Lighting: Global Illumination

DEFINITION
The question of how light is distributed within the space, taking into account the reflective properties of materials and the distribution of light from light sources. Since each surface can be thought of as interacting with every other surface and light, it is a "global" illumination question.

REAL SURFACES REFLECT LIGHT
Real surfaces don't just receive light from light sources in the space; they reflect some of the light they receive. Ray-tracing was developed to address the specific condition of highly reflective surfaces as well as transparent objets that refract light, but most surfaces are at least somewhat diffusely reflective. Completely diffuse environments can be approached with radiosity, but most surfaces are at least somewhat specularly reflective too. To compute the illumination on any one surface, you have to know the illumination on all the others.

Most modern approaches take advantage of the fact that the influence of surfaces on each other is limited to a small number of bounces, and that a suitably large random sample (of light, or image colors, etc) produces a pretty good approximation. By working forward from the lights and backwards from the pixels, a visually acceptable result can be produced.

FURTHER READING
Look into the more advanced area in the "Rendering" readings, including Radiosity", "Photon Mapping", "and "Final Gather". There is also a good, but readable, general discussion of Global Illumination at http://web.cs.wpi.edu/.

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