[tuhopuu-devel] Re: Minnaert diffuse shader

[Jonathan Merritt]

Hi Everyone,

In the absence of further comments, I've played with the Minnaert model 
a bit more and committed my latest changes.  You can see an example of 
the diffuse shading models here:
    http://www.warpax.com/temp/diffuse-test.png
Sorry about my choice of a really gaudy purple.  It's getting late over 
here, so I don't feel like fixing it.  Please make your own demo images, 
and post them to Elysiun! :-)

The darkness parameter of the Minnaert shader can now take on both 
positive and negative values.  Positive values tend to look more like a 
specular model, or a metal, because they darken the "glancing edges" of 
an object.  Negative values look more like velvet, because they lighten 
the edges.  (See the demo image.)

A major problem with all versions of the shader (mine, nVIDIA's and the 
"original"), is that in the "velvet" mode, they all show very large 
peaks in intensity around the edges of the object (caused by raising 
small positive numbers to a negative power).  This caused some quite 
unsightly artifacts in the original patch version.  To combat these 
artifacts, I have used a "clamp and attenuate" scheme.  The algorithm 
for i is:

i = nl * pow( MAX2( nv*nl, 0.1 ), darkness );
if (darkness < 0.0)
    i /= pow( 0.1, darkness );

As you can see, the first part is the ordinary Minnaert shader with 
slight clamping of of low nv*nl.  The second part isolates the "velvet" 
case, and reduces the overall brightness so that the rim will never be 
greater than 1.0 brightness (maybe I've overdone this darkening - it can 
be adjusted if necessary?).  The overall shader is still C_0 continuous 
at darkness = 0, with darkness = 0 being identical to the Lambertian shader.

I'm now wondering: "why limit this kind of attenuation to the Lambertian 
case alone?" ... why not attenuate any diffuse shader (eg: Oren-Nayar, 
Toon, etc.) in the same kind of way? :-)

Anyway, please play with this and tell me what you think! :-)

Jonathan Merritt.