The Fast subsurface scattering shaders do not simulate true volumetric scattering but the "appearance" of scattering. They are producing a reasonable, visually pleasing, easy to use, consistent looking result that is free of artifacts, and have good frame-to-frame coherence (no flickering). One of the design goals of these shaders is to provide a way to efficiently render human skin.
The shaders are wrapped up into two simple Phenomena, which are built from a set of component shaders.
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