Implement Ray Tracing on Metal

[zmarlon]

Raytracing is not yet implemented for Metal? Is it currently being worked on? If not, I might take a look at it. Is there an example that could be tested?

[Vecvec]

See the ray_cube_compute, ray_cube_fragment examples which use the wgpu api built on top of the wgpu-hal api.

[Lichtso]

I implemented it for the most part: https://github.com/Lichtso/wgpu/tree/metal/ray_tracing_acceleration_structures
But it does not work yet. It can be compiled and cargo run --bin wgpu-examples ray_traced_triangle runs, but there are no intersections / hits. Probably missing a encoder.use_resource(blas); somewhere.

A challenge I encontered is that metal does not use raw GPU pointers like all other backends. Instead one has to build the BLASes first and then add a list of their handles to the TLAS descriptor to build it. Then the instances in the TLAS can reference those listed BLASes by index. This could require a rework of the AccelerationStructureInstances across all backends I think.

And then there are these other annoyances like how the flags and the barriers are handled.

[Vecvec]

This could require a rework of the AccelerationStructureInstances across all backends I think

Maybe try MTLIndirectInstanceAccelerationStructureDescriptor? https://developer.apple.com/documentation/metal/mtlindirectinstanceaccelerationstructuredescriptor.

Edit: Also see https://developer.apple.com/documentation/metal/mtlindirectaccelerationstructureinstancedescriptor.
Edit 2: It seems to (unclear from docs) be possible to continue using MTLAccelerationStructureInstanceDescriptor without setting instancedAccelerationStructures by setting instanceDescriptorType to indirect and using MTLIndirectAccelerationStructureInstanceDescriptor in the buffer.

[Lichtso]

Great idea, I will give it a shot after I got something working. I fixed a few more things, the metal API validation and the msl shader validation pass, but so far there still are no intersections ever at all :(

At least I can now see the BLAS, the TLAS instantiations thereof and the shaders acceleration structure buffer binding being active in XCode: https://developer.apple.com/documentation/xcode/inspecting-acceleration-structures

Still, the actual shader produces no hits even if I make all parameters the least restrictive possible (like using a negative minimal distance, forcing opacity and full hit mask etc.).

[Vecvec]

The only other thing I could find is that you bitshift custom_data into the mask (I don't think this would fix it but maybe try?). IIUC rather confusingly, I think that the place custom data should go is in UserID (in MTLAccelerationStructureUserIDInstanceDescriptor, or in the indirect descriptor). This isn't actually the ID of a user but a user provided ID for the instance. This has a confusing name, but is 8 bits sent to the shader and not used anywhere else which is what we need. The only other idea I have is that the rays are going is the wrong direction (e.g. 90 degrees off). Have you tried aligning the acceleration structure viewer camera to the ray direction?

[Lichtso]

Well, turns out it was already working. I just had Xcode configured not to run cargo build, so I was looking at an outdated build.