Tim Nikias wrote:
> I'd expect POV to shoot rays for focal-blur, but what does
> it actually do to judge if the pixel-color traced so far is
> probably reaalllly close to the exact color? How can
> POV be "confident" about that? I guess that variance,
> telling in the docs that its a tolerance on "samples taken
> so far", just tells POV: "See, the most recent samples all
> look pretty much the same, lets stop tracing."
> 

I only know some school book statistics, but based on that,
I'd say that's a pretty close guess. IIRC, the theory is
based on the idea that in a random sampling, the sum of all
samples tend to gravitate toward some mean value. That mean
itself will fluctate as more samples are taken, but will
tend to become increasingly fixed as a sufficiently
representative number of samples have been taken. I think
at this point, average variance from the mean is used to
determine what constitutes "sufficiently representative."

It's a good question, though, because I'm not clear why POV
needs both a variance and confidence keyword. I gathered that
[1.0 - confidence] yields the threshold below which average
variance must fall, I believe, to consititute "close enough."

My guess is that variance as used in POV terms will terminate the
sampling early if the confidence has *not* been reached, *but*
the variance is now so low anyway that it can't possibly contribute
to the pixel color (given the output bit depth). Sort of a double
adaptive method that offers the sampling two possible reasons to
stop to make absolutely sure no more samples are taken than
matter.

-- 
@C[$F];
The Silver Tome ::  http://www.silvertome.com
"You may sing to my cat if you like..."

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