> That's because the darkening you see is an artefact :) It's due to
> the fact that when you don't take many samples, a lot of them
> proportionnaly get taken deep inside the media where it's dark.
> Increasing the number of samples ensures that more and more get taken
> in the thin layer near the surface where the SSS effect occurs which
> leads POV to notice that the result should be bright.

According to this, more samples -> brighter media. But I see the opposite,
as if the influence of the samples was "averaged" (since most samples are
deep inside, the effect of the surface ones is weaker). I would have
thought that the sampling algorithm (and colour calculation) would be
smarter than that, but maybe that's not so easy...

> BTW, you 
> shouldn't increase the number of samples too much. Instead you should
> increase the aa_level and possibly decrease the aa_threshold. This
> will ensure that the additional samples get taken in places where
> they will have a useful impact on the result.

I couldn't get the same effect... Even if the "darkening" due to many
samples (or intervals) is an artifact, I didn't find a reliable way to get
what I needed other than increasing the number of samples.

-- 
light_source{9+9*x,1}camera{orthographic look_at(1-y)/4angle 30location
9/4-z*4}light_source{-9*z,1}union{box{.9-z.1+x clipped_by{plane{2+y-4*x
0}}}box{z-y-.1.1+z}box{-.1.1+x}box{.1z-.1}pigment{rgb<.8.2,1>}}//Jellby

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