I’ve made a simple scene to test the physics of light. for proper light calculation, I used spectral render Indigo and Octane.
Indigo has multiple Engines, standard Spectral Path tracer on CPU or GPU and Bidirectional path tracing with MTL sampling (metropolis Light Transport). Octane has only default Spectral Path tracer on GPU but includes an MTL sampling method. I’v also added Renderman 23 to the test with its unified rendering integrator. It supports bidirectional path, manifold caustics and path guiding on the CPU.
Another render engine like Arnold or Cycles with regular Path tracing would impractical fro complex light calculation tasks.
The base scene is Sphere and squashed Sphere underneath inside a volume box (uniform VDB).
The following image is the result of Indigo Renderer with BiDirectional path tracing and MTL. It was by far the fastest rendering.