[Radiance-general] Talk: "A Decade of Radiance Tricks"

Mark Stock mstock at umich.edu
Sat Apr 9 08:28:30 PDT 2011


Those are good questions. In lieu of actual machinery that would 
return real reflectances, I'd had to come up with a scheme that 
would get get me close. The basis of the scheme is that RGB 
screen values aren't reflectances, and they shouldn't be treated 
as such.

I'll normalize to 0.8 or 0.9 because there is no real "pure 
white" in real life. Even white paint is rarely over 0.8. And 
with models with complicated geometry, even 0.8 requires a hefty 
number of ambient bounces to balance the energy---light really 
can penetrate into some deep spaces with 0.8. Consider that with 
a reflectivity of 0.9, only 10% of the energy is killed after 
every bounce, and you'll see that -ab will need to be 
prohibitively high to get an accurate rendering. And I choose 
Radiance for its accuracy, but I haven't got all month for a 

The squaring of the result is to push the values down closer to 
zero. Most of the interesting colors are in the 0.2 range, and 
that's just a hack to get closer. I am sure there's a better way, 
but that's easy to remember and quick to compute.


On Sat, 9 Apr 2011, Terrance Mc Minn wrote:

> Hello Mark,
> Many wonderful points of inspirations  - thanks for sharing.
> Question - page 11 RGB Estimate:
> 1) Why are you normalising to 0.9 rather than full scale of 1?
> 2) Why are you squaring the result?
> *Terrance Mc Minn
> ***

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