A union of two boxes like

        union {
                box { <0,0,0> <1,1,1> }
                box { <0.5,0,0> <1.5,1,1> }
        }

 should be indistinguishable from a single box

        box { <0,0,0> <1.5,1,1> }

However, when one fills the union with an emitting medium, using code like

----------------- 8< ------------------
#include "colors.inc"

camera {
        location <2,2,2>
        look_at <1,0.5,0.5>
}

object {
 union {
  box { <0,0,0> <1,1,1> }
  box { <0.5,0,0> <1.5,1,1> }
 }
 pigment { rgbt 1 } hollow
 interior {
  media {
   emission <0,3,8,1>
   samples 1 100
  }
 }
}
----------------- 8< ------------------

povray renders three separate boxes filled with emitting media.

Tested with 3.6 and 3.7.

Best regards

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Am 18.04.2015 um 20:18 schrieb afm:
> A union of two boxes like
>
>          union {
>                  box { <0,0,0> <1,1,1> }
>                  box { <0.5,0,0> <1.5,1,1> }
>          }
>
>   should be indistinguishable from a single box
>
>          box { <0,0,0> <1.5,1,1> }

That's only true when the surfaces of the boxes are opaque. To suppress 
internal surfaces, use merge{} instead of union{}.

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