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

nop head nop.head at gmail.com
Sun Nov 17 06:44:18 EST 2019


On Sun, 17 Nov 2019 at 09:30, Michael Marx <michael at marx.id.au> wrote:

> > I frequently ran into non-manifold conditions, and also due to (I
> believe) the small number problem, producing self intersections.
>
>
>
> The problem is OpenSCAD snaps to a grid and then stores the result as a
> polygon soup. That can create non-manifold geometry whenever there are very
> close vertices. It is a massive bug to truncate vertex coordinates and not
> fix the mesh.
>
>
>
> Agreed, however with floats it will still be a problem, perhaps lesser,
> without the grid.
>

Indeed that is why I truncate. Any reduction in resolution from CGALs
rationals can cause a problem.


> The two-cube-edge will be a problem regardless of grids/floats.
>

Yes but I don't regard that as problem, just a reality of this universe I
find myself in. I can easily specify if I want two separate cubes or a
joined pair.

However, if one unions two curved services with a slight overlap then CGAL
will do a perfect job but OpenSCAD might break the result by truncation.
That I regard as a serious bug.

>
>
>
> ------------------------------
>
> *From:* Discuss [mailto:discuss-bounces at lists.openscad.org] *On Behalf Of
> *nop head
> *Sent:* Sun, 17 Nov 2019 19:26
> *To:* OpenSCAD general discussion
> *Subject:* Re: [OpenSCAD] Discuss manifoldness, co-incident faces edges
> etc
>
>
>
>
>
> On Sun, 17 Nov 2019 at 07:29, <arnholm at arnholm.org> wrote:
>
> On 2019-11-17 00:33, Michael Marx wrote:
> > As Doug said 'STL can't represent curved surfaces, it can only
> > represent a polyhedral approximation'.
>
> That's true of anything that comes out of OpenSCAD or any other mesh
> based modeller, it is nothing particular to STL, OBJ, OFF, AMF or
> whatever format you choose. A mesh representation is a finite
> approximation, of course it is.
>
> It is not the file format, is the core modeller.
>
> If you took the CGAL library and replaced it with another library based
> on CAD BREP style datastructure, for example OpenCASCADE
> https://dev.opencascade.org/ the situation would be different, and you
> could output STEP files or other formats with accurate respresentation
> of curved geometry. But as long as the core model is a mesh
> representation, the output will be a polyhedral approximation.
>
> Carsten Arnholm
>
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