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

nop head nop.head at gmail.com
Sun Nov 17 17:33:48 EST 2019


So with 2 cubes sharing an edge, at a point on that edge you can't
decide if it is inside or outside, so it can't be an orientable 2-manifold
regardless of how it is represented on a computer. So it should just be an
error when modelling 3D objects, not something that should be catered for
by rewriting OpenSCAD.

Is is it correct to say polygons soups are fine for representing orientable
2-manifolds and all physical objects are orientable 2-manifolds?

Having edge connection data that contradicts vertex coordinates seems wrong
to me. You can have logically distinct points that are physically
coincident and self intersections which go unnoticed. This seems to be what
3MF encourages as a solution to the hard problem of actually
outputting topologically correct meshes after coordinate truncation.





On Sun, 17 Nov 2019 at 21:21, Carsten Arnholm <arnholm at arnholm.org> wrote:

> On 17.11.2019 21:11, adrianv wrote:
> > It sounds like you
> > want to talk about the face count of an edge, perhaps, the number of
> faces
> > meeting at an edge.
>
> That's how you check for it, so yes.
>
> The main point is that it is not necessarily wrong or ambiguous to have
> an edge referred to from more than two faces.
>
> Carsten Arnholm
>
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