## BSSRDF for dummies

### BSSRDF for dummies

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

Has anyone implemented Christophe Hery shadow buffer technique to
implement BSSRDF Subsurface scattering? I'm trying to find an example
working model I can study. I don't quite understand how this is
implemented.

If you didnt see the Stupid Renderman Tricks for 2002 you can get the
code and information here at
http://www.renderman.org/RMR/Examples/srt2002/PrmanUserGroup2002.ppt

Thanks,

Phillip

3. BSSRDF