Light Tree

Light tree is a method of sampling direct light sources that takes into account their contribution to the overall scene. This is usually based on a combination of factors, such as estimated intensity, distance from the camera, etc.

In order to improve efficiency ray tracers do not sample every light in the scene at once, and instead randomly sample lights. While this avoids needless calculation, random sampling of direct light sources results in additional noise. To work around this, renderers will create a light tree to approximate lighting contribution before the actual render process, and adjust the probability of each light being selected based on those results, allowing more samples to be used on lights that contribute more to the scene. This works best in scenes with larges amounts of light sources.