UV unwrapping makes the same edge different lengths

Something I just encountered when unwrapping:


Why is it that the selected edge has different lengths on either side of the seam?

I noticed that “conformal” mapping makes the edges the same:


The documentation seems to suggest that angle-based is the “better” choice, since conformal “usually gives a less accurate UV mapping than Angle Based, but works better for simpler objects.”

Why is it that the angle-based unwrapping is so inconsistent with the edge length here? Is there a rule of thumb for how “simple” an object needs to be before conformal is better?

In the end it doesn’t matter. You shouldn’t completely rely on automatic unwrapping modes anyway and should set up your UVs/islands according to where you need more detail/less stretching.

This is simply a thing how the algorithms work. Angle based gives more accurate results with organic shapes. And Conformal gives more accurate results with non organic shapes. So use the right tool for the right job :slight_smile:

If you want exact geometry select all edges and mark seam, then unwrap. All faces will be separated and exactly proportional as far as I have seen, except for nonplanar quads and Ngons which are just flattened.

Turn on the stretch checkbox in the N panel to see which UVs are being distorted. The method I described should give you absolutely no stretch.