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

Max Bond max.o.bond at gmail.com
Wed Nov 13 12:39:23 EST 2019


I think it would be helpful to enumerate the use cases for which
nonmanifold geometry is desired/required, so we can look at that list and
ask; is this what OpenSCAD is for? What do these things have in common?
Would they all be served well by 3MF etc?

I think one of OpenSCAD's strengths is that it has chosen a narrow
objective & focused on fulfilling that need well. It doesn't propose to be
a Fusion or FreeCAD replacement; it's a parametric, CSG design tool, with
some GUI sugar to assist in that.

I don't totally understand what it is people want this for, but I think
it's important to ask; is this the mission of OpenSCAD? Or is this
something for another project?

Best,
Max

On Wed, Nov 13, 2019, 7:00 AM nop head <nop.head at gmail.com> wrote:

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