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I thought that face orientation depends on face index, but my last test showed that I was wrong. Here you can see a mesh with 0 faces, extrudes by GN (regular Extrude acts similarly, GN is used for demonstration purposes):

enter image description here

Then I mirror a set of edges, it starts to extrude with different face orientation, but when I reorder indexes, it has no effect. But how does Blender keep that information in a set of edges then?

Moreover, direction is saved even if I connect edge loop with other:

enter image description here

Vertices have normals, but they have no effect also:

enter image description here

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  • $\begingroup$ You reorder vertex indices but not edge indices, so maybe try that as well? $\endgroup$ Sep 22, 2023 at 22:10
  • $\begingroup$ @MarkusvonBroady Just tried - no effect as well $\endgroup$
    – Crantisz
    Sep 22, 2023 at 22:51

1 Answer 1

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Looks like I've found, correct me if I am wrong.

Each edge is represented as a pair of vertex indexes:

enter image description here

One of each vertex in the pair should be first, the other - second. So face orientation depends on which vertex is first in the edge pair.

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  • $\begingroup$ ... so if the question is, how Blender orders vertex-indices into face-corner loops, (anticlockwise to normal), starting with no faces... how to explain the logic here ? All vertex, edge, face indices wind up the same. But the face-loop order is reversed between vertex-extrusion and edge-extrusion. $\endgroup$
    – Robin Betts
    Sep 23, 2023 at 5:00

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