Am 30.04.2017 um 06:56 schrieb dick balaska:

> Thanks. That was it.  I had switched it from union to merge; I was
> getting weird coincidence errors on the main outer box. They've gone
> away too.
> (I think I was doing union {box {color Red} box {color Red}} instead of
> union {box {} box {} color Red}
> )
> 
> Weird that merge has that issue. I would think it shouldn't matter for
> this application, just be slower.

Not so weird if you understand how merge works internally.

In a union, coincident surfaces "only" compete for texture and interior.

In a merge, coincident surfaces _additionally_ make a mess of the
geometry itself.

The reason is that a merge works by ignoring surfaces of any member that
are found to be inside any of the sibling objects. If a surface is
coincident with that of any sibling, it's more or less pure chance
whether that surface will be judged outside that sibling object (=
visible) or inside it (= invisible).

(The same is true for intersection and difference as well.)

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