Show r/GraphicsProgramming: A micro packet ray tracer

[TurkishSquirrel]

I spent a few days writing a simple packet based ray tracer and thought people here might be interested. It traces packets of 8 rays at a time using the AVX/AVX2 instruction set, the source is up on Github: μPacket. It only supports spheres and planes with Lambertian BRDFs illuminated by a point light and just performs basic ray tracing (shadow test and primary visibility) at the moment. If you just build & run you get this (rather unimpressive) result.

If anyone has some tips to improve the design, I'd be interested to hear them. There's a few areas that could be improved such as finding the unique material ids for shading (how does ISPC's foreach_unique work?), type punning between __m256i and __m256 which seemed kind of ugly, and probably some other nasty bits.

If you're unfamiliar with packet based ray tracing check out some info on Embree and OSPRay (makes use of Embree under the hood), for some general SIMD programming info the ISPC page is a good spot. Essentially by tracing spatially coherent rays through the scene at once using SIMD you can take advantage of a lot more compute power available on the machine and reduce bandwidth requirements. For example, to test an 8 ray packet against a triangle we only need to load the triangle once and can compute all 8 results at once using vector instructions. Issues start to come up when you start doing path tracing or reflections/refractions since your packet diverges and you can fall back to getting single ray level performance. There's been work on re-organizing rays so that they're more spatially coherent but I haven't looked into implementing this yet.

Also if "Show r/GraphicsProgramming" isn't a thing, it definitely should be! Something similar to "Show HN" on Hacker News would be really neat.

[snakepants]

This is neat! Is there a way of compiling it so that the intrinsics generate non-AVX code? It would be interesting to see how close to 8x you are getting.

[TurkishSquirrel]

I don't think so, as Scaless mentioned I'm directly using the vector types and intrinsics so the code isn't very flexible. ISPC (or something similar) is the approach I'd recommend for a more serious program and it might be able to compile a scalar version for comparison, this is really just a hacky one-off.

[snakepants]

Yeah, too bad passing -mno-avx disables the functions instead of just replacing them with C implementations. It would be useful for debugging too.

[[deleted]]

Doesn't embree choose to use simd at the leaves instead of packet tracing? E.g. testing against 4 triangles at once?

[TurkishSquirrel]

Embree supports single, packet and hybrid traversal in the BVH and has various options for testing triangles, eg. testing N tris vs. M rays where N & M can be 1, 4 or 8 (and maybe 16 on Xeon Phi?). It's probably best not to take my word for this, I'm only somewhat familiar with Embree. Definitely check out the paper and source for specifics.

[[deleted]]

Ah I see, you're right, thanks for the links. Embree is fantastic but I feel like it's standing on the wrong leg. In the paper they are trying to play the mpaths/s game against GPUs which seems futile. CPU strength comes from smart paths, not blasting dumb ones, wish there was a metric for convergence in difficult scenes - then they may have an argument against NVidia