> I've attached two images: "test1" has samples 10, "test2" has samples 200.

I would guess that what is happening is in test1, the first sample of a ray
is well lit and the other 9 are poorly lit or dark (due to self shadowing).
While in test2, the first sample is well lit and the other 199 are poorly
lit or dark. So the percentage of lit samples in the first is much greater,
causing it to be significantly brighter. This is only happening because the
media is so dense that it becomes dark at a very shallow level.

I would assume that the greater number of samples is producing the more
accurate result.

One thing to try: make two nested spheres, the inner one about 4/5 the
diameter of the outer one. Give them the same media, but give the outer one
much more samples. Then the edge of the media (in the outer sphere) will be
well sampled - as it needs to be to avoid this problem, but the inside of
the media (the inner sphere) will be poorly sampled, which is ok since it's
all dark in there anyway. If this works, it may be the fastest way to get
the effect with accuracy. (Be sure, of course, to avoid having the sphere's
volume's overlap (use a difference on the outer sphere to do this), and to
avoid coincident surfaces.)

> In my experiments, I did not see any significant difference in the output
by
> fiddling with aa. According to your explanation, "samples 10 aa_level 8
> aa_threshold 0.0001" should give similar results to "samples 160 aa_level
4
> aa_threshold 0.0001", and I don't see that with the above example. Could
> you give a more clear example?

When the media has a constant density, the first case will take 10 samples
and the latter case will take 160 samples. Only if the aa_threshold is set
to 0 (or if the media has a very noisy density) will the two samplings be
equivalent.

 - Slime
 [ http://www.slimeland.com/ ]

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