.cpp# Renderer + VoxelRenderPath (sous-classe RenderPath3D)
+│   │   ├── VoxelRenderer.h/.cpp# Renderer + VoxelRenderPath (sous-classe RenderPath3D)
+│   │   └── TopingSystem.h/.cpp # Système de topings (biseaux décoratifs sur faces +Y)
 │   └── app/
 │       └── main.cpp            # Point d'entrée Win32 + crash handler SEH
 ├── shaders/                    # Sources HLSL des shaders voxel (copiés dans engine/ au build)
@@ -26,7 +27,9 @@ bvle-voxels/
 │   ├── voxelVS.hlsl            # Vertex shader (vertex pulling, triple-mode: CPU/MDI/GPU mesh)
 │   ├── voxelPS.hlsl            # Pixel shader (triplanar + lighting)
 │   ├── voxelCullCS.hlsl        # Compute shader frustum+backface cull (Phase 2.3)
-│   └── voxelMeshCS.hlsl        # Compute shader GPU mesher 1×1 (Phase 2.4-2.5)
+│   ├── voxelMeshCS.hlsl        # Compute shader GPU mesher 1×1 (Phase 2.4-2.5)
+│   ├── voxelTopingVS.hlsl      # Vertex shader topings (instanced vertex pulling, t4/t5)
+│   └── voxelTopingPS.hlsl      # Pixel shader topings (triplanar + directional lighting)
 └── CLAUDE.md
 ```
 
@@ -391,12 +394,40 @@ Approche **PS-based** : le pixel shader lit directement les données voxel (pas
 - **Mode debug** (F4) : visualise les zones de blend (rouge=U, bleu=V, vert=pas de blend, rouge vif=data mismatch)
 - **Fonctionne uniquement en GPU mesh path** (1×1 quads) ; CPU/MDI paths ont `blendEnabled=0`
 
-### Phase 4 - Toping [A FAIRE]
+### Phase 4 - Toping [EN COURS]
 
-- TopingSystem avec bitmask d'adjacence 4 bits (16 variantes)
+Système de biseaux décoratifs (« topings ») sur les faces +Y exposées pour adoucir les transitions entre blocs.
-- Instance buffer GPU par chunk
+
-- Instanced draw dans le G-buffer
+#### Phase 4.1 - Infrastructure TopingSystem [FAIT]
-- 2-3 types de test (rebord de pierre, bordure d'herbe)
+
+- **TopingSystem** (`TopingSystem.h/.cpp`) : data structures + mesh generation + instance collection
+- **4-bit adjacency bitmask** : pour chaque face +Y exposée, vérifie 4 voisins cardinaux (±X, ±Z) pour même matériau avec +Y exposée → 16 variantes
+- **Wedge cross-section** : chaque bord ouvert génère un mur vertical (outer wall) + une pente (slope) depuis le peak (y=1+h au bord) vers l'intérieur (y=1, w unités vers le centre)
+- **Priority-based adjacency** : `TopingDef.priority` détermine quel toping cède aux frontières de matériaux. Grass (priority=1) génère des biseaux par-dessus stone (priority=0)
+- **2 types de test** :
+  - Stone : priority=0, h=0.06, w=0.12, 1 segment (biseau simple)
+  - Grass : priority=1, h=0.12, w=0.18, 4 segments (bosses sinusoïdales)
+- **16 variantes × 2 types** : 1920 vertices de mesh procédural, ~191K instances pour ~170 chunks
+
+#### Phase 4.2 - Rendu GPU [FAIT]
+
+- **Shaders dédiés** : `voxelTopingVS.hlsl` (vertex pulling instancié) + `voxelTopingPS.hlsl` (triplanar + lighting directionnel)
+- **Vertex pulling** : `StructuredBuffer<TopingVertex>` (t4) + `StructuredBuffer<float3>` (t5 instances)
+- **Push constants** : `vertexOffset`, `instanceOffset`, `materialID` réutilisent les 3 premiers champs de b999
+- **Per-group DrawInstanced** : instances triées par (type, variant), un `DrawInstanced` par groupe contigu
+- **Render pass séparé** avec `LoadOp::LOAD` : les topings se rendent après les voxels, préservent le RT et depth existants
+- **PSO** : même rasterizer/depth/blend que les voxels (`RSTYPE_FRONT`, `DSSTYPE_DEFAULT`, `BSTYPE_OPAQUE`)
+- **Pièges résolus** :
+  - **Winding CW** : `emitTri()` auto-corrige le winding en comparant la normale géométrique (cross product) à la normale souhaitée. Si `dot(geom, desired) > 0` → swap B↔C pour CW (front-facing dans Wicked Engine)
+  - **Slope normal = inward + up** : la pente monte vers le bord extérieur, donc sa normale pointe vers l'intérieur et vers le haut. Utiliser `(e.ix, e.iz)` (direction inward), PAS `(e.nx, e.nz)` (direction outward)
+  - **sunDirection dans le PS** : `sunDirection` pointe vers le bas (direction de voyage de la lumière). Le PS doit faire `L = normalize(-sunDirection.xyz)` avant `dot(N, L)`, comme le voxel PS
+
+#### Phase 4.3 - Polish et extensions [A FAIRE]
+
+- Plus de types de topings (végétation, neige, etc.)
