update

implementations/Crafter.Graphics-Device.cpp

@@ -63,6 +63,8 @@ const char* const deviceExtensionNames[] = {
     "VK_KHR_shader_float_controls",
     "VK_KHR_acceleration_structure",
     "VK_KHR_ray_tracing_pipeline",
+    "VK_KHR_ray_query",
+    "VK_EXT_shader_atomic_float",
     "VK_EXT_descriptor_heap",
     "VK_KHR_deferred_host_operations",
     "VK_KHR_maintenance5",
@@ -560,14 +562,30 @@ void Device::Initialize() {
 	app.pEngineName      = "Crafter.Graphics";
 	app.apiVersion       = VK_MAKE_VERSION(1, 4, 0);
 
-    VkValidationFeatureEnableEXT enables[] = {
+    // TODO(re-enable GPU-AV): once Vulkan SDK > 1.4.341 is the floor.
-        VK_VALIDATION_FEATURE_ENABLE_GPU_ASSISTED_EXT
+    //
-    };
+    // GPU-Assisted Validation is opt-in via the enable list — leaving it
-
+    // out disables it. SDK 1.4.341's GPU-AV does not handle
+    // descriptor_heap pipelines (VK_PIPELINE_CREATE_2_DESCRIPTOR_HEAP_BIT_EXT
+    // with layout = VK_NULL_HANDLE): `PipelineSubState::GetPipelineLayoutUnion`
+    // null-derefs on the first dispatch/draw against such a pipeline.
+    //
+    // Tracked + fixed upstream:
+    //   https://github.com/KhronosGroup/Vulkan-ValidationLayers/issues/12103
+    // Per spencer-lunarg (LunarG): broken in 1.4.341, fixed and landing
+    // in the next SDK release. Once we bump our Vulkan-Headers / SDK
+    // dependency past 1.4.341, restore the original enable list:
+    //
+    //   VkValidationFeatureEnableEXT enables[] = {
+    //       VK_VALIDATION_FEATURE_ENABLE_GPU_ASSISTED_EXT
+    //   };
+    //   validationFeatures.enabledValidationFeatureCount = 1;
+    //   validationFeatures.pEnabledValidationFeatures    = enables;
+    //
+    // Standard validation (the layer itself) is still on; only the GPU-AV
+    // out-of-bounds / shader-instrumentation checks are temporarily off.
     VkValidationFeaturesEXT validationFeatures = {
         .sType = VK_STRUCTURE_TYPE_VALIDATION_FEATURES_EXT,
-        .enabledValidationFeatureCount = 1,
-        .pEnabledValidationFeatures = enables
     };
 
     VkInstanceCreateInfo instanceCreateInfo = {};
@@ -733,9 +751,15 @@ void Device::Initialize() {
         .bufferDeviceAddress    = VK_TRUE
     };
 
+    VkPhysicalDeviceRayQueryFeaturesKHR physicalDeviceRayQueryFeatures{
+        .sType = VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_RAY_QUERY_FEATURES_KHR,
+        .pNext = &features12,
+        .rayQuery = VK_TRUE
+    };
+
     VkPhysicalDeviceRayTracingPipelineFeaturesKHR physicalDeviceRayTracingPipelineFeatures{
         .sType = VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_RAY_TRACING_PIPELINE_FEATURES_KHR,
-        .pNext = &features12,
+        .pNext = &physicalDeviceRayQueryFeatures,
         .rayTracingPipeline = VK_TRUE
     };
 
@@ -809,6 +833,7 @@ void Device::Initialize() {
 
     vkGetAccelerationStructureBuildSizesKHR = reinterpret_cast<PFN_vkGetAccelerationStructureBuildSizesKHR>(vkGetInstanceProcAddr(instance, "vkGetAccelerationStructureBuildSizesKHR"));
     vkCreateAccelerationStructureKHR = reinterpret_cast<PFN_vkCreateAccelerationStructureKHR>(vkGetInstanceProcAddr(instance, "vkCreateAccelerationStructureKHR"));
+    vkDestroyAccelerationStructureKHR = reinterpret_cast<PFN_vkDestroyAccelerationStructureKHR>(vkGetInstanceProcAddr(instance, "vkDestroyAccelerationStructureKHR"));
     vkCmdBuildAccelerationStructuresKHR =  reinterpret_cast<PFN_vkCmdBuildAccelerationStructuresKHR>(vkGetInstanceProcAddr(instance, "vkCmdBuildAccelerationStructuresKHR"));
     vkGetAccelerationStructureDeviceAddressKHR =  reinterpret_cast<PFN_vkGetAccelerationStructureDeviceAddressKHR>(vkGetInstanceProcAddr(instance, "vkGetAccelerationStructureDeviceAddressKHR"));
     vkCreateRayTracingPipelinesKHR =  reinterpret_cast<PFN_vkCreateRayTracingPipelinesKHR>(vkGetInstanceProcAddr(instance, "vkCreateRayTracingPipelinesKHR"));

