My UVs are not in the same scale when I apply 'Average Island Scale'

Question

I cannot make the UVs the same dimension, they shrink when I apply Average Island Scale. Is this behavior normal or is it my way of thinking?

Here's the steps to reproduce the problem I found:

1. Start a new project, default settings, and recreate the mesh in photo 1, also apply the same Seams.

2. Using project from view or cube projection, create the UVs for the front and side.

3. You can see the UVs match the height and are at the desired scale.

4. Apply "Average Islands Scale", the uvs get's squished, and there's a clear difference in scale.

5. If Textured, you can see visually the difference, how is that "Average Islands Scale"?

My problem was that, in a project with a object, I had multiple separate pieces, so I unwrapped it okay, with correct seams and everything, and needed to average the different islands, but if I apply the "Average Island Scale", it reproduces the same effect in step 4, it squishes and break the UVs proportion. And yes, I am well aware that objects out of scale 1,1,1, give errors, my objects are always have applied rotation and scale.

I am a fan of the Textools Add-on for Blender, but it seems that the function "Apply Texel Density by Scaling the UVs to match the ratio." gives the same result as Average Island Scale.

Finally, my question is, this effect is clearly undesired, shouldn't the Average Islands Scale take into account the dimensions? Or is it normal to consider angles and geometry when Averaging islands?

Answer 1

It works with Areas

Average Island Scale is (to my kowledge) based on the 3D faces surface area, not it's dimensions. This would make more sense with more complex, irregular figures coming out from the unwrapping of a character, for example.

Take as instance the following example: two faces with the same area (1 square unit) and individual UV island. Running the operator doesn't affect the UV mapping in any way, wether the two figures have different dimension.

If I alter the scale of one of the two islands, and re-run the operator, all the selected faces are being scaled in order to show the correct ratio: they have the same surface in the 3D viewport, so their corresponding islands have the should cover the same area in the UV.

In the example you are providing, everything is going fine in my opinion. The portion with the trapezoidal face has less surface in the 3D viewport, so it should be smaller in the UV map.

If you would to get UV island that shares the same units, the faster way is to unwrap them at the same time (your figure n°3 is probably what you were looking for).