[Radiance-general] BRTDFunc material tutorial?

[Greg Ward]

Hi Kirk,

Probably the simplest example using the BRTDfunc type is the  
glaze.csh script output and its associated glaze1.cal and glaze2.cal  
auxiliary files.  These may be found in the ray/src/gen directory of  
the latest Radiance distribution.  If this is not enough for you, you  
can look at the attached He.cal file for ideas.

It might be possible to make use of the Curet database, but no one  
has done it yet with Radiance, as far as I know.  You would probably  
want to apply the plasdata or transdata primitives, rather than  
BRTDfunc, which is awkward to use with a large dataset directly.

I'm sorry I don't have any better examples of BRDF use, but the  
common method is to fit the data to one of the Gaussian distribution  
models, which offer more complete and efficient ray interactions.   
Specifically, the BRDF types do not account for indirect specular  
components, though I believe there is some support for this in the  
photon map extension.

At the 2005 Eurographics Symposium on Rendering, Addy Ngan presented  
the following paper:

Experimental Analysis of BRDF Models
Addy Ngan, FrŽdo Durand, and Wojciech Matusik

which is online at:

	http://people.csail.mit.edu/addy/research/brdf/

The "supplemental document" linked above contains a very nice list of  
BRDF materials and their fits to the "Ward model," which is used in  
Radiance.  If you are applying this to a recent version of Radiance  
(3.6 or later), then you'll actually want to use the coefficients for  
the "Ward-Duer model," as it includes the correction to the  
normalization of my original model, pointed out by Arne Duer of the  
Univ. of Innsbruck (and also Don Walters of Boeing Corporation).   
Unfortunately, Ngan et al. only cover isotropic materials in this  
document.  They did measure a couple of anisotropic materials, and I  
have further information on those, but there isn't really enough and  
the fit wasn't good enough to make them worth using in Radiance.

I hope this helps.
-Greg

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> From: Kirk Thibault <kthibault at biomechanicsinc.com>
> Date: January 27, 2006 12:39:03 PM PST
>
> Interesting site:
>
> http://www1.cs.columbia.edu/CAVE/curet/
>
> is this kind of database directly applicable to a BRTDFunc?
> ------------------------------
>
> Kirk L. Thibault, Ph.D.
>
> On Jan 27, 2006, at 1:02 PM, Kirk Thibault wrote:
>
>> Hi folks,
>>
>> I'm trying to do some simple tests within Radiance of material  
>> parameters and their appearance and I was wondering if anyone has  
>> a good explanation of the BRTDFunc and how one goes about  
>> structuring a .cal file for it, etc.
>>
>> Thanks,
>>
>> kirk