If the geometry of the scene is such that no light can travel between two points, then whatever illumination that x’ provides cannot be absorbed and/or reflected at x. From a geometric standpoint this makes perfect sense. The value of g(x, x’) is exactly zero if there is no straight line-of-sight from x’ to x and vice versa. The quantity g(x, x’) represents the occlusion between point x’ and point x. Once all calculations are done, the numerical value of I(x, x’) is usually normalized to the range. White, for example, is considered a hot (or high intensity) color while deep blues, purples and very dark shades of grey are cool (or low intensity) colors. Instead, more descriptive terms are used. In many cases, computer graphics researchers do not deal with joules of energy when talking about intensity of light. I(x, x’) is that energy of radiation per unit time per unit area of source dx’ per unit area dx of the target. The intensity of light that travels from point x’ to point x assumes there are no surfaces between to deflect or scatter the light.
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