( screencast of the issue in action )

Trying to get a better understanding of face creation. I've got some very high detailed faces I'm trying to make that will later be used with a subdivision (multires) for animating a displacement map.

I'm nearly zoomed in 100% ... a few steps out ... and trying to create some Ngons. Most of the time it works perfectly but occasionally I will get a face that seems to ignore one of the vertices. When this happens the edges are created using the errant vertex but the face seems to skip it.

The faces are all co-planar (0 distance on the Y axis). I even selected everything and re-scaled to 0 on Y to be sure but it didn't change anything.

I've made a video showing what I'm doing. Basically trying to make sure I don't have any faces that are below a certain size near my seam. To do so I'm selecting nearby vertices and moving them to the seam edge vertices with auto-merge on. Then I'm deleting sub-optimal faces and re-creating them with the newly positioned vertices.

Take a look at the short screencast linked above to see the face being created with the strange edge. Perhaps this is just a wireframe preview bug? But I haven't seen it happen elsewhere in the model.

The precision I'm going for is sort of silly but it's one of those labor-of-love projects. I've found ways to make the ngon the way I want it to work, the video shows one workaround, but I've had most of my workaround methods fail and it becomes awfully intensive (I'm optimizing a couple thousand faces this way).

NOTE: I'm using Gooseberry with wireframe colors on a different mesh that is adjacent to the one being used. Not mentioning this because I think it is the issue (this occurs when switching to the standard distribution). Just letting folks who might be confused by seeing nearby edges in an unusual color.


1 Answer 1


It's a kind of warning that you are creating a concave quad face.

Ther's an article that explains they should be avoided.

Remember that render engine base their calculations on triangled meshes. Concave quad faces, as non planar quad faces, are not automatically splitted in Blender.

You can see how it works by adding a plane and move the vertex with fixed z till is placed inside the triangle made by the other 3 vertices

enter image description here

  • $\begingroup$ Thanks :) I'm planning to do a "poke" once I have the rest lined up. Am I safe in this case? If it is a warning then once I poke to make the non-concave faces will the overlap area go away for the final geo? $\endgroup$
    – Jahf
    Jul 20, 2015 at 3:30
  • $\begingroup$ Also for follow-up, is the concave warning fairly reliable? I managed to re-make the various ngons where this appeared, and visually they were identical except the overlap warning. $\endgroup$
    – Jahf
    Jul 20, 2015 at 3:31
  • $\begingroup$ Poke 1 quads creates 4 triangular faces by adding a vertex. Triangles can't be ambigiuous, the are fully descripted with the position of their vertex. So YES, it will work. $\endgroup$
    – Carlo
    Jul 20, 2015 at 10:31
  • $\begingroup$ Notice that this happens only with quad faces, Ngons are allowed to be concave. At the end of the video you somewhat manage to get rid of the overlapping... I'm not able to tell you why was that possible. It's interesting! $\endgroup$
    – Carlo
    Jul 20, 2015 at 10:44
  • $\begingroup$ I'm leaning towards thinking the rules on creating a quad vs. ngon with 4 edges might be coming in to play. I've repeated that process a number of times and it seems almost random when fixing one of those faces whether or not I'll get the overlap again on the first attempt to fix. I wonder if Blender has the ability to create a 4-sided ngon. Either way, since the final result will be triangulated it won't be a problem for this case. Thanks for the replies :) Keeping it tagged as answered and leaving the demo video on Youtube for anyone else who stumbles over this. $\endgroup$
    – Jahf
    Jul 20, 2015 at 22:05

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