[OpenSCAD] Discuss manifoldness, co-incident faces edges etc

arnholm at arnholm.org arnholm at arnholm.org
Thu Nov 14 11:11:02 EST 2019

On 2019-11-14 16:46, Doug Moen wrote:
> On Thu, Nov 14, 2019, at 10:32 AM, arnholm at arnholm.org wrote:
>> Attempting to automatically repair the kind of self intersection you
>> describe, i.e. two faces intersecting will give "interesting" results.
>> If you split the two faces into say 4 triangles it implies creating a
>> 4-manifold edge... so an automatic repair attempt of such issues is
>> mostly self defeating.
> Thanks for correcting me on that.
> Actually, the repair would result in a 2-manifold mesh *if* topology
> information is part of the mesh, because the repair would create
> duplicated vertexes in the vertex list.

I cannot see how this can be true. If the self intersection happens 
because a 2-manifold body (with full topology representation) somehow 
folds in on itself and creating overlapping volumes, the face splitting 
will 4-manifold edges in all cases. The only way to avoid that is if you 
could device a scheme where the volume overlap could be eliminated by 
deleting faces that was inside the volume, i.e some kind of boolean 
union with itself. That however, is a challenging task.

So I maintain that self intersection is a symptom of some other problem 
that needs to be fixed. Trying to fix it by attacking the symptom is 
likely not leading to a desirable result in most cases.

Carsten Arnholm

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