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

A. Craig West acraigwest at gmail.com
Wed Nov 13 07:40:18 EST 2019


STL certainly supports non-manifold shapes, it has pretty much no validity
checking at all... The problem is they don't tend to be very meaningful 😊

On Wed, 13 Nov 2019, 07:23 nop head, <nop.head at gmail.com> wrote:

> >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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