[Bf-cycles] Velvet Shaders Sigma

[Greg Zaal]

Hey guys,

I'm curious about the implementation of the Velvet shader - I can see that
it only reflects light at grazing angles and that the Sigma value is a sort
of IOR of some fresnel calculation, but how is it different from simply
connecting fresnel/facing ratio to a diffuse shader?

Why the name "Sigma" and not something a little more artist-friendly? Do
other renderers use the same term?

With only a single light source, there's a big black patch near the
0-degree grazing angle, facing directly at camera - is it possible to
control the size of this big black patch or is it a fixed constant due to
the concept of a velvet shader?

But really I'd just like to know why this shader exists - what it's
intended use is and why we can't just create the same effect using other
shaders and some fresnel/light vector calculations.

The wiki for the Sigma value is empty, so I'll update it with anything you
can tell me.

Cheers,
Greg
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