+- LOD : supprimer les topings à distance
+- Animation subtile (vent sur l'herbe)
+- Optimisation : compute shader pour le instance collection
 
 ### Phase 5 - Rendu smooth [A FAIRE]
 

shaders/voxelTopingPS.hlsl

@@ -0,0 +1,42 @@
+// BVLE Voxels - Toping Pixel Shader (Phase 4.2)
+// Simplified version of voxelPS: triplanar texture sampling + basic lighting.
+// No height-based blending (topings are small decorative meshes).
+
+#include "voxelCommon.hlsli"
+
+Texture2DArray<float4> materialTextures : register(t1);
+SamplerState texSampler : register(s0);
+
+struct PSInput {
+    float4 position : SV_POSITION;
+    float3 worldPos : WORLDPOS;
+    float3 normal   : NORMAL;
+    nointerpolation uint materialID : MATERIALID;
+};
+
+[RootSignature(VOXEL_ROOTSIG)]
+float4 main(PSInput input) : SV_TARGET0 {
+    float3 N = normalize(input.normal);
+    float tiling = textureTiling;
+
+    // Material texture index (materialID 1-5 → array layer 0-4)
+    uint texIdx = clamp(input.materialID - 1u, 0u, 4u);
+
+    // Triplanar sampling (same as voxel PS)
+    float3 blend = abs(N);
+    blend = blend / (blend.x + blend.y + blend.z + 0.001);
+
+    float4 xSample = materialTextures.Sample(texSampler, float3(input.worldPos.yz * tiling, (float)texIdx));
+    float4 ySample = materialTextures.Sample(texSampler, float3(input.worldPos.xz * tiling, (float)texIdx));
+    float4 zSample = materialTextures.Sample(texSampler, float3(input.worldPos.xy * tiling, (float)texIdx));
+
+    float3 texColor = (xSample.rgb * blend.x + ySample.rgb * blend.y + zSample.rgb * blend.z);
+
+    // Basic directional lighting + ambient
+    float3 L = normalize(-sunDirection.xyz);
+    float NdotL = max(dot(N, L), 0.0);
+    float3 ambient = float3(0.15, 0.18, 0.25);
+    float3 lit = texColor * (sunColor.rgb * NdotL + ambient);
+
+    return float4(lit, 1.0);
+}

shaders/voxelTopingVS.hlsl

@@ -0,0 +1,53 @@
+// BVLE Voxels - Toping Vertex Shader (Phase 4.2)
+// Instanced vertex pulling: reads mesh vertices from t4, instance positions from t5.
+// Push constants carry per-draw-group offsets.
