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

Doug Moen doug at moens.org
Wed Nov 13 06:39:06 EST 2019

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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