<div dir="ltr">That makes sense, thanks Brecht.</div><div class="gmail_extra"><br><br><div class="gmail_quote">On 12 August 2013 16:18, Brecht Van Lommel <span dir="ltr"><<a href="mailto:brechtvanlommel@pandora.be" target="_blank">brechtvanlommel@pandora.be</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">It could indeed use a better name, I don't know what other renderers<br>
call it. It's sigma as in the the variance of the normal distribution,<br>
controlling the sharpness of the peak. It's a kind of roughness I<br>
guess.<br>
<br>
Velvet is made up of small threads along the normal of the surface,<br>
which means if you look straight at the surface there is no light<br>
reflected, only when viewing them from the side do you get light<br>
reflection. In practice threads aren't quite placed perfectly and<br>
there's also scattering going on between them, so this idealized model<br>
isn't enough and you would usually mix it with a diffuse shader.<br>
<br>
There is no light vector available for shading nodes so you couldn't<br>
do these calculations there.<br>
<span class="HOEnZb"><font color="#888888"><br>
Brecht.<br>
</font></span><div class="HOEnZb"><div class="h5"><br>
On Mon, Aug 12, 2013 at 11:06 AM, Greg Zaal <<a href="mailto:gregzzmail@gmail.com">gregzzmail@gmail.com</a>> wrote:<br>
> Hey guys,<br>
><br>
> I'm curious about the implementation of the Velvet shader - I can see that<br>
> it only reflects light at grazing angles and that the Sigma value is a sort<br>
> of IOR of some fresnel calculation, but how is it different from simply<br>
> connecting fresnel/facing ratio to a diffuse shader?<br>
><br>
> Why the name "Sigma" and not something a little more artist-friendly? Do<br>
> other renderers use the same term?<br>
><br>
> With only a single light source, there's a big black patch near the 0-degree<br>
> grazing angle, facing directly at camera - is it possible to control the<br>
> size of this big black patch or is it a fixed constant due to the concept of<br>
> a velvet shader?<br>
><br>
> But really I'd just like to know why this shader exists - what it's intended<br>
> use is and why we can't just create the same effect using other shaders and<br>
> some fresnel/light vector calculations.<br>
><br>
> The wiki for the Sigma value is empty, so I'll update it with anything you<br>
> can tell me.<br>
><br>
> Cheers,<br>
> Greg<br>
><br>
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