+
+#include "voxelCommon.hlsli"
+
+// Toping mesh vertex (must match C++ TopingVertex, 24 bytes)
+struct TopingVtx {
+    float3 position; // local to voxel [0,1]^3
+    float3 normal;
+};
+
+// Toping instance (just the world position, 12 bytes)
+struct TopingInst {
+    float3 worldPos;
+};
+
+StructuredBuffer<TopingVtx>  topingVertices  : register(t4);
+StructuredBuffer<TopingInst> topingInstances : register(t5);
+
+// Push constants — repurposed fields for toping draws:
+//   chunkIndex → vertexOffset (into t4)
+//   quadOffset → instanceOffset (into t5)
+//   flags      → materialID
+struct TopingPush {
+    uint vertexOffset;
+    uint instanceOffset;
+    uint materialID;
+    uint pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7, pad8;
+};
+[[vk::push_constant]] ConstantBuffer<TopingPush> push : register(b999);
+
+struct VSOutput {
+    float4 position : SV_POSITION;
+    float3 worldPos : WORLDPOS;
+    float3 normal   : NORMAL;
+    nointerpolation uint materialID : MATERIALID;
+};
+
+[RootSignature(VOXEL_ROOTSIG)]
+VSOutput main(uint vertexID : SV_VertexID, uint instanceID : SV_InstanceID) {
+    TopingVtx vtx   = topingVertices[push.vertexOffset + vertexID];
+    TopingInst inst  = topingInstances[push.instanceOffset + instanceID];
+
+    float3 worldPos = inst.worldPos + vtx.position;
+
+    VSOutput output;
+    output.position  = mul(viewProjection, float4(worldPos, 1.0));
+    output.worldPos  = worldPos;
+    output.normal    = vtx.normal;
+    output.materialID = push.materialID;
+    return output;
+}

src/voxel/TopingSystem.cpp

@@ -38,16 +38,33 @@ static const EdgeDef kEdges[4] = {
     { 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 1.0f, 0.0f,-1.0f }, // bit 3: -Z edge
 };
 
-// ── Helper: emit one triangle (3 vertices, shared normal) ───────
+// ── Helper: emit one triangle with auto-corrected winding ───────
+// Compares the geometric normal (from vertex cross product) to the
+// desired shading normal. If they disagree, swaps B and C to flip
+// the winding so the triangle is front-facing (CCW in Wicked Engine).
 static void emitTri(std::vector<TopingVertex>& v,
     float ax, float ay, float az,
     float bx, float by, float bz,
     float cx, float cy, float cz,
     float nx, float ny, float nz)
 {
+    // Geometric normal = cross(AB, AC)
+    float abx = bx - ax, aby = by - ay, abz = bz - az;
+    float acx = cx - ax, acy = cy - ay, acz = cz - az;
+    float gnx = aby * acz - abz * acy;
+    float gny = abz * acx - abx * acz;
+    float gnz = abx * acy - aby * acx;
+
+    // If geometric normal disagrees with desired normal, swap B↔C
+    if (gnx * nx + gny * ny + gnz * nz > 0.0f) {
+        v.push_back({ ax, ay, az, nx, ny, nz });
+        v.push_back({ cx, cy, cz, nx, ny, nz });
+        v.push_back({ bx, by, bz, nx, ny, nz });
+    } else {
         v.push_back({ ax, ay, az, nx, ny, nz });
         v.push_back({ bx, by, bz, nx, ny, nz });
         v.push_back({ cx, cy, cz, nx, ny, nz });
+    }
 }
 
 // ═════════════════════════════════════════════════════════════════
@@ -65,10 +82,12 @@ void TopingSystem::registerDefs() {
 
     // Type 0: Stone bevel — clean angular ridge along open edges
     // Applied to stone (materialID=3), face +Y
+    // Priority 0 (low): yields to grass at material boundaries
     {
         TopingDef def{};
         def.materialID = 3;
         def.face       = FACE_POS_Y;
+        def.priority   = 0;
         def.height     = 0.06f;  // subtle bevel
         def.width      = 0.12f;
         def.segments   = 1;      // single smooth segment per edge
@@ -77,10 +96,12 @@ void TopingSystem::registerDefs() {
 
     // Type 1: Grass edge — organic bumpy tufts along open edges
     // Applied to grass (materialID=1), face +Y
+    // Priority 1 (high): generates bevels over stone at boundaries
     {
         TopingDef def{};
         def.materialID = 1;
         def.face       = FACE_POS_Y;
+        def.priority   = 1;
         def.height     = 0.12f;  // taller, more visible
         def.width      = 0.18f;
         def.segments   = 4;      // subdivided for bumpy profile
@@ -159,45 +180,33 @@ void TopingSystem::generateVariant(TopingDef& def, uint8_t bitmask) {
 
             // ── Outer wall face (vertical, facing outward) ──────
             // Normal points outward: (nx, 0, nz)
-            // CW winding from outside: peak0→outerBot0→peak1, peak1→outerBot0→outerBot1
+            // Winding: emit both orderings — CW from outside view
+            // Empirically CW = front-facing in our engine (see CLAUDE.md)
             emitTri(vertices_,
-                x0, pk0y, z0,   x0, 1.0f, z0,   x1, pk1y, z1,
+                x0, pk0y, z0,   x1, pk1y, z1,   x0, 1.0f, z0,
                 e.nx, 0.0f, e.nz);
             emitTri(vertices_,
-                x1, pk1y, z1,   x0, 1.0f, z0,   x1, 1.0f, z1,
+                x1, pk1y, z1,   x1, 1.0f, z1,   x0, 1.0f, z0,
                 e.nx, 0.0f, e.nz);
 
             // ── Slope face (from peak down to inner edge) ───────
-            // Compute normal via cross product of two edge vectors.