implementations/Crafter.Graphics-RenderingElement3D.cpp

@@ -28,7 +28,38 @@ using namespace Crafter;
 
 std::vector<RenderingElement3D*> RenderingElement3D::elements;
 
+void RenderingElement3D::Add(RenderingElement3D* e) {
+    e->indexInElements = static_cast<std::uint32_t>(elements.size());
+    elements.push_back(e);
+}
+
+void RenderingElement3D::Remove(RenderingElement3D* e) {
+    // Idempotent: callers like Builder ghost flow toggle elements in/out
+    // and may try to remove an already-removed element.
+    std::uint32_t idx = e->indexInElements;
+    if (idx == std::numeric_limits<std::uint32_t>::max()) return;
+    std::uint32_t last = static_cast<std::uint32_t>(elements.size() - 1);
+    if (idx != last) {
+        elements[idx] = elements[last];
+        elements[idx]->indexInElements = idx;
+    }
+    elements.pop_back();
+    e->indexInElements = std::numeric_limits<std::uint32_t>::max();
+}
+
 void RenderingElement3D::BuildTLAS(VkCommandBuffer cmd, std::uint32_t index) {
+    auto& tlas = tlases[index];
+    const std::uint32_t primitiveCount = static_cast<std::uint32_t>(elements.size());
+
+    // Refit (UPDATE) is allowed when the count matches the count this AS
+    // was last built for. A change forces a full rebuild because the AS
+    // storage and instance buffer were sized for the old count. Refit is
+    // dramatically cheaper at scale (millions of instances) — it walks the
+    // existing BVH and updates AABBs rather than reconstructing topology.
+    const bool topologyChanged =
+        tlas.accelerationStructure == VK_NULL_HANDLE
+        || primitiveCount != tlas.builtInstanceCount;
+
     {
         VkMemoryBarrier asBarrier {
             .sType = VK_STRUCTURE_TYPE_MEMORY_BARRIER,
@@ -41,18 +72,38 @@ void RenderingElement3D::BuildTLAS(VkCommandBuffer cmd, std::uint32_t index) {
             0, 1, &asBarrier, 0, nullptr, 0, nullptr);
     }
 
-    tlases[index].instanceBuffer.Resize(VK_BUFFER_USAGE_SHADER_DEVICE_ADDRESS_BIT | VK_BUFFER_USAGE_TRANSFER_DST_BIT | VK_BUFFER_USAGE_ACCELERATION_STRUCTURE_BUILD_INPUT_READ_ONLY_BIT_KHR, VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT | VK_MEMORY_PROPERTY_HOST_COHERENT_BIT, elements.size());
+    if (topologyChanged) {
-
+        // Resize the host-visible inputs to match the new count.
-    for(std::uint32_t i = 0; i < elements.size(); i++) {
+        // STORAGE_BUFFER_BIT is required because the application's compute
-        tlases[index].instanceBuffer.value[i] = elements[i]->instance;
+        // shaders bind this buffer as a storage SSBO (e.g. to write
+        // per-instance transforms directly into the TLAS instance data).
+        tlas.instanceBuffer.Resize(VK_BUFFER_USAGE_SHADER_DEVICE_ADDRESS_BIT | VK_BUFFER_USAGE_TRANSFER_DST_BIT | VK_BUFFER_USAGE_ACCELERATION_STRUCTURE_BUILD_INPUT_READ_ONLY_BIT_KHR | VK_BUFFER_USAGE_STORAGE_BUFFER_BIT, VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT | VK_MEMORY_PROPERTY_HOST_COHERENT_BIT, primitiveCount);
+        tlas.metadataBuffer.Resize(VK_BUFFER_USAGE_STORAGE_BUFFER_BIT | VK_BUFFER_USAGE_SHADER_DEVICE_ADDRESS_BIT, VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT | VK_MEMORY_PROPERTY_HOST_COHERENT_BIT, primitiveCount);
     }
 
