[OpenSCAD] CGAL error depends on order of unioning

nop head nop.head at gmail.com
Wed Jul 24 17:19:08 EDT 2019


Unless you do a CSG operation CGAL isn't involved. So yes you can create
any old polyhedron and make an STL but only manifold STLs allow the
topology to be reconstructed unambiguously.

When I said you can print a non-mainlined object I mean they cannot exist
in reality so can't be printed. Yes you can send a non-manifold STL to a 3D
printer and something may come out, depending on the slicer, but it will
always be a manifold object and not what the file represents.

On Wed, 24 Jul 2019 at 19:31, Ronaldo Persiano <rcmpersiano at gmail.com>
wrote:

> nop head <nop.head at gmail.com> written:
>
>> Basically everything that needs to be rendered to a solid has to be
>> manifold, including all the intermediate steps, because the library used,
>> CGAL, only handles manifold objects. It isn't a bug, it is designed that
>> way.
>>
>
> That is not precisely true. When we have just one polyhedron() primitive,
> it can generate a STL file even if the polyhedron is not a manyfold.
> For instance:
>
> polyhedron([[0,0,0], [1,0,0],[0,1,0]],[[0,1,2]]);
>
>
> is just a triangle and it renders fine and generate a STL file. Even
> polyhedron() with self-intersection are accepted by CGAL when are alone.
>
>  STL files can not represent non-manifold geometry and 3D printers can not
>> print it. These things will never change.
>>
>
> That is not also true. The above example shows that OpenSCAD is able to
> generate non-manifold STL files. After all, STL files are just a soup of
> triangles.
>
> I don't agree either that non-manifold can't be 3D printed. The object
> bellow is printable:
>
> union()
> {
>   cube(1);
>   translate([1,1,0]) cube(1);
>   translate([0,0,0.9]) cube(2);
>   translate([0,0,-1.9]) cube(2);
> }
>
> and it will be solid in the sense that it will not fall apart easily. It
> can't be rendered by OpenSCAD (CGAL) as is but it could be represented in a
> unique polyhedron and then a STL file could be generated by OpenSCAD.
>
>
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