[Radiance-general] Question on Setting ambient parameters

[Gregory J. Ward]

Hi Rick,

The AVGREFL macro notwithstanding, you shouldn't be converging to the  
wrong result when you increase the -ab setting under any  
circumstances.  This sounds suspiciously like a bug, and it may be  
related to the sampling bias problem that Rick Mistrick and Younju  
Yoon ran across in their work at State College (Pennsylvania).  It  
only shows up in very particular situations, but maybe yours is one  
of them.

If you don't mind, please send me your test scene privately, and the  
commands you used to generate your results.

Thanks,
-Greg

> From: "Boettger, Richard G. \(JSC-SF\)[LM]"  
> <richard.g.boettger at nasa.gov>
> Date: April 6, 2006 8:55:18 AM PDT
>
> Hi All,
>
> We have been using Radiance in our lab for many years performing
> lighting analyses mainly for exterior (on orbit) activities involving
> the shuttle and the space station. We are now beginning to do more
> interior lighting evaluations such as the space station laboratory
> module.
>
> The interiors we evaluate tend to have a lot of white, non specular
> surfaces. So diffuse interreflections seem to be important. I've been
> looking into the parameters that control the ambient calculation in
> Radiance and I am baffled by the "-ab" ambient bounces parameter. I
> thought, by keeping all other parameters the same and increasing the
> ambient bounces, the results should be more accurate. That is not  
> what I
> see from our test case.
>
> We built a box 17" x 17" x 36" with white walls. The wall material
> properties were measured. A calibrated light source was placed in a  
> hole
> at one end of the box. Illuminance measurements were taken at various
> points within the closed box. We then modeled the environment in
> Radiance and took illuminance values at the same points. After some
> experimentation I settled on the following ambient settings:
>
>       av: 0.0 0.0 0.0
>       aa: 0.08
>       ab: 3
>       ad: 1024
>       as: 512
>       ar: 64
>
> The errors between Radiance and measured values were less than 10%.
>
> But as I increased the ambient bounces beyond 3, the illuminance  
> values
> at the points kept increasing. By 5 bounces the errors were averaging
> over 60%.
>
> It appears that I am missing something fundamental. I'm hoping the  
> group
> can help me out.
>
> Thanks,
> rick