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

Doug Moen doug at moens.org
Mon Nov 11 10:04:26 EST 2019


Hi Ronaldo.

The STL file format is ambiguous. There is no formal written standard. Different slicers interpret the same STL file in different ways.

OpenSCAD tries to work around this ambiguity by restricting its output to an unambigous subset of STL, which is 2-manifold, watertight, and non-self-intersecting.

The 3MF standard is unambigous. There are rules for what constitutes a valid mesh, and there are rules for how a slicer should interpret a valid mesh. Since it is very difficult, in general, to avoid generating meshes that are free from self-intersection, 3MF even has rules for how a slicer should interpret self-intersection.

3MF meshes are required to be 2-manifold, but this means something subtly different from what it means in OpenSCAD. An STL mesh is just a list of triangles, where each triangle is 3 vertices, and each vertex is 3 numbers. In 3MF, there is a level of indirection, as I said before. There is a vertex list, which assigns a number (a vertex id) to each vertex, and there is a list of triangles, where each triangle is 3 vertex IDs. In 3MF, the 2-manifold restriction is defined in terms of vertex IDs. It is legal in some cases for two distinct vertex IDs to have the same coordinates. This loophole exists so that you can represent 2 cubes sharing an edge.

Doug Moen.

On Mon, Nov 11, 2019, at 12:32 PM, Ronaldo Persiano wrote:
> 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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