7.4 Photons

From [pov:6.10]:
The basic goal of this implementation of the photon map is to render true reflective and refractive caustics. The photon map was first introduced by Henrik Wann Jensen.

Photon mapping is a technique which uses a backwards ray-tracing pre-processing step to render refractive and reflective caustics realistically. This means that mirrors can reflect light rays and lenses can focus light.

Photon mapping works by shooting packets of light (photons) from light sources into the scene. The photons are directed towards specific objects. When a photon hits an object after passing through (or bouncing off of) the target object, the ray intersection is stored in memory. This data is later used to estimate the amount of light contributed by reflective and refractive caustics.

Figure: Example using photons

Figure: Another example using photons