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

Doug Moen doug at moens.org
Thu Nov 14 08:53:42 EST 2019


On Thu, Nov 14, 2019, at 7:36 AM, Dan Shriver wrote:
> Ideally I'd like to see some way to fix manifold issues.
> 
> For instance, I like to play with epicycles and the like. Unfortunately, these shapes, while aesthetically pleasing tend to do a lot of intersecting loops etc. The end result is they are unprintable.

Hi Dan. If you sweep a shape through an epicycle, then technically, the resulting mesh will be manifold, but it will be self intersecting.

I know nophead said something slightly different, but my definition of "self intersecting" is: there are two faces, anywhere in the model, that intersect each other.

Normally, a slicer program should be able to print a model that is manifold but self intersecting. But you may run into problems that prevent the model from being printable.

One reason why an epicycle mesh might be unprintable is because it becomes accidently non-manifold as a result of export to STL. At the point where two regions of the swept path intersect each other, there might be two vertices that coincide. If the mesh representation contains topology information, then the topology will tell the 3D printer slicer that those vertices are actually distinct, and there's no problem. However, when you export to STL, then the topology information is lost, and those coinciding vertices will be identified as the same vertex, and that makes the mesh non-manifold. Which means you can't print it.

I think that we need to do two things to fix this problem. We need to change OpenSCAD to preserve topology information when processing and exporting meshes. And we need to deprecate the STL file format. Obviously we can't remove STL support. But we need to educate our users that exporting to STL destroys topology information, which can sometimes make a mesh unprintable. We need to explain the benefits of alternate file formats that support topology information, such as OBJ and 3MF.

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I think that OpenSCAD should automatically repair self intersections, but only in the case where CGAL fails with a self-intersection error. Otherwise, we should leave the model the way it is, in order to satisfy users who don't want OpenSCAD to mess around with their carefully specified mesh for unnecessary reasons. OpenSCAD should also provide a `repair_self_intersections` module, so that you can do it yourself. You might need to repair self intersections if they are causing problems upstream. For example, you might do this if you are exporting to STL.

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