[Feature Request] Support getting hit vertex positions when raytracing.

[Vecvec]

Is your feature request related to a problem? Please describe.
When ray-tracing the vertex positions of the hit triangle are almost always useful (e.g for getting the normal). Obtaining vertex positions currently requires binding extra resources to the shader even though acceleration structures should have this stored in them (technically an acceleration structure does not have a layout, but there doesn't seem to be a way to avoid storing the positions)

Describe the solution you'd like
HLSL to support something like Vulkan's OpRayQueryGetIntersectionTriangleVertexPositionsKHR/HitTriangleVertexPositionsKHR

Describe alternatives you've considered
Just getting this from the vertex buffer - unoptimal as previously described.

Additional context
I'm quite new to HLSL, so have I missed some way already in HLSL that can do this better than binding an index and vertex buffer?
Vulkan proposal: https://github.com/KhronosGroup/Vulkan-Docs/blob/main/proposals/VK_KHR_ray_tracing_position_fetch.adoc#problem-statement.

[Vecvec]

This may also require a flag in DirectX like in Vulkan. Should that be opened as a seperate issue, if it should be opened where should I open it?

[damyanp]

Thanks - no need for a separate issue.

[damyanp]

Marking as a candidate for SM 6.9.

[Vecvec]

Thank you!

[Vecvec]

I would like to write a proposal for this, is there anything which I should discuss first? I think the dxil ops would work the same as Vulkan's so the names would be similar.

[damyanp]

We have an internal proposal already, I'll try see if we can make it public yet.

Note that DXIL and SPIR-V have some unique differences that mean we shouldn't expect a 1:1 mapping from SPIR-V to DXIL operations here. IIRC, some of the work in #361 will impact this, since the right design may be able to build on the long-vectors feature (I remember us thinking that before we designed the long-vectors feature so that thinking may be out of date.

[Vecvec]

OK, it will be nice to have the proposal public so I can figure out how to integrate it.