In the past, other techniques have sought to approximate the effect of diffusely reflected light in a scene by applying a constant 'ambient' value of illumination indiscriminately through the entire scene. Radiosity is essentially a technique which correctly computes the distribution of this 'ambient' light, accounting for light reaching a surface after being reflected from one or more diffuse surfaces.
Radiosity techniques originated in thermal engineering, where they were used to simulate radiative heat transfer between surface elements. In recent years, computer graphics researchers have shown how radiosity can be used to simulate the transport of visible radiation in an environment. The physical accuracy of radiosity solutions results in images with a stunning level of realism. If physically correct input data is provided, then the results are faithful representations of the real world. This allows aesthetic judgements to be made from radiosity images, and also allows quantitative measurements of illumination levels to be obtained from the simulation results.
The technical definition of the term 'radiosity' is that it is a measure of radiant energy, in fact "the amount of energy leaving a surface, per unit area, per unit time". The principle underlying the technique is to divide all the surfaces in the model into small areas (called 'patches'), and to calculate this 'radiosity value' for each. This gives a value for the 'brightness' of each patch, which can then be used to produce an image made up of all the patches visible from the viewpoint, each rendered lighter if they are 'bright' or darker if they are 'dim'.
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