[OpenSCAD] User Poll: What do you want to see from OpenSCAD development?

Ronaldo Persiano rcmpersiano at gmail.com
Mon Nov 11 07:32:13 EST 2019


On Mon, 11 Nov 2019 at 04:17, Doug Moen <doug at moens.org> wrote:

>
> The specific problem being referenced here is that if you have two
> polyhedra that don't intersect (with a non-zero intersection volume), but
> they touch at a vertex or touch at an edge, then you can't represent that
> in an STL file, because the polygon mesh is not 2-manifold.
>

Sorry but STL file format can represent objects (in a broad sense) that are
not 2-manifolds. As a soup of triangles, STL file format can represent the
faces of the two cubes sharing just an edge and even flaps. What STL cannot
represent is its topology as an aggregate of 2-manifolds like 3MF seems to
do.

I don't know the specification of 3MF but if it is able to represent the
following:

cube(10);
translate([5,5,0]) cube(10);


not as an union but as an aggregate of two cubes (the same way it would
represent if the intersect only in an edge), then we may have troubles
trying to print it because slicers seems to be non consistent in their way
to interpret that aggregate. A long time ago I have done a test how
different slicers interpret such aggregate by writing an STL file that
represent it. And I have found two distinct interpretations: in the former,
the union was produced; in the later, the common points of the two cubes
were off the resulting object. Perhaps now they had converged to a common
interpretation by using the same rule to recognize what points are inside
the object.

On 11 Nov 2019 at 07:34, nop head <nop.head at gmail.com> wrote:

>
> Two cubes cannot share a zero width edge. They either overlap by at least
> one atom or are separate. It is no coincidence that STL can represents all
> physically realisable objects.
>

The STL file format is able to represent two cubes sharing a zero width
edge. To do it yourself, join the two STL files (keeping only one header
and one tail) representing each cube. The 2-manifold concept is not a
physical one but a mathematical abstraction so not necessarily able to be
physically realizable.
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