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Crafter.Graphics/examples/VulkanTriangle/README.md
catbot 45ecc91424 fix(vulkan-rt): merge TLAS push constant into existing block (#18) 
The NVIDIA descriptor-heap AS-read workaround (#15) rewrote heap
acceleration-structure reads into a load of the TLAS device address from
a push-constant block. It always *synthesized a new* push-constant block,
so any ray-tracing shader that already declared one ended up with two —
which SPIR-V forbids ("at most one push constant block statically used per
entry point"), and vkCreateShaderModule's spirv-val check rejected:

    Entry point id '4' uses more than one PushConstant interface.

WorkaroundNvidiaAS::Patch now detects an existing PushConstant variable and,
when present, appends a single ulong member (the TLAS address) to that
block instead of adding a second one, reading the address through the
shader's own push-constant variable. The append offset is the end of the
user's block, computed from the members' explicit Offset/ArrayStride/
MatrixStride decorations (correct under both scalar and std140 layout) and
rounded up to 8. Shaders with no push constant of their own keep getting a
freshly synthesized single-member block at offset 0, exactly as before.

That offset is published via Device::workaroundTlasPushOffset and RTPass
feeds it to vkCmdPushDataEXT so the address lands where the rewritten load
reads it (0 for the synthesized case, preserving prior behaviour).

Verified on the affected driver (NVIDIA 610.43.02, RTX 4090): VulkanTriangle
ray-traces correctly and validation-clean both with and without a
user-declared raygen push constant.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
2026-06-03 02:28:02 +00:00

4.6 KiB
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VulkanTriangle

The minimal ray-traced example. Renders a single static triangle through vkCmdTraceRaysKHR. No UI.

What it shows

  • Device::Initialize() + Window + swapchain bring-up.
  • A DescriptorHeapVulkan sized for one image + one buffer slot, with slot ranges allocated via the bump-allocator API (AllocateImageSlots, AllocateBufferSlots).
  • A PipelineRTVulkan built from raygen / miss / closesthit SPIR-V shaders compiled at build time.
  • Mesh::Build constructing a BLAS and RenderingElement3D::BuildTLAS the per-frame TLAS.
  • Direct descriptor writes via vkWriteResourceDescriptorsEXT for the swapchain views and TLAS device addresses.
  • RTPass{&pipeline} plugged into window.passes — the canonical way to add ray tracing to a window in this library.

It's the smallest sensible test of the bindless VK_EXT_descriptor_heap

  • ray-tracing path.

Run

cd examples/VulkanTriangle
crafter-build -r

You should see a 1280×720 window with an RGB-barycentric triangle filling roughly the centre. On the NVIDIA driver this works through an engine-side workaround for a driver fault — see below.

Native status — NVIDIA driver fault, worked around

On NVIDIA driver 610.43.02 (Vulkan 1.4) reading the acceleration structure through VK_EXT_descriptor_heap aborts the device with VK_ERROR_DEVICE_LOST on the first frame. This is a driver-side fault in the brand-new descriptor-heap acceleration-structure path, not an engine bug (full investigation in #7, summarised below).

The engine works around it transparently (issue #15). On the NVIDIA proprietary driver only, VulkanShader rewrites the compiled SPIR-V at module-load time so that every OpLoad of an accelerationStructureEXT out of the heap becomes a load of the TLAS device address (from a push-constant block) followed by OpConvertUToAccelerationStructureKHR — which reads no descriptor and so never touches the faulting path. RTPass feeds the active frame's TLAS address in as push data. SPIR-V allows only one push-constant block per entry point, so when a shader already declares one the TLAS address is appended to that block (rather than adding a second, which would fail validation — issue #18); shaders without a push constant get a freshly synthesized single-member block. raygen.glsl and the example code are unchanged; acceleration structures still bind into the heap normally. On every other driver the workaround is inert. It's gated on Device::workaroundDescriptorHeapAS and confined to one fenced block in interfaces/Crafter.Graphics-ShaderVulkan.cppm so it can be deleted wholesale once a fixed NVIDIA driver ships.

The underlying fault (#7)

The fault was traced to the acceleration-structure read through VK_EXT_descriptor_heap, not to the engine's RT setup:

  • The BLAS/TLAS build is correct and finishes before rendering (Window::FinishInit does vkQueueWaitIdle). The built TLAS instance has an identity transform, mask = 0xFF, and the correct BLAS device address.
  • The AS descriptor is written correctly — vkWriteResourceDescriptorsEXT stores the TLAS device address at the expected heap byte offset (verified by dumping the raw heap bytes after the write).
  • The Khronos validation layers (1.4.350, current) report zero errors for the whole frame, including the SBT regions handed to vkCmdTraceRaysKHR.
  • Storage images and buffers bound through the same descriptor heap work — with traceRayEXT removed, the raygen shader's imageStore renders correctly, so the heap binding / image path is sound.
  • Both the ray-tracing pipeline (traceRayEXT) and inline ray query (rayQueryEXT, which uses no shader binding table) fault identically when they read the acceleration structure from the heap. That isolates the fault to the AS-via-heap read, not the SBT or the RT pipeline.
  • The fault reproduces even with the AS descriptor written at heap byte 0 and read at shader index 0 (no descriptor offset/stride ambiguity), and is unaffected by the pAddressRange size.
  • VK_EXT_descriptor_heap is brand new; on this machine NVIDIA is the only implementation that advertises it (llvmpipe does not), so there is no second conformant implementation to cross-check against.

Conclusion: this is a driver-side fault in NVIDIA's VK_EXT_descriptor_heap acceleration-structure path, not an engine bug, and it should be reported to NVIDIA. Until a fixed driver ships, the SPIR-V rewrite above keeps the native RT path working; once it does, remove the workaround and the heap AS read becomes the direct path again.