[Tunabrain]

I think that an implementation targeting real-time would need an entirely different design than an offline implementation, especially if it had to run on the GPU.

The main challenge is probably achieving full utilization of SIMD units (CPU) or streaming multiprocessors (GPU), which would require tracing and shading multiple rays within one unit. There's lots of papers on how to do this for geometry intersection (i.e. packet tracing), and I think there's a few slides from nvidia that give hints on how to do it for shading (avoid megakernels, sort all shading points by material id).

Brigade is a path tracer that is close to the real-time ideal (for outdoor scenes and using high-end GPUs), so looking into their code would certainly be interesting.

[Svenstaro]

As far as I know, Brigade isn't open source though. :/