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

Ronaldo Persiano rcmpersiano at gmail.com
Wed Nov 13 14:50:28 EST 2019


 nop head <nop.head at gmail.com> wrote:

> >I'm not asking for OpenSCAD to support non-manifold shapes, I'm asking
> for OpenSCAD to more fully support the 3MF file format.
>
> Isn't that a self contradictory statement? If 3MF supports non-manifold
> shapes then OpenSCAD will need to support them in order to read them.
> OpenSCAD only supports manifolds for the exact same reason STL files only
> support them. Its mesh representation is a polygon soup.
>

It is not. I see contradiction in your arguments favoring STL file format
against 3MF when this last format is able to enforce the 2-manifold
condition. It is insane to me that a CAD program has to discard all
topological information present in its internal structures to write a STL
file to be consumed by other programs that will have to guess the topology
from an unstructured pool of triangles in order to do their job. If
something is representable by a 3MF file but not admissible by the reader
it can just reject the file as OpenSCAD does with malformed (meaning non
2-manifold) STL files. Reading and writing a well structured file format
should be not only easier but safer.

The reason it has lasted so long as a format for 3D printing is that is all
> that is needed. It's a shame the standard didn't contain that rule.


It has lasted so long because it was the first (1987) and unique file
format available at the beginning of rapid prototyping and not by its
merits. Other formats, like  3MF (2015), AMF (2011), are much more recent.
It is a shame that STL is still in use.
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