Jorijn van der Graaf
449d5da553
The heap acceleration-structure read still needs the rewrite on 615.78.08; the READMEs say so and point at HeapASRead --no-workaround for checking a newer driver. Dynamic descriptor indexing in hit shaders is fixed there, which the README and the Sponza and SponzaBench notes now say, and the VulkanTriangle investigation no longer contradicts itself about llvmpipe, which advertises VK_EXT_descriptor_heap now. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
106 lines
5.2 KiB
Markdown
106 lines
5.2 KiB
Markdown
# 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
|
||
|
||
```bash
|
||
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.
|
||
|
||
### Status on later drivers
|
||
|
||
On `615.78.08` the fault is still there. With the rewrite off
|
||
(`CRAFTER_GRAPHICS_NO_AS_WORKAROUND` set), `traceRayEXT` still loses the device,
|
||
and an inline ray query that reads the TLAS from the heap no longer faults but
|
||
renders nothing. Mesa's llvmpipe now advertises `VK_EXT_descriptor_heap` too
|
||
and renders the plain heap read correctly, which is a second implementation
|
||
backing the conclusion below. `tests/HeapASRead` renders this triangle through
|
||
both paths and checks the pixels; `HeapASRead --no-workaround` is the quick
|
||
check for a new driver, reporting each path on its own.
|
||
|
||
### 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; at the time NVIDIA was the only
|
||
implementation on this machine that advertised it (llvmpipe did not), so
|
||
there was no second conformant implementation to cross-check against.
|
||
Mesa's llvmpipe has it now; see "Status on later drivers" above.
|
||
|
||
**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.
|