I've been meaning to report this problem forever. The other problem that I
had noticed with Round_Box is that if the radius is exactly half the length
of one of the edges, you get "Parse Error: Degenerate cylinder, base point =
apex point". For example:
  #include "shapes.inc"
  Round_Box_Union (<5,0,0>, <10,15,5>, 2.5)

FWIW, here's my corrected version of the Round_Box macro in shapes.inc,
which fixes both problems.

// -----BEGIN-----
#macro Round_Box(A, B, EdgeRadius, UseMerge)
   #local AA = <min(A.x, B.x), min(A.y, B.y), min(A.z, B.z)> + EdgeRadius;
   #local BB = <max(A.x, B.x), max(A.y, B.y), max(A.z, B.z)> - EdgeRadius;
   #local LBF = AA;
   #local RBF = < BB.x, AA.y, AA.z>;
   #local RBB = < BB.x, AA.y, BB.z>;
   #local LBB = < AA.x, AA.y, BB.z>;
   #local LTF = < AA.x, BB.y, AA.z>;
   #local RTF = < BB.x, BB.y, AA.z>;
   #local RTB = BB;
   #local LTB = < AA.x, BB.y, BB.z>;

   #if(UseMerge)
      merge {
   #else
      union {
   #end
      sphere {LBF, EdgeRadius}
      sphere {RBF, EdgeRadius}
      sphere {RBB, EdgeRadius}
      sphere {LBB, EdgeRadius}

      sphere {LTF, EdgeRadius}
      sphere {RTF, EdgeRadius}
      sphere {RTB, EdgeRadius}
      sphere {LTB, EdgeRadius}

      #if (AA.x != BB.x)
         cylinder {LBF, RBF, EdgeRadius}
         cylinder {RBF, RBB, EdgeRadius}
         cylinder {RBB, LBB, EdgeRadius}
         cylinder {LBB, LBF, EdgeRadius}
      #end

      #if (AA.y != BB.y)
         cylinder {LBF, LTF, EdgeRadius}
         cylinder {RBF, RTF, EdgeRadius}
         cylinder {RBB, RTB, EdgeRadius}
         cylinder {LBB, LTB, EdgeRadius}
      #end

      #if (AA.z != BB.z)
         cylinder {LTF, RTF, EdgeRadius}
         cylinder {RTF, RTB, EdgeRadius}
         cylinder {RTB, LTB, EdgeRadius}
         cylinder {LTB, LTF, EdgeRadius}
      #end

      box {AA-EdgeRadius*x, BB+EdgeRadius*x}
      box {AA-EdgeRadius*y, BB+EdgeRadius*y}
      box {AA-EdgeRadius*z, BB+EdgeRadius*z}
   }
#end
// -----END-----

Anders

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