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I have a problem while exporting and importing vertex groups data through OBJ format. I have certain vertex groups in my object which consist of only single vertex (as shown in the following two images)Vertex Group - Corner1 with just one vertexVertex Group - Corner2 with just one vertex

Now I export it by checking the 'polygroups' option and the 'keep vertex order'option in the OBJ export options and then import it by checking the 'Polygroups' option under 'Keep Vertex Order' tab in the OBJ import options. I see that, as expected, the vertex order is maintained, but when I tried selecting the vertex groups, I am selecting faces (as shown in the following images). How can I prevent this from happening? Why is the vertex group with single vertex changing into a vertex group with 4 vertices and the corresponding face is getting selected when the vertex group is selected?enter image description hereenter image description here

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  • $\begingroup$ Did you by any chance have a subdivide modifier in your mesh prior to exporting? It mat happen that whatever modifier you had was subdividing your mesh and propagating your vertex group assignments to neighboring vertex. $\endgroup$ – Duarte Farrajota Ramos Apr 3 '16 at 22:25
  • $\begingroup$ No. I didn't use subdivide modifier, but I subdivided the mesh in edit mode. I have no active modifiers while exporting. $\endgroup$ – Sum-Al Apr 5 '16 at 18:24
  • $\begingroup$ Did you subdivide prior to assigning the vertex to a group or after? Subdividing after might propagate the vertex group to the newly created adjacent vertex . $\endgroup$ – Duarte Farrajota Ramos Apr 5 '16 at 18:30
  • $\begingroup$ Oh, let me check that. $\endgroup$ – Sum-Al Apr 5 '16 at 20:35
  • $\begingroup$ I was subdividing before assigning the vertex groups. But like u said, subdividing after creating vertex groups was propagating it to adjacent vertices. This behavior is also seen in my case wherein in spite of subdividing before creating the vertex groups, it is propagating to adjacent vertices upon exporting and then importing. $\endgroup$ – Sum-Al Apr 5 '16 at 20:41
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Ok, I've just been testing myself, basically it's what I said before. OBJ file format specification probably doesn't support "vertex groups" feature. There is no such structure or way to store that information in this file format.

What blender does apparently try to map vertex groups to the "poly groups" feature of OBJ. Since polygroups - as the name implies - only stores groups of faces it as a few caveats as it can't unequivocally map vertex groups to faces, so it can't store certain cases like single isolated vertex from vertex groups.

The minimum supported is probably three vertex, which is the least possible to define a face. This is what my four-corner vertex group (to the left) was imported like, two separate meshed with the four corners separated from the main one (colors for clarity purposes).

Vertex to polygroups

So short answer is it is not supported, either find another file format that supports this, recreate you vertex groups in your destination application, or fix them by hand there.

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