BSSRDF for dummies

BSSRDF for dummies

Post by marsplast » Sun, 02 Sep 2001 09:53:04



I have been following treads regarding the subject for a while and
I have have worked for a a week or so to implement the BSSRDF
algorithm in renderman shading language.  I have actually written and
compiled a shader that simulate one layer of skin but it's not giving
any interesting result. It's actually kind of wacky. So I went through
the paper one more time and realized that may be some of the
assumptions
that I made weren't correct. I would like to know if there is
someone out there that can kindly verify my theories.
To calculate the diffuse term of the BSSRDF I'm generating a bunch of
point in a plane around P. I'm not compensating for any curvature
cause
reading the paper seemed to me that the all theory olds together
because
we assume that we are dealing with a flat surface. When I have a new
P I compute equation (4) of the paper  and than I use a Monte Carlo
method to approximate the diffuse factor of the BSSRDF function on the
original point P.
I understand that to compute the diffuse light I have to use a dipole
method, but even if I create a light under the surface, what is his
intensity? The intensity of the original light shining at that point?
And even if this is the case, where I have to put it? Equation (4)
doesn't have any light! What is "alpha one" in equation (4) of the
paper? I didn't find any explanation so I assumed that was the reduced
albedo coef ( Reduced scatering/absorption coef).
To calculate the single scattering I calculate the transmitted ray T
at
the point Xi ( one of my generated points) where the light enter in
the surface . Then, knowing the
thickness of the layer I calculate the lent of T traveling inside the
layer and from there I create a new vector from Xo, where the light
exit
( my original P) to the intersection point of the T vector and the end
of the layer. this new vector is going to became my outgoing
refracted ray where I sample the single scattering term. Does it make
sense?
Sorry if all this is terribly wrong or stupid, any help is greatly
appreciated.

Mars

 
 
 

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http://www.renderman.org/RMR/Examples/srt2002/PrmanUserGroup2002.ppt

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