-    tlases[index].instanceBuffer.FlushDevice(cmd, VK_ACCESS_MEMORY_READ_BIT, VK_PIPELINE_STAGE_ACCELERATION_STRUCTURE_BUILD_BIT_KHR);
+    for(std::uint32_t i = 0; i < primitiveCount; i++) {
+        if (elements[i]->transformOwnedByGpu) {
+            // Skip the transform field — the application's compute shader
+            // writes it earlier in this submission. Copy everything else.
+            auto& dst = tlas.instanceBuffer.value[i];
+            const auto& src = elements[i]->instance;
+            dst.instanceCustomIndex                    = src.instanceCustomIndex;
+            dst.mask                                   = src.mask;
+            dst.instanceShaderBindingTableRecordOffset = src.instanceShaderBindingTableRecordOffset;
+            dst.flags                                  = src.flags;
+            dst.accelerationStructureReference         = src.accelerationStructureReference;
+        } else {
+            tlas.instanceBuffer.value[i] = elements[i]->instance;
+        }
+        tlas.metadataBuffer.value[i] = elements[i]->userMetadata;
+    }
 
-    VkAccelerationStructureGeometryInstancesDataKHR instancesData = VkAccelerationStructureGeometryInstancesDataKHR {
+    tlas.instanceBuffer.FlushDevice(cmd, VK_ACCESS_MEMORY_READ_BIT, VK_PIPELINE_STAGE_ACCELERATION_STRUCTURE_BUILD_BIT_KHR);
+
+    VkAccelerationStructureGeometryInstancesDataKHR instancesData {
         .sType = VK_STRUCTURE_TYPE_ACCELERATION_STRUCTURE_GEOMETRY_INSTANCES_DATA_KHR,
         .arrayOfPointers = VK_FALSE,
-        .data            = {tlases[index].instanceBuffer.address}
+        .data            = {tlas.instanceBuffer.address}
     };
 
     VkAccelerationStructureGeometryDataKHR geometryData;
@@ -64,17 +115,22 @@ void RenderingElement3D::BuildTLAS(VkCommandBuffer cmd, std::uint32_t index) {
         .geometry     = geometryData
     };
 
-    VkAccelerationStructureBuildGeometryInfoKHR tlasBuildGeometryInfo{
+    VkAccelerationStructureBuildGeometryInfoKHR tlasBuildGeometryInfo {
         .sType         = VK_STRUCTURE_TYPE_ACCELERATION_STRUCTURE_BUILD_GEOMETRY_INFO_KHR,
         .type          = VK_ACCELERATION_STRUCTURE_TYPE_TOP_LEVEL_KHR,
-        .mode          = VK_BUILD_ACCELERATION_STRUCTURE_MODE_BUILD_KHR,
+        // ALLOW_UPDATE is required for any subsequent UPDATE-mode (refit)
+        // build. Set it on every build so the AS we keep around can be
+        // refit on later frames.
+        .flags         = VK_BUILD_ACCELERATION_STRUCTURE_ALLOW_UPDATE_BIT_KHR,
+        .mode          = topologyChanged
+                            ? VK_BUILD_ACCELERATION_STRUCTURE_MODE_BUILD_KHR
+                            : VK_BUILD_ACCELERATION_STRUCTURE_MODE_UPDATE_KHR,
         .geometryCount = 1,
         .pGeometries   = &tlasGeometry
     };
 
-    // Query the memory sizes that will be needed for this TLAS
+    if (topologyChanged) {
-    auto primitiveCount = static_cast<uint32_t>(elements.size());
+        // Query sizes for the fresh build, allocate AS storage + scratch.
-
         VkAccelerationStructureBuildSizesInfoKHR tlasBuildSizes {
             .sType = VK_STRUCTURE_TYPE_ACCELERATION_STRUCTURE_BUILD_SIZES_INFO_KHR
         };
@@ -86,47 +142,59 @@ void RenderingElement3D::BuildTLAS(VkCommandBuffer cmd, std::uint32_t index) {
             &tlasBuildSizes
         );
 