+            // Slope normal: the slope rises from inner (y=1) to peak (y=1+h) at the edge.
-            // v_along_slope = inner - peak = (w*ix, -h, w*iz) at midpoint
+            // Since it tilts outward, the normal points INWARD and UP.
-            // v_along_strip = strip direction = (dx/segs, 0, dz/segs)
+            // Normal = inward_dir * (h/L) + up * (w/L), where L = sqrt(h²+w²).
             const float avgH = (h0 + h1) * 0.5f;
-            const float asx  = w * e.ix;
+            float slopeLen = sqrtf(avgH * avgH + w * w);
-            const float asy  = -avgH;
+            if (slopeLen < 0.0001f) slopeLen = 1.0f;
-            const float asz  = w * e.iz;
+            float cnx = e.ix * (avgH / slopeLen);
-            const float adx  = dx / segs;
+            float cny = w / slopeLen;
-            const float adz  = dz / segs;
+            float cnz = e.iz * (avgH / slopeLen);
 
-            // normal = cross(v_along_strip, v_along_slope)
+            // Slope: peak → inner, strip direction
-            float cnx = /* 0*asz - adz*asy = */  adz * avgH;
+            // CW winding from normal direction
-            float cny = /* adz*asx - adx*asz = */ adz * asx - adx * asz;
-            float cnz = /* adx*asy - 0*asx = */ -adx * avgH;
-
-            float clen = sqrtf(cnx * cnx + cny * cny + cnz * cnz);
-            if (clen > 0.0001f) {
-                cnx /= clen; cny /= clen; cnz /= clen;
-            } else {
-                cnx = 0.0f; cny = 1.0f; cnz = 0.0f;
-            }
-            // Ensure normal points upward (slope is visible from above)
-            if (cny < 0.0f) { cnx = -cnx; cny = -cny; cnz = -cnz; }
-
-            // CW winding from above: peak0→peak1→inner0, inner0→peak1→inner1
             emitTri(vertices_,
-                x0, pk0y, z0,   x1, pk1y, z1,   in0x, 1.0f, in0z,
+                x0, pk0y, z0,   in0x, 1.0f, in0z,   x1, pk1y, z1,
                 cnx, cny, cnz);
             emitTri(vertices_,
-                in0x, 1.0f, in0z,   x1, pk1y, z1,   in1x, 1.0f, in1z,
+                x1, pk1y, z1,   in0x, 1.0f, in0z,   in1x, 1.0f, in1z,
                 cnx, cny, cnz);
         }
     }
@@ -246,28 +255,29 @@ void TopingSystem::collectInstances(const VoxelWorld& world) {
                         if (!world.getVoxel(wx, wy + 1, wz).isEmpty()) continue;
 
                         // Face exposed. Compute 4-bit adjacency bitmask.
-                        // A neighbor contributes if: same material AND its +Y face is also exposed.
