KhronosGroup / KhronosGroup/Vulkan-Samples
the shader in the sample mobile_nerf_rayquery probably produce underfined behaviour
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Description
texture() compiles to OpImageSampleImplicitLod, which needs screen-space derivatives of the UV. Derivatives in a fragment shader are computed by differencing across the 2×2 quad, so they're only meaningful when all four invocations in the quad execute the instruction with comparable values.
Vulkan spec, "Derivative Operations": derivative operations (and implicit-LOD image ops, which are defined in terms of them) have undefined results unless invoked in quad-uniform control flow.
if (rayQueryGetIntersectionTypeEXT(...) != ...NoneEXT) is per-pixel: one pixel of a quad can hit while its neighbor misses. So texture sampling is inside non-quad-uniform control flow.
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Research direction
Locate the shader used by the mobile_nerf_rayquery sample and inspect the texture() call guarded by rayQueryGetIntersectionTypeEXT(...). Confirm that the sample no longer performs an implicit-LOD texture sample in non-quad-uniform control flow; the issue does not name a test or validation command.
Written by the indexing model from the issue text.
Assessment
- Domain
- computer-graphics
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Active
- Clarity
- Needs clarification
- Newbie friendliness
- 38/100