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

MathLover mathlover at w-p.dds.nl
Fri Nov 15 10:48:57 EST 2019

Not really. But the problem is that the lines are a representation of the
boundary of an object. If the line pieces end at the "x", you have two
separate close loops that could be printed or constructed. However, if the
line pieces cross each other, this is no longer possible.

This is because the boundary lines have a "material" side and an "outside"
size. 3D triangular faces have something similar: they have an outward
facing normal that follows from the order of the corner points.

In the crossing lines example, the figure may look just the same to you, but
it is impossible to know on which side of the lines you would have to put
the material. For the crossing lines, the "material" and "outside" side swap
at the crossing. But even without crossing lines or faces you could make
impossible figures like the  Moebius band
or the  Klein bottle <https://en.wikipedia.org/wiki/Klein_bottle>  .

So you can invent more boundaries that do not define objects. The
manifoldness is a way to ensure that there is actually something that could
exist as an object. We might think of that as "3D printability", but even
the preview window wants to show you what your code looks like as an object.
So if that object cannot be defined, the preview window cannot show you
anything useful either.

Sent from: http://forum.openscad.org/

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