[Bf-cycles] Velvet Shaders Sigma

Brecht Van Lommel brechtvanlommel at pandora.be
Mon Aug 12 16:18:09 CEST 2013

It could indeed use a better name, I don't know what other renderers
call it. It's sigma as in the the variance of the normal distribution,
controlling the sharpness of the peak. It's a kind of roughness I

Velvet is made up of small threads along the normal of the surface,
which means if you look straight at the surface there is no light
reflected, only when viewing them from the side do you get light
reflection. In practice threads aren't quite placed perfectly and
there's also scattering going on between them, so this idealized model
isn't enough and you would usually mix it with a diffuse shader.

There is no light vector available for shading nodes so you couldn't
do these calculations there.


On Mon, Aug 12, 2013 at 11:06 AM, Greg Zaal <gregzzmail at gmail.com> wrote:
> 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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