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

Dan Shriver tabbydan at gmail.com
Thu Nov 14 09:58:57 EST 2019

If you want to see it, before I get back to the box where it is saved,
check this list for my posts.  I posted questions about it over a year ago
(ignore posts from me about ogee / arches, hyperbolic trig, etc) might even
be two years ago.

On Thursday, November 14, 2019, Tim V. Shaporev <tim.shaporev at auriga.ru>

> I am interested in the epicycles sample too because I could not comprehend
> what the problem is (a minute of self-advertising :-)
> https://www.thingiverse.com/thing:3483143
> and I'd appreciate new idea(s)
> On 11/14/2019 5:36 PM, Dan Shriver wrote:
>> Thanks Doug. I can provide an example next month when I have access to
>> the code again.  I did epicycles and things like them and one surface
>> looped from the outside into the inside.
>> There probably are tools / algorithms for "fixing" stuff like this since
>> it is a general problem in making 3D volumes (when one has loops, voids).
>> I don't believe I was using sweep at the time.  I think I did some ugly
>> homegrown thing where I did a linear extrude of an epicycle.  I would
>> "gradually" add a term by tacking it on (with a tiny fraction multiplier)
>> doing another extrude, increasing the multiple.... Until the term was at
>> 100% then I'd add a new term....
>> Deprecating STL support sounds like curing a headache with hemlock.  I'm
>> not going to argue that STL isn't a seriously flawed format, but it is
>> accepted by a wide range of 3D printers.  Maybe print a warning to the
>> console that suggests another format(s) on export to STL, nudge people in
>> the right direction but don't force them to abandon something they might
>> require.  You might have meant that yourself as you said you can't remove
>> it but the phrase "deprecate STL" was also used, which spooks me.
>> On Thursday, November 14, 2019, Doug Moen <doug at moens.org <mailto:
>> doug at moens.org>> wrote:
>>     __
>>     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.
>>     ---
>>     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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