Global illumination ( GI ), or indirect illumination , is a group
of algorithms used in 3D computer graphics that are meant to add more
realistic lighting to 3D scenes. Such algorithms take into account not only
the light that comes directly from a light source ( direct illumination ),
but also subsequent cases in which light rays from the same source are
reflected by other surfaces in the scene, whether reflective or not (
indirect illumination ).
Theoretically, reflections, refractions, and shadows are all examples of
global illumination, because when simulating them, one object affects the
rendering of another (as opposed to an object being affected only by a direct
source of light).