+                        // A neighbor "connects" (bit SET → no bevel on that edge) if:
+                        //   - Same material AND its +Y face is also exposed (same toping type)
+                        //   - OR different material with a toping of HIGHER or EQUAL priority
+                        //     AND its +Y face is exposed (the dominant toping wins at the boundary)
                         uint8_t adj = 0;
+                        const uint8_t myPriority = def.priority;
 
-                        // bit 0: +X
+                        auto checkNeighbor = [&](int nx, int nz) -> bool {
-                        if (world.getVoxel(wx + 1, wy, wz).getMaterialID() == mat &&
+                            uint8_t nMat = world.getVoxel(nx, wy, nz).getMaterialID();
-                            world.getVoxel(wx + 1, wy + 1, wz).isEmpty())
+                            if (nMat == 0) return false; // empty
-                            adj |= 1;
+                            if (!world.getVoxel(nx, wy + 1, nz).isEmpty()) return false; // +Y not exposed
+                            // Same material → connect (no bevel)
+                            if (nMat == mat) return true;
+                            // Different material with toping: check priority
+                            int8_t nDefIdx = matToDef[nMat];
+                            if (nDefIdx >= 0 && defs_[nDefIdx].priority >= myPriority) return true;
+                            return false;
+                        };
 
-                        // bit 1: -X
+                        if (checkNeighbor(wx + 1, wz)) adj |= 1; // +X
-                        if (world.getVoxel(wx - 1, wy, wz).getMaterialID() == mat &&
+                        if (checkNeighbor(wx - 1, wz)) adj |= 2; // -X
-                            world.getVoxel(wx - 1, wy + 1, wz).isEmpty())
+                        if (checkNeighbor(wx, wz + 1)) adj |= 4; // +Z
-                            adj |= 2;
+                        if (checkNeighbor(wx, wz - 1)) adj |= 8; // -Z
-
-                        // bit 2: +Z
-                        if (world.getVoxel(wx, wy, wz + 1).getMaterialID() == mat &&
-                            world.getVoxel(wx, wy + 1, wz + 1).isEmpty())
-                            adj |= 4;
-
-                        // bit 3: -Z
-                        if (world.getVoxel(wx, wy, wz - 1).getMaterialID() == mat &&
-                            world.getVoxel(wx, wy + 1, wz - 1).isEmpty())
-                            adj |= 8;
 
                         instances_.push_back({
                             (float)wx, (float)wy, (float)wz,

src/voxel/TopingSystem.h

@@ -34,6 +34,7 @@ struct MeshSlice {
 struct TopingDef {
     uint8_t  materialID;       // voxel material that triggers this toping
     uint8_t  face;             // Face enum (FACE_POS_Y, etc.)
+    uint8_t  priority;         // higher priority topings generate bevels over lower ones at boundaries
     float    height;           // bevel peak height (voxel units)
     float    width;            // bevel inward extent (voxel units)
     int      segments;         // subdivisions per edge strip (1=smooth, 3+=bumpy)

src/voxel/VoxelRenderer.cpp

@@ -177,6 +177,24 @@ void VoxelRenderer::createPipeline() {
     psoDesc.pt = PrimitiveTopology::TRIANGLELIST;
 
     device_->CreatePipelineState(&psoDesc, &pso_);
+
+    // ── Toping pipeline (Phase 4) ────────────────────────────────
+    wi::renderer::LoadShader(ShaderStage::VS, topingVS_, "voxel/voxelTopingVS.cso");
+    wi::renderer::LoadShader(ShaderStage::PS, topingPS_, "voxel/voxelTopingPS.cso");
+
+    if (topingVS_.IsValid() && topingPS_.IsValid()) {
+        PipelineStateDesc topingPsoDesc;
+        topingPsoDesc.vs = &topingVS_;
+        topingPsoDesc.ps = &topingPS_;
+        topingPsoDesc.rs = wi::renderer::GetRasterizerState(wi::enums::RSTYPE_FRONT);
+        topingPsoDesc.dss = wi::renderer::GetDepthStencilState(wi::enums::DSSTYPE_DEFAULT);
+        topingPsoDesc.bs = wi::renderer::GetBlendState(wi::enums::BSTYPE_OPAQUE);
+        topingPsoDesc.pt = PrimitiveTopology::TRIANGLELIST;
+        device_->CreatePipelineState(&topingPsoDesc, &topingPso_);
+        wi::backlog::post("VoxelRenderer: toping pipeline created");
+    } else {
+        wi::backlog::post("VoxelRenderer: toping shader loading failed", wi::backlog::LogLevel::Warning);
+    }
 }
 
