Global illumination (GI) is illumination reflected more than once inside a scene.
After light was emitted, it has to be reflected before it can arrive at the eye of an observer or a sensor. When this reflection occurs once, it is called local or direct illumination. Illumination which has been reflected more than once is referred to as global or indirect illumination (GI). Reflection of light is sometimes called a bounce. If light is reflected two times from a surface, it is called the one-bounce indirect illumination; if it is reflected three times, it is called two-bounce; etc. As physical reflectance is below one, only a finite number of bounces contribute to the scene illumination.
A simple example of GI is best visible when putting a rectangular achromatic cardboard next to a perpendicular chromatic (e.g., red) piece of cardboard. When directional light is cast...
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