Join does not merge vertices after splitting mesh with "line" selection

Quick notes: Joining two meshes after a split using “line selection” does not merge vertices properly. Doing the same with rectangular selection does the job.

I raised this issue a couple of months ago in this thread :

The problem still seems to exist I did some experiments (screenshots from the webdemo included)

Is this simply a bug or is there an explanation for it? Or am I just doing something wrong?

Not really a bug as relying on vertex merge after split is not really an official workflow.
But I can easily adjust it so that it works with line as well.

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That would be great. Thank you!.
Is there a particular reason, why this happens?

Yes the trim line uses a different logic than the other shapes.

The trim line cuts polygons with a 3d plane, while the other shapes do something a bit weird in 2d projected space (you can only create new point on edges, not inside a face, that’s why you can’t have clear corner).

But the main reason is numerical consistency, the 2d logic does the exact same operation for both shapes, while the 3d cut plane is slightly different for both shapes.

Basically it does:

  • « split edge [i1, i2] with plane [o, n] »
  • « split edge [i2, i1] with plane [o, n] »

Mathematically that’s the same thing but numerically it often results in a slightly different intersection.

Thanks a lot for these insights. This shows that it is always easy to ask for stuff without knowing what’s actually going on under the hood. Keep up the great work.

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