Am 03.11.2018 um 14:12 schrieb Robert Munyer:

> I'm trying to see if any other CSG-related programs have this problem.
> I tried the three other CSG programs that happen to be installed on my
> system.  I asked people at the local hackerspace who have other CSG
> programs.  None of the other programs seem to have this problem.

That's probably because CSG-capable programs usually come from the CAD 
modelling world, not the 3D rendering world.

POV-Ray has a bit of a handicap in that it isn't totally committed to 
solid geometry, and instead supports a random mix of patch and solid 
primitives.

As a consequence, it can't just use the "direction" of the surface to 
determine where the volume begins or where it ends; instead, it uses a 
separately defined notion of "insideness" of a given point.

So the case you describe does pretty much fall into POV-Ray's equivalent 
of this one:

> If two coincident surfaces have opposite orientation (i.e. one of
> them is trying to show a surface whose interior side faces the camera,
> and the other one is trying to show a surface whose exterior side faces
> the camera) most CSG programs will render unintended geometry:

Because POV-Ray doesn't distinguish surface orientation in its 
implementation of CSG.


I have toyed with some ideas of how to change that and do make use of 
surface orientation to address the issue at least for certain standard 
cases, but it would require some major changes, and my supply of 
king-size Round Tuits(TM) is running low, as always.

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