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

Doug Moen doug at moens.org
Sat Nov 16 08:03:13 EST 2019

On Fri, Nov 15, 2019, at 9:59 PM, Michael Marx wrote:
> On the topic of, can we detect and/or fix it. I started with an edge.


> Netfabb reports two 10x10x10 cubes with a shared edge, as having two holes, and also highlights the edge as a self-intersection.

> GUI repair doesn't fully fix it, the holes go away (ie the count in the repair stats = 0), but there is still a self-intersection, on export it shows:


The Netfabb export dialog says "Some file formats do not include enough information. This may result in errors."

Translated, that means "The STL file format does not include topology information. This may result in errors.". Most (all?) of the later mesh file formats used for 3D printing include topology information. So we might expect different results if we use a different file format.

I only have Netfabb Basic, which doesn't do automatic repairs, so I used MeshLab instead for my first test.

In OpenSCAD, I exported this model:
 cube(10); translate([10,10,0]) cube(10);
to STL.

When import into MeshLab, it detects corruption, and asks if I want to repair it. The dialog says "Unify duplicated vertices?".

I say yes, and MeshLab performs the repair. Then I export to an OBJ file, which includes topology information. Then, when I import that same OBJ file into MeshLab, it does not report any errors.

If use OpenSCAD to export this model as OFF (which includes topology information), then I can import the OFF file into MeshLab, and it does not report any errors.

When I import the STL file into Netfabb Basic, it reports an error. When I import the OBJ file into Netfabb, the mesh is clean, no error. (My version of Netfabb doesn't read OFF files.)

Based on these tests, it appears that the problem is with the STL file format, not with the model.
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