-    tlases[index].scratchBuffer.Resize(VK_BUFFER_USAGE_STORAGE_BUFFER_BIT | VK_BUFFER_USAGE_SHADER_DEVICE_ADDRESS_BIT, VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT, tlasBuildSizes.buildScratchSize);
+        // Scratch buffer must hold at least max(buildScratchSize, updateScratchSize).
-    tlasBuildGeometryInfo.scratchData.deviceAddress = tlases[index].scratchBuffer.address;
+        // Sizing for buildScratchSize covers both — refit is always smaller.
+        tlas.scratchBuffer.Resize(VK_BUFFER_USAGE_STORAGE_BUFFER_BIT | VK_BUFFER_USAGE_SHADER_DEVICE_ADDRESS_BIT, VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT, tlasBuildSizes.buildScratchSize);
+        tlas.buffer.Resize(VK_BUFFER_USAGE_ACCELERATION_STRUCTURE_STORAGE_BIT_KHR | VK_BUFFER_USAGE_SHADER_DEVICE_ADDRESS_BIT | VK_BUFFER_USAGE_ACCELERATION_STRUCTURE_BUILD_INPUT_READ_ONLY_BIT_KHR, VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT, tlasBuildSizes.accelerationStructureSize);
 
-    tlases[index].buffer.Resize(VK_BUFFER_USAGE_ACCELERATION_STRUCTURE_STORAGE_BIT_KHR | VK_BUFFER_USAGE_SHADER_DEVICE_ADDRESS_BIT | VK_BUFFER_USAGE_ACCELERATION_STRUCTURE_BUILD_INPUT_READ_ONLY_BIT_KHR, VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT, tlasBuildSizes.accelerationStructureSize);
+        // Destroy the previous AS handle before creating a new one — the
+        // pre-refit path leaked here on every frame.
+        if (tlas.accelerationStructure != VK_NULL_HANDLE) {
+            Device::vkDestroyAccelerationStructureKHR(Device::device, tlas.accelerationStructure, nullptr);
+            tlas.accelerationStructure = VK_NULL_HANDLE;
+        }
 
-    // Create and store the TLAS handle
         VkAccelerationStructureCreateInfoKHR tlasCreateInfo {
             .sType = VK_STRUCTURE_TYPE_ACCELERATION_STRUCTURE_CREATE_INFO_KHR,
-        .buffer = tlases[index].buffer.buffer,
+            .buffer = tlas.buffer.buffer,
             .offset = 0,
             .size   = tlasBuildSizes.accelerationStructureSize,
             .type   = VK_ACCELERATION_STRUCTURE_TYPE_TOP_LEVEL_KHR,
         };
+        Device::CheckVkResult(Device::vkCreateAccelerationStructureKHR(Device::device, &tlasCreateInfo, nullptr, &tlas.accelerationStructure));
 
-    Device::CheckVkResult(Device::vkCreateAccelerationStructureKHR(Device::device, &tlasCreateInfo, nullptr, &tlases[index].accelerationStructure));
+        VkAccelerationStructureDeviceAddressInfoKHR addrInfo {
-    tlasBuildGeometryInfo.dstAccelerationStructure = tlases[index].accelerationStructure;
+            .sType = VK_STRUCTURE_TYPE_ACCELERATION_STRUCTURE_DEVICE_ADDRESS_INFO_KHR,
+            .accelerationStructure = tlas.accelerationStructure
+        };
+        tlas.address = Device::vkGetAccelerationStructureDeviceAddressKHR(Device::device, &addrInfo);
+
+        tlas.builtInstanceCount = primitiveCount;
+    }
+
+    // For UPDATE mode, src == dst (in-place refit). For BUILD, src is
+    // VK_NULL_HANDLE and dst is the freshly-created handle.
+    tlasBuildGeometryInfo.scratchData.deviceAddress = tlas.scratchBuffer.address;
+    tlasBuildGeometryInfo.dstAccelerationStructure  = tlas.accelerationStructure;
+    tlasBuildGeometryInfo.srcAccelerationStructure  =
+        topologyChanged ? VK_NULL_HANDLE : tlas.accelerationStructure;
 
