ShaderToy already has GPU ray tracers and the main problem is branching control flow. When you draw triangles you can give the same exact operations to a chunk

ShaderToy already has GPU ray tracers and the main problem is branching control flow. When you draw triangles you can give the same exact operations to a chunk of cores and GPUs are very good at that. When you ray trace, or at least when you path trace (multiple bounces per ray), each ray can go in any number of directions, and they diverge very quickly with any kind of reflection or diffusion.