Scattering media behaves quite strangely when you put
a light_source directly into it.

Consider following scene, a box containing scattering media
and a light source just above it:

camera { location -30*z look_at 0 }
plane { y, -15 pigment { checker rgb<1,0,0> rgb<0,1,0> scale 10 } }
light_source { 10.1*y, 1 media_attenuation on }
box {
  <-10,-10,-10>, <10,10,10> hollow pigment { color rgbt<1,1,1,1> }
  interior { media { scattering { 1, 0.2 extinction 1 } } }
}

Rendering it gives what one expects: glow in the upper part
of the box, the lower part being almost dark since little
light come there (most is scattered).

But changing the third line to

light_source { 9.9*y, 1 media_attenuation on }

i.e. putting the light source inside the box, makes the media
to emit an abusrd amount of light.  If fact, it seems the
amount of light increases with distance from light source.

Observed on:

  PoV-Ray 3.5 (3.50a)
  RedHat Linux 7.3, linux-2.4.18, glibc-2.2.90
  AMD Athlon(tm) Processor stepping 02, 512MB of RAM

Yeti

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