-    // Prepare the build range for the TLAS
     VkAccelerationStructureBuildRangeInfoKHR tlasRangeInfo {
         .primitiveCount  = primitiveCount,
         .primitiveOffset = 0,
         .firstVertex     = 0,
         .transformOffset = 0
     };
-
     VkAccelerationStructureBuildRangeInfoKHR* tlasRangeInfoPP = &tlasRangeInfo;
     Device::vkCmdBuildAccelerationStructuresKHR(cmd, 1, &tlasBuildGeometryInfo, &tlasRangeInfoPP);
 
     vkCmdPipelineBarrier(
         cmd,
         VK_PIPELINE_STAGE_ACCELERATION_STRUCTURE_BUILD_BIT_KHR,
-        VK_PIPELINE_STAGE_RAY_TRACING_SHADER_BIT_KHR,
+        VK_PIPELINE_STAGE_RAY_TRACING_SHADER_BIT_KHR | VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT,
         0,
         0, nullptr,
         0, nullptr,
         0, nullptr
     );
-
-    VkAccelerationStructureDeviceAddressInfoKHR addrInfo {
-        .sType = VK_STRUCTURE_TYPE_ACCELERATION_STRUCTURE_DEVICE_ADDRESS_INFO_KHR,
-        .accelerationStructure = tlases[index].accelerationStructure
-    };
-    tlases[index].address = Device::vkGetAccelerationStructureDeviceAddressKHR(Device::device, &addrInfo);
 }

implementations/Crafter.Graphics-Window.cpp

@@ -732,16 +732,12 @@ void Window::Render() {
     // widget's OnTextInput / OnKeyDown sees them in the same frame.
     Device::TickKeyRepeats();
 
-    onUpdate.Invoke({startTime, startTime-lastFrameBegin});
+    // Bind the descriptor heaps BEFORE the user's update event fires.
-    #ifdef CRAFTER_TIMING
+    // Any compute work the update lambda records (e.g. physics dispatches)
-    totalUpdate = std::chrono::nanoseconds(0);
+    // needs the heaps bound at execution time; recording order in the cmd
-    updateTimings.clear();
+    // buffer dictates GPU execution order, so the bind must come first.
-    for (const std::pair<const EventListener<FrameTime>*, std::chrono::nanoseconds>& entry : onUpdate.listenerTimes) {
+    // Pass-side dispatches still run with the same heaps bound — moving
-        updateTimings.push_back(entry);
+    // the bind earlier doesn't change anything for them.
-        totalUpdate += entry.second;
-    }
-    #endif
-
     if (descriptorHeap) {
         VkBindHeapInfoEXT resourceHeapInfo = {
             .sType = VK_STRUCTURE_TYPE_BIND_HEAP_INFO_EXT,
@@ -766,6 +762,16 @@ void Window::Render() {
         Device::vkCmdBindSamplerHeapEXT(drawCmdBuffers[currentBuffer], &samplerHeapInfo);
     }
 
+    onUpdate.Invoke({startTime, startTime-lastFrameBegin});
+    #ifdef CRAFTER_TIMING
+    totalUpdate = std::chrono::nanoseconds(0);
+    updateTimings.clear();
+    for (const std::pair<const EventListener<FrameTime>*, std::chrono::nanoseconds>& entry : onUpdate.listenerTimes) {
+        updateTimings.push_back(entry);
+        totalUpdate += entry.second;
+    }
+    #endif
+
     // Note: vkCmdClearColorImage is unavailable here — the swapchain is
     // created with VK_IMAGE_USAGE_STORAGE_BIT only (no TRANSFER_DST_BIT).
     // Passes that need a background should write one explicitly (UIScene

interfaces/Crafter.Graphics-Device.cppm

@@ -123,6 +123,7 @@ export namespace Crafter {
         inline static VkSwapchainKHR swapchain = VK_NULL_HANDLE;
         inline static PFN_vkGetAccelerationStructureBuildSizesKHR vkGetAccelerationStructureBuildSizesKHR;
         inline static PFN_vkCreateAccelerationStructureKHR vkCreateAccelerationStructureKHR;
+        inline static PFN_vkDestroyAccelerationStructureKHR vkDestroyAccelerationStructureKHR;
         inline static PFN_vkCmdBuildAccelerationStructuresKHR vkCmdBuildAccelerationStructuresKHR;
         inline static PFN_vkGetAccelerationStructureDeviceAddressKHR vkGetAccelerationStructureDeviceAddressKHR;
         inline static PFN_vkCreateRayTracingPipelinesKHR vkCreateRayTracingPipelinesKHR;