 // ── Procedural texture generation ───────────────────────────────
@@ -1130,6 +1148,178 @@ void VoxelRenderer::render(
     dev->RenderPassEnd(cmd);
 }
 
+// ── Phase 4: Toping GPU upload + rendering ─────────────────────
+
+void VoxelRenderer::uploadTopingData(const TopingSystem& topingSystem) {
+    if (!device_ || !topingPso_.IsValid()) return;
+
+    // Upload mesh vertices (done once, meshes are static)
+    const auto& verts = topingSystem.getVertices();
+    if (!verts.empty() && !topingVertexBuffer_.IsValid()) {
+        GPUBufferDesc vbDesc;
+        vbDesc.size = verts.size() * sizeof(TopingVertex);
+        vbDesc.bind_flags = BindFlag::SHADER_RESOURCE;
+        vbDesc.misc_flags = ResourceMiscFlag::BUFFER_STRUCTURED;
+        vbDesc.stride = sizeof(TopingVertex);
+        vbDesc.usage = Usage::DEFAULT;
+        device_->CreateBuffer(&vbDesc, verts.data(), &topingVertexBuffer_);
+
+        char msg[128];
+        snprintf(msg, sizeof(msg), "Toping: uploaded %zu vertices (%zu bytes)",
+            verts.size(), verts.size() * sizeof(TopingVertex));
+        wi::backlog::post(msg);
+    }
+
+    // Upload instance positions (re-upload when world changes)
+    const auto& instances = topingSystem.getInstances();
+    if (instances.empty()) return;
+
+    // GPU instances are just float3 (12 bytes), sorted by (type, variant) for batched draws.
+    // We sort a copy and build a draw group table.
+    struct SortedInst {
+        float wx, wy, wz;
+        uint16_t type, variant;
+    };
+    std::vector<SortedInst> sorted(instances.size());
+    for (size_t i = 0; i < instances.size(); i++) {
+        sorted[i] = { instances[i].wx, instances[i].wy, instances[i].wz,
+                       instances[i].topingType, instances[i].variant };
+    }
+    std::sort(sorted.begin(), sorted.end(), [](const SortedInst& a, const SortedInst& b) {
+        if (a.type != b.type) return a.type < b.type;
+        return a.variant < b.variant;
+    });
+
+    // Pack GPU instance data (just float3 positions)
+    struct GPUTopingInst { float x, y, z; };
+    uint32_t instCount = (uint32_t)std::min(sorted.size(), (size_t)MAX_TOPING_INSTANCES);
+    std::vector<GPUTopingInst> gpuInsts(instCount);
+    for (uint32_t i = 0; i < instCount; i++) {
+        gpuInsts[i] = { sorted[i].wx, sorted[i].wy, sorted[i].wz };
+    }
+
+    // Create or recreate instance buffer
+    GPUBufferDesc ibDesc;
+    ibDesc.size = instCount * sizeof(GPUTopingInst);
+    ibDesc.bind_flags = BindFlag::SHADER_RESOURCE;
+    ibDesc.misc_flags = ResourceMiscFlag::BUFFER_STRUCTURED;
+    ibDesc.stride = sizeof(GPUTopingInst);
+    ibDesc.usage = Usage::DEFAULT;
+    device_->CreateBuffer(&ibDesc, gpuInsts.data(), &topingInstanceBuffer_);
+}
+
+void VoxelRenderer::renderTopings(
+    CommandList cmd,
+    const TopingSystem& topingSystem,
+    const Texture& depthBuffer,
+    const Texture& renderTarget
+) const {
+    if (!topingPso_.IsValid() || !topingVertexBuffer_.IsValid() ||
+        !topingInstanceBuffer_.IsValid()) return;
+
+    const auto& instances = topingSystem.getInstances();
+    const auto& defs = topingSystem.getDefs();
+    if (instances.empty()) return;
+
+    auto* dev = device_;
+
+    // Open render pass with LOAD (preserve voxel render output)
+    RenderPassImage rp[] = {
+        RenderPassImage::RenderTarget(
+            &renderTarget,
+            RenderPassImage::LoadOp::LOAD,
+            RenderPassImage::StoreOp::STORE,
+            ResourceState::SHADER_RESOURCE,
+            ResourceState::SHADER_RESOURCE
+        ),
+        RenderPassImage::DepthStencil(
+            &depthBuffer,
+            RenderPassImage::LoadOp::LOAD,
+            RenderPassImage::StoreOp::STORE,
+            ResourceState::DEPTHSTENCIL,
+            ResourceState::DEPTHSTENCIL,
+            ResourceState::DEPTHSTENCIL
+        ),
+    };
+    dev->RenderPassBegin(rp, 2, cmd);
+
+    // Viewport & scissor
+    Viewport vp;
+    vp.top_left_x = 0; vp.top_left_y = 0;
+    vp.width  = (float)renderTarget.GetDesc().width;
+    vp.height = (float)renderTarget.GetDesc().height;
+    vp.min_depth = 0.0f; vp.max_depth = 1.0f;
+    Rect scissor = { 0, 0, (int)renderTarget.GetDesc().width, (int)renderTarget.GetDesc().height };
+    dev->BindViewports(1, &vp, cmd);
+    dev->BindScissorRects(1, &scissor, cmd);
+
+    // Bind toping pipeline (MUST be before PushConstants!)
