Hi,

In developing my first POV-Ray scene I encountered a subtle artifact and I am
not sure how to get rid of it. I have a 100% transmissive cylinder containing
identically-transformed emissive media with a cylindrical distribution. When in
front of a dark reflective texture, the edges of the cylinder appear dark (a
little bit like a shadow, but obviously not a shadow). When in front of a
brighter or flatter texture, the edges of the cylinder do not appear darker than
the surroundings--the desired effect. This artifact seems a bit stronger when
the cylinder is partially embedded in the dark-textured object. This image shows
what I mean:

http://www.freeimagehosting.net/image.php?923dd7c03e.png

Could anyone suggest what causes this, and how I might get rid of it?

The following code reproduces this scene:

background { rgb 0 }
global_settings {
      max_trace_level 256 //just in case
}

//includes
#include "golds.inc"
#include "colors.inc"

//declare textures
#declare gold = texture
{
   T_Gold_1C
}

camera { location <2, 2, -7> look_at <0,0,0> }

light_source {<20, 40, 10>, rgb 1.5 media_interaction off shadowless }

plane { y,-3
        pigment { Blue }
        finish { ambient 0 diffuse .3}
}

box { <-1,-1,-1>, <1,1,1>
   texture {gold}
}

#declare med1 = media {
   emission 7
   intervals 1
   samples 10
   method 3
   density {
        cylindrical color_map {
         [0 rgb <0,0,0>]
         [1 rgb <1,0,0>]
      }
   }
}

#declare med1cont = cylinder {
         <0,1,0>, <0,-1,0>, 1
         pigment {rgbt 1}
}

merge {
   object { med1cont
      scale <.1,1.1,.1>
      translate <1,0,-1>
   }
   object { med1cont
      scale <.1,1.1,.1>
      translate <0,0,-2>
   }
   interior {
      media { med1
         scale <.1,1.1,.1>
         translate <1,0,-1>
      }
      media { med1
         scale <.1,1.1,.1>
         translate <0,0,-2>
      }
   }
   hollow
   no_shadow
}

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