In article <Rgk### [at] econymdemoncouk>, 
nos### [at] econymdemoncouk says...
> Yes. It's more complicated than I thought.
> 
> The effect doesn't appear when using a box, but it does appear when
> using "difference {  plane {y, -5}  plane {y, -1000}" when the lower
> plane is sufficiently distant. When using this difference, the sphere
> can be replaced with "background {rgb 1}" and the effect remains exactly
> the same.
> 
Yeah.. This:

//POV-Ray 3.6.1.icl8.win32

global_settings {
  assumed_gamma 1
  max_trace_level 170

  radiosity {
    pretrace_start 0.08
    pretrace_end   0.04
    recursion_limit 1
    low_error_factor 0.5
    minimum_reuse 0.015
    adc_bailout 0.01/2
    media on
  }
}

camera {
   location  <6, 8, 10>
   up        <0, 1, 0>
   right     <1, 0, 0>
   look_at   <0, -1, 0>
}

#declare fsqr = function(a) {a*a}

plane {
  y, -5
  hollow

  pigment { rgbf 1 }
  finish { diffuse 0
           ambient 0 }

  interior{
   media{
    intervals 20
    emission 0.09
    scattering { 4, 1 }
    density{
     function { fsqr(sin(x/4)*sin(y/4)*sin(z/4)) }
    }
   }
  }
}

plane {
  y, -100
  hollow
  pigment {rgbf 1}
  inverse
} 

Probably comes a lot closer to what it 'should' look like. It still gets 
artifacts if the second place is at -200, so you may have been partly 
right, or maybe completely, I didn't let the previous 'working' version 
with the sphere finish so can't compare them. Likely the light bounces 
around so make times that anything 'thinker' than about 100 units 
generates artifacts. Probably an effect of trace level or some other 
setting, though increasing it to 'fix' the problem would slow the media 
down even more. The artifacts this produces btw are very different when 
you place the second plane at -15000, where the sphere limit would have 
been, but with some similarities, so the 'shape' does effect the 
artifacts when they do appear.

-- 
void main () {

    call functional_code()
  else
    call crash_windows();
}

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