[OpenSCAD] Visually and by inspecting code: surface looks perfect - But: "Object may not be a valid 2-manifold and may need repair"

kaefert at gmail.com kaefert at gmail.com
Mon May 18 16:53:50 EDT 2015

2015-05-18 22:30 GMT+02:00 Ed Nisley <ed.nisley at pobox.com>:
> It's not exactly *wrong*, but the order in which you specify the points of a
> polygon / polyhedron determine which way the surface normals point:
> http://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Primitive_Solids
> "When looking at the face from the outside inwards, the points must be
> clockwise. You can rearrange the order of the points or the order they are
> referenced in each tuple. [...] Note that if your polygons are not all
> oriented the same way OpenSCAD will either print an error or crash
> completely"
> Given the way that you're generating the points, at least some of the
> polyhedrons may be wound backwards.
> The reason this doesn't show up in thrown-together mode may be due to the
> way the polyhedrons combine: everything looks fine from the outside, but
> some inside (invisible) faces wind backwards. Perhaps assembling one
> polyhedron at a time will be productive.

hmm, well the "if else if else" variant with 3 different polyhedrons
being drawn (1 for begin, 1 for middle 1 for end) does not have any
faces on the inside as far as I can tell.

> Using Meshlab (or something similar) to check your work may also help pin
> this down.

Yes, I've found by trial and error that I can produce a STL that I can
slice into something useful with slic3r by opening the STL produced by
OpenSCAD in MeshLab and apply the Filter "Cleaning and Repairing" ->
"Remove Duplicate Faces". It still gives errors on some triangles
where the wings hit the center cylinder but who cares if I can slice
it ;)

I had to increasing the calculation resolutions of my loop from 15 to
30 (variables rad_res & chord_res) for those errors at the
intersection between wings and the center cylinder to be small enough
to not have a negative effect on the slicing process.

> It's a nice-looking prop, though!

Yes! Thats what I thought as well ;) But any congratulations need to
go to user DMeyer from the thingiverse.com - he did the hard part of
designing that propeller wing.

Thanks & Regards,
Thomas K.

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