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iddm/vkrt_ hit_variables.md

Last active January 26, 2023 19:49
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Vulkan Ray Tracing API documentation regarding gl_GeometryIndexID, gl_InstanceID, gl_PrimitiveID in the hit shader
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vkrt_ hit_variables.md

Missing documentation found empirically for the aforementioned variables.

We can traverse only one TLAS at at time. While traversing it via traceRayEXT, we can hit just one BLAS at at a time. Hence, all the *ID references here are relative to the BLAS we hit (except gl_InstanceID):

  1. gl_GeometryIndexID - Geometry index within the BLAS this ray has hit.
  2. gl_InstanceID - BLAS index within the TLAS.
  3. gl_PrimitiveID - Triangle (or other primitive) index within the BLAS.

The gl_GeometryIndexID and all the mentioned variables are set automatically for the hit shader. If a BLAS is created with several geomeotries, then the gl_GeometryIndexID is the zero-based index of the geometry used during the BLAS creation.

When a TLAS is created from one or many of BLAS instances, the order of these BLAS instances is preserved and when a BLAS is hit, for the hit shader the gl_InstanceID is set.

I wish we had those documented better or it was easier to find.

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