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

nop head nop.head at gmail.com
Sun Nov 17 06:45:16 EST 2019


Indeed that is why I *said *truncate.

On Sun, 17 Nov 2019 at 11:44, nop head <nop.head at gmail.com> wrote:

>
>
> 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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>
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