> Ah, I checked those.  It works as expected.  light_groups are meant to place
> local fill-lights.  There is no other interaction defined by a light_group.
> The problem in the given scenes is essentially the same misconception as
> assuming a group-based shadow interaction.  This is not what light_groups
> do.

I still do not understand my misconception (and I do understand why shadows
and light groups interact the way they do).
Do you mean that scattering media should not be placed in light groups?
If yes, then I would like to know why that restriction was chosen (and it
should be in the docs). You refer to fill-lights. According to the docs
those are shadowless. If it is really the case that light groups are meant
to only work with shadowless lights then either all lights in light groups
should automatically be made shadowless or the docs should at least say so.
Otherwise I see no reason why scattering media should be disallowed in light
groups.
If no (meaning that scattering media is supposed to work with light groups),
then I do not understand the following scene. I do not understand
  - why absorbing and scattering media behave differently without a light source
  - why the scattering medium behaves differently according to which surfaces the
    ray traverses.

Please explain,

   Mark



plane {y (-5) pigment {rgb 1}}

light_source {<-1,1,-1>*10 1}

light_group {
#local i=0;
#while (i<=10)
   sphere {
     <2*i-10,i/5-5.5,15> 1
     pigment {transmit 1}
     hollow
     #if (mod (i,2))
       interior {media {absorption 1}}
     #else
       interior {media {scattering {1 1}}}
     #end
   }
   #local i=i+1;
#end
global_lights off
}


-- 
merge{#local i=-11;#while(i<11)#local
i=i+.1;sphere{<i*(i*i*(.05-i*i*(4e-7*i*i+3e-4))-3)10*sin(i)30>.5}#end
pigment{rgbt 1}interior{media{emission x}}hollow}//  Mark Weyer

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