photonmapping

Photon mapping is a two-pass global illumination algorithm used in computer graphics to simulate the behavior of light. The first pass, known as the photon tracing pass, simulates the emission of photons from light sources. These photons travel through the scene, bouncing off surfaces according to their material properties, and are stored in a data structure called a photon map. This map effectively records the distribution and intensity of light energy throughout the scene.

The second pass, the rendering pass, calculates the final image. For each pixel being rendered, a ray is cast from the camera into the scene. When this ray hits a surface, its color is not solely determined by the direct illumination. Instead, the algorithm queries the photon map to find nearby photons that have accumulated at that point. The contribution of these photons, weighted by their proximity and intensity, is combined with the direct illumination and specular reflections to produce the final pixel color. This approach allows for the realistic rendering of complex lighting effects such as caustics (light patterns created by reflection or refraction) and soft shadows, which are difficult to achieve with simpler illumination models. Photon mapping is computationally intensive but produces visually appealing results.