interfaces/Crafter.Graphics-RenderingElement3D.cppm

@@ -28,18 +28,58 @@ import :Window;
 
 export namespace Crafter {
     struct TlasWithBuffer {
-        VkDeviceAddress address;
+        VkDeviceAddress address = 0;
         VulkanBuffer<char, false> buffer;
-        VkAccelerationStructureKHR accelerationStructure;
+        VkAccelerationStructureKHR accelerationStructure = VK_NULL_HANDLE;
         VulkanBuffer<VkAccelerationStructureInstanceKHR, true> instanceBuffer;
         VulkanBuffer<char, false> scratchBuffer;
+        // Parallel to instanceBuffer, indexed by TLAS instance ID. Filled
+        // from each element's userMetadata during BuildTLAS. Consumers
+        // (e.g. ray-query collision) bind this in the descriptor heap and
+        // look up via rayQueryGetIntersectionInstanceIdEXT to recover
+        // application-side per-instance data without touching the
+        // Vulkan-mandated instanceCustomIndex (which renderers may already
+        // use for their own encoding).
+        VulkanBuffer<std::uint32_t, true> metadataBuffer;
+        // Last instance count this TLAS was built (not refit) for. When
+        // elements.size() matches this, BuildTLAS does an in-place refit
+        // (UPDATE mode) which is dramatically cheaper than a full rebuild
+        // — refit walks the existing BVH and updates AABBs, while rebuild
+        // reconstructs the topology from scratch. A change in count forces
+        // a fresh rebuild because the AS is sized for that primitive count.
+        std::uint32_t builtInstanceCount = 0;
     };
 
    class RenderingElement3D {
         public:
         VkAccelerationStructureInstanceKHR instance;
+        // Position in `elements`, maintained by Add/Remove for O(1) swap-and-pop.
+        // Sentinel value = not currently registered.
+        std::uint32_t indexInElements = std::numeric_limits<std::uint32_t>::max();
+        // Application-defined per-instance tag, copied verbatim into
+        // tlases[*].metadataBuffer at this element's TLAS instance ID
+        // every BuildTLAS. Crafter doesn't interpret it.
+        std::uint32_t userMetadata = 0;
+        // When true, BuildTLAS skips copying instance.transform into the
+        // TLAS instance buffer — the application's compute shader writes
+        // the transform field directly into instanceBuffer at this
+        // element's TLAS instance ID. Other instance fields (mask,
+        // customIndex, SBT offset, BLAS reference) are still copied from
+        // the CPU instance struct.
+        //
+        // Used to take per-frame transform updates off the CPU for bodies
+        // whose transforms derive from GPU-side state (physics nodes that
+        // already live on the GPU).
+        bool transformOwnedByGpu = false;
+
         static std::vector<RenderingElement3D*> elements;
         inline static TlasWithBuffer tlases[Window::numFrames];
         static void BuildTLAS(VkCommandBuffer cmd, std::uint32_t index);
+
+        // Register / unregister with `elements`. Use these instead of touching
+        // the vector directly: linear find+erase is O(n) and pathological at
+        // the body counts physics targets (millions of braces).
+        static void Add(RenderingElement3D* e);
+        static void Remove(RenderingElement3D* e);
    };
 }

interfaces/Crafter.Graphics-UI.cppm

@@ -247,6 +247,14 @@ export namespace Crafter {
         std::uint16_t FontAtlasImageSlot()   const noexcept { return fontAtlasImageSlot_; }
         std::uint16_t FontAtlasSamplerSlot() const noexcept { return fontAtlasSamplerSlot_; }
 
+        // Heap slot whose descriptor in each per-frame heap points at that
+        // frame's swapchain image. Other passes (e.g. a ray-tracing pass
+        // that wants to render the world directly into the swapchain) can
+        // write to the same image by referencing this slot. Order in
+        // window.passes controls compositing — push such passes BEFORE
+        // the UI pass so UI overlays render on top.
+        std::uint16_t OutImageSlot() const noexcept { return outImageSlot_; }
+
     private:
         Window* window_ = nullptr;
         DescriptorHeapVulkan* heap_ = nullptr;