Global Illumination: Photon Mapping

There are some effects that simple raytracing cannot render properly: e.g., caustics:

What we need to add: Rays traced FROM the various light sources.

From each light source, emit “photons” in random directions and trace them into the environment;

Store records (of brigtness / photon density) at intersections with surfaces;

Emit more photons from these locations . . .