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The two top left-images in this paper exhibit an effect called "subsurface scattering", where light enters a surface at one point and leaves it elsewhere. (Hold your fingers up to a flashlight and you'll see a good example of this effect.) An efficient method for rendering images like skin and marble that exhibit this complex effect won a technical oscar in 2004 due to it's importance in realistic rendering.

Here's the paper:


Fixed graphics pipeline is able to handle diffuse and specular reflections easily because the color of a pixel can be calculated given the material and the lights' properties.

Subsurface scattering is a level more difficult to implement because the ray penetrates through the surface and leaves elsewhere. One pixel's luminosity might depend on other pixels'. To render a scene with subsurface scattering, one might have to solve the rendering equation using material property functions such as BSSRDF.


Yes but as long as you are close does it really matter, as we saw latter in the class. In graphics sometimes speed is more important then accuracy. I guess what I am saying is a near infinite number of materials may exist in the world but you can get close to all of them with a much smaller set of materials.