To add some challenges to your list: - implement a bidirectional path tracer to increase convergence speed - implement multiple importance sampling. It's a si

To add some challenges to your list: - implement a bidirectional path tracer to increase convergence speed - implement multiple importance sampling. It's a simple concept that I found astonishingly hard to implement in practice. - implement an acceleration structure (like a BVH) that builds and caches efficiently. Can you make it fast enough to have animation? - implement shader based unit tests to ensure correctness. Thus is also surprisingly hard.