I've made an extremely simple scene, as a pretty minimal example to reproduce this issue. I'd like to know what I was supposed to do differently to get reasonable results. To make this scene:

  1. Make a plane.
  2. Extrude one edge up to make a wall.
  3. Knife a little cave entrance in the wall, touching the bottom edge of the wall.
  4. Extrude this cave in a little.

I want to UV unwrap this structure, and I did so with the following seams (the selected edge is also a marked seam, by the way), followed by Unwrap:

Cave with seams marked

From these marked seams it's incredibly clear to a human what the four islands should be. From this image it's clear that Blender messed up the complicated concave face; that's not even really my problem, I'm okay with splitting up that face with more edges (although I'd appreciate tips on making unwrap go less crazy with complex planar ngons).

The real issue is that the selected edge gets mapped down to being incredibly small for the large square ngon floor face. To see how bad it is, observe when a color grid is overlayed:

Small edge in UV map distorts image

It's easy to fix manually in this case, but what am I doing wrong in general?

The .blend file is here.

  • $\begingroup$ What type of UV unwrap did you use? Perhaps Smart UV Project would work? $\endgroup$ – Gliderman Jun 10 '16 at 1:46
  • $\begingroup$ I simply used U > Unwrap before, but U > Smart UV Project did way better, thanks! It also correctly untangled the big planar concave ngon. I didn't realize that Smart UV Project respected marked seams. $\endgroup$ – SceneLinearColorSpace Jun 10 '16 at 2:23
  • $\begingroup$ It may be result of concave Ngons. While Smart UV Project in this case will handle with that, it's best to avoid such faces because of possible problems further. It's enough to split 2 faces converting them to quads and UV unwrap works just fine. $\endgroup$ – Mr Zak Jun 10 '16 at 12:16

As per Gilderman's comment, the problem is that I was using U>Unwrap. Simply using U>Smart UV Project fixed both of the issues I mention above, producing this much more reasonable UV mapping:

Reasonable mapping

Observe how the concave planar ngon is correctly mapped isometrically (that is, both conformally and authalically) into UV space, and the same is true of the big square floor ngon. In fact, every single face ended up being mapped isometrically; exactly what I wanted.


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