This shader is an updated and slightly more physical version of the fast scatter shaders. The primary differences is that scatter radii are defined per color (with individual scatter radii for the red, green, and blue components). Also, the falloff in energy of the light is always exponential, unlike the misss_fast_shader which has an explicit parameter to modify the falloff.
The primary benefit with misss_fast_shader2 is that it allows a more realistic effect, since scattering in the real world is exponential, and wavelength-dependent. This also lowers the number of scattering layers needed for a realistic effect; in the past at least two layers were needed to simulate skin, to allow the red color to scatter further. This is no longer necessary with this shader (although multiples can still be used for greater artistic control.)
For information on this shader, see the mental ray Documentation at www.autodesk.com/mentalray-help-2013-enu.
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