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

nop head nop.head at gmail.com
Wed Nov 13 07:22:43 EST 2019


>I'm not asking for OpenSCAD to support non-manifold shapes, I'm asking for
OpenSCAD to more fully support the 3MF file format.

Isn't that a self contradictory statement? If 3MF supports non-manifold
shapes then OpenSCAD will need to support them in order to read them.
OpenSCAD only supports manifolds for the exact same reason STL files only
support them. Its mesh representation is a polygon soup.

On Wed, 13 Nov 2019 at 11:40, Doug Moen <doug at moens.org> wrote:

> On Wed, Nov 13, 2019, at 3:34 AM, nop head wrote:
>
> Yes OpenSCAD uses a Polygon soup as one of its internal representations,
> so it can't handle non-manifold shapes just the same as STL. I don't wee
> why that is a problem. What practical use are non-manifold shapes?
>
>
> I'm not asking for OpenSCAD to support non-manifold shapes, I'm asking for
> OpenSCAD to more fully support the 3MF file format. The practical use is
> the ability to download an arbitrary 3MF file from the internet, transform
> it in some way, export back to 3MF, and then print the results. As long as
> the 3MF file contains a valid mesh, as defined by the 3MF standard, I
> expect this to work.
>
> If you send two cubes with a shared edge to a slicer what do you expect it
> to produce? Since it can't generate gcode for an object with a shared
> edge... Why send a design that can never be printed ever with any
> technology ever, even in the distance future because it doesn't make sense
> at a physical level.
>
>
> No, this is a straw man argument. I'm asking for better 3MF support. If I
> send a valid 3MF file to a slicer, then I expect it to print the model
> without any problem. The 3MF standard provides unambiguous instructions on
> how to slice a valid 3MF mesh.
>
> I once helped out at a MiniMakerfair printing some giveaway objects. I was
> given an STL file and just sliced for my machine and filament and started
> printing. I thought the design was very weak but I had printed dozens
> before I realised it contained self intersections and when sliced with a
> different sliced it made a totally different object.. Whatever CAD tools
> was used didn't automatically union objects and allowed a non-manifold
> design to be sent to an STL file.
>
>
> This is why you should embrace the 3MF standard. It contains rules
> defining what is and is not a valid mesh. There is open source code for
> rapidly validating a 3MF mesh. Given a valid mesh, there are rules that
> define exactly how the mesh should be sliced. This means that model files
> can be portable between slicers. This also means that we can test a slicer
> or 3D modelling tool for conformance to the 3MF standard, and report a bug
> if it misinterprets a valid model. Unambiguous rules, validation, and model
> portability are a big selling point of 3MF.
>
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>
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