Consider an object as
#declare obj = union {
cylinder{ <-1,-1,-1>,<1,1,1>,.1
cylinder{ <-1.5,-1.5,-1.5>,<1,1,1>,.1
}
I can center the object by its bounding box using max_extent and min_extent.
Evidently, the bounding box is centered not the visible object.
In this case, the right side is empty because there's nothing in the bounding
(the cylinder side is at z=1, the farthest point).
