The BRDF expresses a simple model of light interaction
with a surface; it states what fraction of the incoming light from a
particular direction ω' is sent off in the direction ω :
The simpler models of diffuse reflection, specular reflection, and Phong reflection are idealized special cases of the general 4-dimensional BRDF. Highly anisotropic BRDF: Brushed Aluminum If we also consider the dependency on wave length, the BRDF becomes 5-dimensional. |
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Subsurface Scattering: A more sophisticated model of reflection also takes into consideration subsurface scattering (important for realistic rendering of faces, or marble statues). In this case photons may make a few bounces inside the matrial before they pop out again; thus they may emerge in a slightly different location from where they entered. Alabaster Horse, by M. Goesele et al Face Comparison, by H.W. Jensen et al |
This is of mostly academic/theoretical use. It calculates the amount of reflected light in any direction given full knowledge of the BRDF of the surface and of the incoming light distribution. It serves as the most abstract formal expression of the non-perceptual (physical) aspect of rendering. All specific rendering algorithms can understood as finding (approximate) solutions to some parts of this equation.