Page "Photon mapping" Paragraph 6

With photon mapping, light packets called photons are sent out into the scene from the light sources.

Whenever a photon intersects with a surface, the intersection point and incoming direction are stored in a cache called the photon map.

Typically, two photon maps are created for a scene: one especially for caustics and a global one for other light.

After intersecting the surface, a probability for either reflecting, absorbing, or transmitting / refracting is given by the material.

A Monte Carlo method called Russian roulette is used to choose one of these actions.

If the photon is absorbed, no new direction is given, and tracing for that photon ends.

If the photon reflects, the surface's bidirectional reflectance distribution function is used to determine the ratio of reflected radiance.

Finally, if the photon is transmitting, a function for its direction is given depending upon the nature of the transmission.