Bi-reflectance distribution function. A way to model surface properties for computer graphics applications. When light hits a surface, the light is scattered in a hemisphere around the object. The BRDF models this. Both the incoming and outgoing light are represented in spherical coordinates, ie. as theta and phi. Theta represents the angle from the normal to the surface, and phi represents the angle from the binormal or tangent on the surface. Hence the BRDF is a function that has four parameters: the incoming theta and phi (typically from a light source), and the outgoing theta and phi (typically towards a viewer). Further, the BRDF is usually different for different frequencies.

It can be shown that conventional illumination models, like Phong and Lambert, as well as some of the more complex ones, like Cook-Torrance can be modelled as BRDFs.

It is not usually used for real-time polygon rendering, but it is used quite a lot for ray tracing and off-line polygon rendering systems, such as RenderMan.

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