+    dev->BindPipelineState(&topingPso_, cmd);
+    dev->BindConstantBuffer(&constantBuffer_, 0, cmd);
+    dev->BindResource(&textureArray_, 1, cmd);
+    dev->BindResource(&topingVertexBuffer_, 4, cmd);   // t4
+    dev->BindResource(&topingInstanceBuffer_, 5, cmd); // t5
+    dev->BindSampler(&sampler_, 0, cmd);
+
+    // Build sorted draw groups (same sort order as uploadTopingData)
+    struct DrawGroup {
+        uint16_t type, variant;
+        uint32_t instanceOffset, instanceCount;
+    };
+    struct SortKey { uint16_t type, variant; };
+    std::vector<SortKey> sortedKeys(instances.size());
+    for (size_t i = 0; i < instances.size(); i++) {
+        sortedKeys[i] = { instances[i].topingType, instances[i].variant };
+    }
+    std::sort(sortedKeys.begin(), sortedKeys.end(), [](const SortKey& a, const SortKey& b) {
+        if (a.type != b.type) return a.type < b.type;
+        return a.variant < b.variant;
+    });
+
+    // Identify contiguous groups
+    std::vector<DrawGroup> groups;
+    uint32_t instCount = (uint32_t)std::min(sortedKeys.size(), (size_t)MAX_TOPING_INSTANCES);
+    if (instCount > 0) {
+        DrawGroup g = { sortedKeys[0].type, sortedKeys[0].variant, 0, 1 };
+        for (uint32_t i = 1; i < instCount; i++) {
+            if (sortedKeys[i].type == g.type && sortedKeys[i].variant == g.variant) {
+                g.instanceCount++;
+            } else {
+                groups.push_back(g);
+                g = { sortedKeys[i].type, sortedKeys[i].variant, i, 1 };
+            }
+        }
+        groups.push_back(g);
+    }
+
+    // Issue one DrawInstanced per group
+    topingDrawCalls_ = 0;
+    struct TopingPush {
+        uint32_t vertexOffset;
+        uint32_t instanceOffset;
+        uint32_t materialID;
+        uint32_t pad[9];
+    };
+
+    for (const auto& g : groups) {
+        if (g.type >= defs.size()) continue;
+        const TopingDef& def = defs[g.type];
+        const MeshSlice& slice = def.variants[g.variant];
+        if (slice.count == 0) continue; // empty mesh (all neighbors present)
+
+        TopingPush pushData = {};
+        pushData.vertexOffset = slice.offset;
+        pushData.instanceOffset = g.instanceOffset;
+        pushData.materialID = def.materialID;
+        dev->PushConstants(&pushData, sizeof(pushData), cmd);
+
+        dev->DrawInstanced(slice.count, g.instanceCount, 0, 0, cmd);
+        topingDrawCalls_++;
+    }
+
+    dev->RenderPassEnd(cmd);
+}
+
 // ── VoxelRenderPath (custom RenderPath3D) ───────────────────────
 
 void VoxelRenderPath::Start() {
@@ -1152,9 +1342,12 @@ void VoxelRenderPath::Start() {
         renderer.updateMeshes(world);
     }
 
-    // Phase 4: Initialize toping system and collect instances
+    // Phase 4: Initialize toping system, collect instances, upload to GPU
     topingSystem.initialize();
     topingSystem.collectInstances(world);
+    if (renderer.isInitialized()) {
+        renderer.uploadTopingData(topingSystem);
+    }
     {
         char msg[256];
         snprintf(msg, sizeof(msg),
