> It looks like povray does some optimizations in the case of a
> constant density media. I did a quick check by adding the following
> density to your code:
> 
> density {
> ~  spherical
> ~  color_map {
> ~    [ 0.0 rgb 1.0 ]
> ~    [ 1.0 rgb 1.0 ]
> ~  }
> }
> 
> Since the colormap is rgb 1.0 everywhere, this should give the same
> results as the original but it doesn't. The result is pretty much
> what I described in my previous post (except that samples 129 takes
> much longer to render since there are a lot of pure black samples
> taken that get culled in the samples 3 case). You may take a look for
> example at those pictures done with aa_threshold 1e-200:

Which POV-Ray version and platform are you using? I could not reproduce your
results, I got quite the same effect I was getting with constant density:
overbright media with "aa_level 8 samples 3" even if I introduce a slope in
the color_map (then the media is different, of course, but still not
similar when the samples and aa_level are varied).

-- 
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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