Photon mapping
From DmWiki
Photon mapping is a Monte Carlo-based algorithm for solving the transport equation. That allows for more complex light interaction, such as diffuse reflection and caustics. The first step of the algorithm iterates through all the light sources in a scene. From which, a large number of rays or "photons" are emitted. The direction of these rays is randomly chosen. These photons are traced through the environment and whenever a photon hits a surface, a packet of data describing the photon and the location of the hit is stored in a volume based data structure known as a kd-tree. The second step is a standard raytracer, but when a ray hits a surface the color used is or is augmented by the average of the photons lying near the hit.
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