Unless I'm missing something, when using multiple media statements, a medium
with a color of rgb<0,0,0> should have no effect on prior media.  Instead, the
scene below seems to show contrary behavior.

This happens with 3.6.1.msvc8.win64, 3.7.0.beta.34.msvc9.win64, and with Linux
beta.34.

~David

/*
These two spheres should appear exactly the same,
according to sections 2.6.2.1.1 - 2.6.2.1.3.
A media color of rgb<0,0,0> means no light is effected.
However, including a second media statement with a
color of rgb<0,0,0> does effect the object's appearance.
In this example, the effect is more subtle, but still
discernible, for black absorption and emission media.
*/

sphere{<0,0,0>,1 hollow pigment{rgbt<1,1,1,1>}
  interior{
    media{ density{onion} scattering{ 4, rgb<1,0,0> } }

  }
  translate -x
}

sphere{<0,0,0>,1 hollow pigment{rgbt<1,1,1,1>}
  interior{
    media{ density{onion} scattering{ 4, rgb<1,0,0> } }
    media{ density{onion} scattering{ 2, rgb<0,0,0> } }
    //media{ density{onion} absorption rgb<0,0,0>}
    //media{ density{onion} emission rgb<0,0,0>}
  }
  translate x
}


sky_sphere {pigment {color <0.3,0.3,1> } }

camera {
  location  <0, 0, -4>
  direction 1.5*z
  right     x*image_width/image_height
  look_at   <0,0,0>
}

// The light source is directly behind the camera.
light_source {<0, 0, 4000> color rgb <1,1,1> }

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