Global illumination is the technique used to capture indirect illumination, the natural phenomenon where light bounces off anything in its path until it is completely absorbed.
Global illumination lets you achieve realistic, real-world lighting conditions, which is particularly good for:
mental ray for Maya creates Global illumination by tracing the paths of photons and recording their bounce points in a photon map.
Caustics, focused light effects, are a form of global illumination.
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