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This page provides information about Global Illumination (GI) methods including ones supported by V-Ray.
The Rendering Equation
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Virtually all modern GI renderers are based on the rendering equation introduced by James T. Kajiya in his 1986 paper "The Rendering Equation". This equation describes how light is propagated throughout a scene. In his paper, Kajiya also proposed a method for computing an image based on the rendering equation using a Monte Carlo method called path tracing.
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Different formulations of the rendering equation are possible, but the one proposed by Kajiya looks like this:
where:
L(x, x1) is related to the light passing from point x1 to point x;
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What the equation means: the light arriving at a given point x in the scene from another point x1 is the sum of the light emitted from all other points x2 towards x1 and reflected towards x:
Except for very simple cases, the rendering equation cannot be solved exactly in a finite amount of time on a computer. However, we can get as close as we want to the real solution - given enough time. The search for global illumination algorithms has been a quest for finding solutions that are reasonably close, for a reasonable amount of time.
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path tracing (exact)
irradiance caching (e.g. final gathering in mental ray), (approximate).
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Gathering methods and some hybrid methods allow for both view-dependent and view-independent solutions.
View-dependent solutions
Advantages:
Only the relevant parts of the scene are taken into consideration (no time is wasted on regions that are not visible).
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Different combinations of view-dependent and view-independent techniques can be combined.
Examples:
photon mapping (deprecated) and Light Cache in V-Ray.
photon mapping and final gathering in mental ray.
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