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

nop head nop.head at gmail.com
Wed Nov 13 15:05:37 EST 2019

There is no guesswork needed to read an STL if the geometry it contains is
manifold. 3MF and AMF presumably can represent non-manifold geometry
unambiguously but I don't see that as an advantage for 3D printing. They
have other advantages, like being able to handle multiple materials.
Nothing compelling enough for me to scrap my 3D tool chain because
multiple STL files can be used to represent multiple materials. I still
slice everything with Skeinforge!

If STL files had a rule to say the geometry they contained has to be
manifold and writers and readers enforced it there would not be a problem.

OpenSCAD doesn't reject bad STLs. It just feeds the soup to CGAL and CGAL
barfs if it is not manifold and produces and incomprehensible exception.
OpenSCAD might as well catch all CGAL exceptions and say the geometry has
become non-manifold. It would be a more meaningful error message.

On Wed, 13 Nov 2019 at 19:51, Ronaldo Persiano <rcmpersiano at gmail.com>

>  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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