Natural caustics in backward path tracing

In this paper we introduce a natural method for producing both refraction and reflection caustics in backward path tracing (e.g. bouncing around an eye ray in the scene until a light is found).

These caustics do not rely on a light location, and as such, do not rely on bidirectional or forward path tracing.

As such, these caustics allow for as many light sources as one would care for; the shapes and positions of the lights are completely arbitrary (can be bunny shaped, etc).

We use the standard Cornell box for testing the backward path tracer.