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

nop head nop.head at gmail.com
Wed Nov 13 08:59:45 EST 2019


Well you could say it supports them write only. It only works as a
non-lossy representation when the contents are manifold. The reason it has
lasted so long as a format for 3D printing is that is all that is needed.
It's a shame the standard didn't contain that rule.

On Wed, 13 Nov 2019 at 12:41, A. Craig West <acraigwest at gmail.com> wrote:

> 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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>>> Discuss at lists.openscad.org
>>> http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org
>>>
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