@@ -1351,6 +1544,9 @@ void VoxelRenderPath::Render() const {
 
         auto tRender0 = std::chrono::high_resolution_clock::now();
         renderer.render(cmd, *camera, voxelDepth_, voxelRT_);
+
+        // Phase 4: render topings (separate render pass, preserves voxel output)
+        renderer.renderTopings(cmd, topingSystem, voxelDepth_, voxelRT_);
         auto tRender1 = std::chrono::high_resolution_clock::now();
         profRender_.add(std::chrono::duration<float, std::milli>(tRender1 - tRender0).count());
     }
@@ -1440,8 +1636,8 @@ void VoxelRenderPath::Compose(CommandList cmd) const {
         stats += "GPU Cull: " + std::string(cullStr) + " ms | Draw: " + std::string(drawStr) + " ms\n";
     }
     stats += "Topings: " + std::to_string(topingSystem.getInstanceCount())
-           + " instances (" + std::to_string(topingSystem.getDefCount()) + " types, "
+           + " instances, " + std::to_string(renderer.getTopingDrawCalls())
-           + std::to_string(topingSystem.getVertexCount()) + " verts)\n";
+           + " draws (" + std::to_string(topingSystem.getDefCount()) + " types)\n";
     stats += "WASD+Space/Ctrl: move | Shift: fast | Right-click: capture mouse\n";
     stats += "F2: console | F3: anim [" + std::string(animatedTerrain_ ? "ON" : "OFF")
         + "] | F4: dbg [" + std::string(renderer.debugBlend_ ? "ON" : "OFF") + "]";

src/voxel/VoxelRenderer.h

@@ -67,12 +67,21 @@ private:
 
     wi::graphics::GraphicsDevice* device_ = nullptr;
 
-    // Shaders & Pipeline
+    // Shaders & Pipeline (voxels)
     wi::graphics::Shader vertexShader_;
     wi::graphics::Shader pixelShader_;
     wi::graphics::PipelineState pso_;
     wi::graphics::Shader cullShader_; // Frustum cull compute shader
 
+    // Shaders & Pipeline (topings, Phase 4)
+    wi::graphics::Shader topingVS_;
+    wi::graphics::Shader topingPS_;
+    wi::graphics::PipelineState topingPso_;
+    wi::graphics::GPUBuffer topingVertexBuffer_;   // StructuredBuffer<TopingVertex>, SRV t4
+    wi::graphics::GPUBuffer topingInstanceBuffer_; // StructuredBuffer<float3>, SRV t5
+    static constexpr uint32_t MAX_TOPING_INSTANCES = 256 * 1024; // 256K instances max
+    mutable uint32_t topingDrawCalls_ = 0;
+
     // Texture array for materials (256x256, 5 layers for prototype)
     wi::graphics::Texture textureArray_;
     wi::graphics::Sampler sampler_;
@@ -186,6 +195,16 @@ public:
     float getGpuDrawTimeMs() const { return gpuDrawTimeMs_; }
     bool isGpuMeshEnabled() const { return gpuMeshEnabled_ && gpuMesherAvailable_; }
     uint32_t getGpuMeshQuadCount() const { return gpuMeshQuadCount_; }
+
+    // Phase 4: Toping rendering
+    void uploadTopingData(const TopingSystem& topingSystem);
+    void renderTopings(
+        wi::graphics::CommandList cmd,
+        const TopingSystem& topingSystem,
+        const wi::graphics::Texture& depthBuffer,
+        const wi::graphics::Texture& renderTarget
+    ) const;
+    uint32_t getTopingDrawCalls() const { return topingDrawCalls_; }
 };
 
 // ── Custom RenderPath that integrates voxel rendering ───────────