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

nop head nop.head at gmail.com
Wed Nov 13 03:34:49 EST 2019

Yes OpenSCAD uses a Polygon soup as one of its internal representations, so
it can't handle non-manifold shapes just the same as STL. I don't wee why
that is a problem. What practical use are non-manifold shapes?

If you send two cubes with a shared edge to a slicer what do you expect it
to produce? Since it can't generate gcode for an object with a shared edge
it probably will either make two separate cubes, each with the own outline
or perhaps two joined cubes with a single outline. Why send a design that
can never be printed ever with any technology ever, even in the distance
future because it doesn't make sense at a physical level. Why not be forced
to send a model that explicitly indicates whether it should be two shapes
or one? Then there are no surprises.

I once helped out at a MiniMakerfair printing some giveaway objects. I was
given an STL file and just sliced for my machine and filament and started
printing. I thought the design was very weak but I had printed dozens
before I realised it contained self intersections and when sliced with a
different sliced it made a totally different object.. Whatever CAD tools
was used didn't automatically union objects and allowed a non-manifold
design to be sent to an STL file.

On Wed, 13 Nov 2019 at 03:18, Doug Moen <doug at moens.org> wrote:

> nop head wrote:
> > It would mean I could accidentally design non-printable object instead
> > of being told it was non-manifold.
> This has nothing to do with my original request.
> What I want is better support for the 3MF file format.
> The 3MF file format provides a legal, supported way to encode a model
> containing two cubes that touch along an edge, but OpenSCAD doesn't support
> this feature, in the way that 3MF files are currently written. So I'd like
> the 3MF export code to be fixed, so that when I export such a model, it is
> written as a valid 3MF file.
> I don't think that better 3MF export will interfere with nop head's
> ability to use OpenSCAD.
> The other thing I'd like is better 3MF import, with the assurance that if
> a *valid* 3MF model is imported, then the topology of the model is
> preserved, so that when the model is presented to CGAL for boolean
> operations, I don't get errors. Right now, during 3MF import, the model is
> converted to a PolySet, which is polygon soup. The topology information is
> discarded, and that's needed to convert the model to a Nef Polyhedron that
> CGAL can perform boolean operations on without reporting an error.
> I don't think that better 3MF import will interfere with nop head's
> ability to use OpenSCAD.
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