On 06/17/2018 09:12 PM, jr wrote:
> 
> "the idea" is very simple, I would like to animate some visible, emissive media
> objects such that they move through other, not directly seen (contained) media.
> the effect I'm after is that the additive effect of the differently coloured
> media when intersecting should reveal the outline of the "hidden" objects
> through the colour change(s).
> 
> 

With v3.8 user_defined{} density is an option. The scene below does 
something close to what you want - I think.

Bill P.

//------------------------------------------
#version 3.8;
global_settings { assumed_gamma 1 }
#default { finish {ambient 0.000 } }
#declare Grey20 = srgb <0.2,0.2,0.2>;
background { color Grey20 }
#declare Camera00 = camera {
     perspective
     location <3,3,-3.001>
     sky y
     angle 35
     right x*(image_width/image_height)
     look_at <0,0,0>
}
#declare White = srgb <1,1,1>;
#declare Light00 = light_source { <50,150,-250>, White }
#declare Red = srgb <1,0,0>;
#declare CylinderX = cylinder { -1*x, 1*x, 0.01 pigment { Red } }
#declare Green = srgb <0,1,0>;
#declare CylinderY = cylinder { -1*y, 1*y, 0.01 pigment { Green } }
#declare Blue = srgb <0,0,1>;
#declare CylinderZ = cylinder { -1*z, 1*z, 0.01 pigment { Blue } }

#declare SphereMediaContainer = sphere { <0,0,0>, 1 }

#declare SphereHidden = sphere { <0.2,0.1,0.2>, 0.3 }

#declare BoxForReveal = box { <-0.5,-0.5,-0.5>,<0.3,0.4,0.3> }

#include "functions.inc"
#declare FnSphereHidden = function {
     pattern { object { SphereHidden } }
}
#declare FnBoxForReveal = function {
     pattern { object { BoxForReveal } }
}
#declare FnZero = function (x,y,z) { 0 }
#declare Density00 = density {
     user_defined {
         function { FnBoxForReveal(x,y,z) },
         function { FnSphereHidden(x,y,z)*FnBoxForReveal(x,y,z) },
         function { FnZero(x,y,z) },,
     }
}
#declare Media00 = media {
     method       3
     samples      100
     emission     rgb <1,1,1>
     density { Density00 }
}
#declare Clear100 = srgbt <1,1,1,1>;
#declare Texture00 = texture { pigment { color Clear100 } }
#declare Interior00 = interior { ior 1 media { Media00 } }
#declare Material00 = material {
     texture { Texture00 }
     interior { Interior00 }
}
#declare ObjectMediaContainer = object {
     SphereMediaContainer
     material { Material00 }
     hollow
}

//--- scene ---
    camera { Camera00 }
    light_source { Light00 }
    object { CylinderX }
    object { CylinderY }
    object { CylinderZ }
    object { ObjectMediaContainer }

//------------------------------------------

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