[Radiance-general] better results on genBSDF

[Greg Ward]

The genBSDF program produces a limited resolution matrix representation using the options you've given.  This is appropriate if you want to use it with WINDOW (although I think some modifications to the output are still required) or if your material is fairly diffusing.  It will not resolve the direct peak to anything finer than 10 degrees, which is the resolution of the full Klems matrix basis.  Some spreading of the direct is unavoidable.

You can improve upon this using the tensor tree formulation by setting the -t3 or -t4 option.  If your system produces an isotropic distribution (i.e., you can rotate the system about its center with no change to the output), you can try "genBSDF -t3 6" or so.  In the more general case, you can use "genBSDF -t4 6", which will resolve the direct component to within a few degrees.  You can increase to "-t4 7" to get twice the resolution, but you'll have to increase the -c parameter as well, and I can't predict when the calculation will finish.

The real solution is to incorporate and employ the actual system geometry using the proxy method described near slide 7 in my 2011 workshop presentation:

	http://www.radiance-online.org/community/workshops/2011-berkeley-ca/presentations/day2/GW5_BSDFFirstClass.pdf

Cheers,
-Greg

> From: Germán Molina Larrain <gmolina1 at uc.cl>
> Date: February 16, 2013 11:49:09 AM PST
> 
> Hi Lars, 
> 
> considering that the specular transmission is that light that does not touch any surface, the analytical solution would be the exact one (am I right?). Also, HERE, those results were compared agains TracePro, and the results of the forward ray-tracing gave, basically, the same results as the model I am using.
> 
> THANKS
> 
> German 
> 
> 2013/2/16 Lars O. Grobe <grobe at gmx.net>
> Hi German!
> 
> > I am using genBSDF to get the BTDF of a shading system, comparing it to
> > an analytical model using the radiosity method. No surprise, the
> > calculations seem to be really good, nevertheless, there are some
> > important differences on directly transmitted (specular) calculations. I
> 
> Why do you think that "an analytical model using the radiosity method"
> results in "better" results than raytracing does for specular transmission?
> 
> Cheers, Lars.
> 

---------------------------
> From: Germán Molina Larrain <gmolina1 at uc.cl>
> Date: February 16, 2013 7:06:26 AM PST
> 
> Dear list, 
> 
> I am using genBSDF to get the BTDF of a shading system, comparing it to an analytical model using the radiosity method. No surprise, the calculations seem to be really good, nevertheless, there are some important differences on directly transmitted (specular) calculations. I am using the following parameters:
> 
> genBSDF -n 4 -c 4000 -r '-ab 4 -ad 512 -as 0 -aa 0 -ds 0.01 -dj 0' material.mat geometry.rad > ../TMX/file.xml
> 
> I tried modifying the -c option, but it did not show any improvements, so I stayed on 4000. Any suggestions from gurus?
> 
> THANKS
> 
> German
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