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

A. Craig West acraigwest at gmail.com
Wed Nov 13 03:49:38 EST 2019

Although I primarily use OpenSCAD for creating 3D printable files, I
consider it a programmatic model generator. There are a great many use
cases for the software where the desired output is essentially an abstract
model. One example in my own experience is where I have a design which is a
cooling duct for a 3d printer. I use openSCAD both to generate the STL for
the duct, but also to generate a model representing the airspace inside the
duct, which I import into another program to model the actual airflow
through it at various fan speeds.
Other use cases are to generate mathematical models for illustrative
purposes. In both of these cases, arbitrarily restricting the output to
'realisable' objects restricts my ability to produce the output I need.
This restriction appears to be rooted in the implicit assumption that the
final output will always be to an STL file, but as was pointed out, other
formats have well defined behaviour for many of the edge cases that are
explicitly blocked in openSCAD.
Colour is another attribute which is arbitrarily discarded when a model is
rendered, apparently because STL doesn't support it.
Related issues come into play when you wish to use openSCAD to produce
output for a laser cutter. In that case, due to the current work flow used
by the majority of people doing laser cutting, you need to output 2d
representations of the laser path. It's all well and good to model the
actual shape you wish to produce, but if what you need is tool paths, then
that doesn't actually do you much good... Laser cutting is an area where
what you need is more CAM oriented than CAD. Whenever you are producing an
actual physical output, eventually you need software that produces a more
abstract tool path, and the actual physical tool determines what will be
output. The job of a slicer, for example, is to predict what the actual
extruder will produce, and try to turn the 'realisable' model into tool
paths that will create that shape, but there is a very definite use case
for allowing the actual designer to just create the path and see what
I have spent more years writing industrial CAD and CAM software than the
majority of people on this list have been alive, I suspect. There are
always use cases that were not anticipated by the writer of the software.
It is best to not enforce restrictions based on one use case on every user

On Tue, 12 Nov 2019, 22:18 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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