Greg wrote: You can get single scatter by using a scattering distance of 0 in the regular material..I honesty never use the hair and skin shaders as I don't do people.
-G
Just short glossary. For these hair shaders, "scatter" means just an ordinary specular highlight. Could be replaced by ray-traced reflection, but nobody wants this (slow). Early single scatter Marcshner model, or XSI MuhHair, or later implementation called P_HairTK, consist about two or three specular highlights, shifted along hair, by angle (not distance). That's a fast approximation of what you usually see on middle tone hair, at high angle between hair and light ray, you see exaggerated base color, brown became reddish. At low angle there's white "reflection".
Dual scattering is well known from "Tangled" movie. It has a few highlights more, shifted to opposite side of light, invoked by first ones. There's recognizable slight greasy look. For those who play with hair shading, it has "I want this" effect, like ring in LOTR
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In order to get seccond scatter to work properly, there's need for a few steps more in shadow evaluation. In Modo, nice proof of "real" dual scattering model, is a plain black hair, when light don't cast shadow. Usually in Renderman compliant, deep shadow map is used, so shader evaluates just a bit longer.
Now in Modo 601, by word of author of shader, ray-trace shadows are only that works with this one - for now. My wild guess, they had no time to adapt renderer for deep shadow map in a way this shader likes. So there is a long render time, even Modo can do much better, I'd believe.
P.S. so, if Modo guys would be able to get some faster hair shader (even not so easy to tweak), I already have enough for.... one month of Modo gallery full of "L'Oréal" type of design
