UV unwrap not symmetrical

Symmetrical objects are distorted when UV unwrapped.

-Create for example a UV sphere with 8 segments and 8 rings
-Delete half of it (so that you have something that looks like half of an eye, with the iris in the center)
-Select all faces, then unwrap

The UV map is not symmetrical.

Notes:
-applying the Scale/Rotation does not help
-when unwrapping, changing any of the F6 options (Angle Based, …) does not make it symmetrical
-trying to work around this by pinning a line in the UV map after straightening it with Scale -> X -> 0 does not help, because this will only straighten a line, but the line’s segments will still not be symmetrical

It’s not symmetrical because the unwrap is trying to minimize stretching. If you want symmetrical unwrap of such an object, do a Project From View when looking at the iris, and then scale the outer UV loops.

Hi Stan_Pancakes thanks for your response. I know that Project From View will create a symmetrical unwrap, but I do not want to project from view for many reasons.

It’s not symmetrical because the unwrap is trying to minimize stretching

I think this is incorrect. Distorting a perfect circle of quads does not minimize stretching. There will always be stretching in a normal unwrap, but the problem here is the distortion.

I should have posted an image so that it is clearer. Here is an image of a perfectly symmetrical loop of quads, with nothing else, taken from a UV sphere. Look at the distortion in the UV unwrap. There is no good reason to have it behave like this.

Method: Conformal

Method: Angle Based

There’s a very good reason. There’s no perfect circle of quads here, since those rings aren’t flat. Laying out any ring from that hemisphere flat on the UV grid while keeping it connected is not possible without distorting it somehow. The “Unwrap” methods attempt to compromise between distorting the shape and area of the faces. Which is not what you want, it seems. You seem to want the unwrapping to disregard the shape wholesale and make a planar projection, which is exactly what Project From View does.

Ok let me try to be more clear. The way I used the word “distortion” was confusing. It’s clearer if I just say “not symmetrical”. Like I said I know that it is impossible to unwrap normally without stretching/distortion. But what I am saying, is that there is no reason for it to be asymmetrical, that does not help in any way. If you see the UV map on the right of my image, you can see that it is very easy for it to be symmetrical. If I move the vertices slightly, just to make it manually symmetrical, there will still be stretching/distortion, but it will be even, as it should be.
I am not saying that there should not be stretching/distortion of the texture, I am saying that it should be symmetrical. There is no relation between unwrap distortion and the problem of symmetry that I am talking about here.

I understand what you’re saying, you’re just being too categorical in your conclusions. These operators are designed to work with arbitrary shapes, symmetrical or otherwise. Undoubtedly some extra controls for these operators would be helpful, but we don’t have them. As such, some manual intervention would be required either way, either in the form of different tool, or cleanup in UV editor.

What are your reasons against using Project From View? It does give a much nicer starting point when straight edges is what you’re after.

Because Project From View only works from our current view, then you would have to manually guess how to move the edge loops to decrease distortion. But also the sphere is just an example. I work with more complex objects, and even though it is not the end of the world, it can be a problem sometimes, so I thought I would mention it.
For example I am modeling a human head, and even though the model is symmetrical, the UV map will not be symmetrical. So for instance the texture is slightly more stretched or distorted on the right side of the mouth. In this case it is not a problem because it is barely visible on the organic skin, but with other types of objects or clothes it could be more visible and require unnecessary work. I know it is possible to somewhat work around this maybe by mirroring the model …, but that would be a lot of work and not worth it.
I downloaded the trial version of Maya to see if it has the same issue (don’t worry I’m not moving to Maya). It is not completely straight either, but it is much more symmetrical, on the other hand it seems like the quads of the UV are too thin in Maya which will cause more distortion.

Here is the exact same unwrap but in Maya:

Anyway, it is a minor issue and I just thought there could be an option to make it symmetric automatically.

Thanks for the help

The switch “Fill Holes” has tooltip “Virtual fill holes in mesh before unwrapping, to better avoid overlaps and preserve symmetry”. If you turn it off, the result gets much less symmetric.

Apparently somebody was thinking about this issue.

From my experience, mirror symmetry is always preserved. Do you have an example where unfolding a human face produces a mirror-asymmetric uvmap?