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In this tutorial by Blender Guru, Andrew re-unwraps his plane (because he changes the UV to be a 2*1 aspect ratio and so it stretched his UV). He also mentions that with cycles, it won't re-unwrap it. You have to be in blender internal to re-unwrap it.

In Blender 2.8, there is no blender internal anymore. (and with EEVEE and Workbench it doesn't sucessfully re-unwrap it either)

How do you re-unwrap an object and have it be the same scale?

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    $\begingroup$ UV's are squared. IMO this is a bad recommendation and there is no need to setup a 2:1 ratio (even for a plane like this). The more UV islands you have the better the resolution of the texture... And if you need more space, you can use UDIMs as of 2.82. $\endgroup$ – brockmann Jan 17 '20 at 18:39
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Well, this is a fun one.

What is going on
It seems, unwrapping depends on your shader's image texture, not the texture in the UV editor. Again. It's not about the UV editor, it's about the shader editor.

I have three textures with different aspect ratios. 1:1, 2:1, 3:1.
But the UV map ratio is always based on the image selected in the shader editor.

enter image description here

So, in some cases, there's no way to match the correct aspect ratio.

Example:
2:1 in UV editor, 3:1 in shader editor. And no way to match the correct ratio.
Correct aspect ✔ doesn't work either - it only switches to 1:1 ratio.

enter image description here

Solution
Check what image is selected in the shader editor. That's what drives the UV map aspect ratio.
And then match it to the image in your UV editor.

Note
The above is a reason why it sometimes works, and sometimes doesn't.
Still not sure if it's a bug or not, but either way it's pretty messed up :).

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  • $\begingroup$ Thank You for explaining! $\endgroup$ – Pro Effects MAX Jan 18 '20 at 1:00
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Cycles CAN re-unwrap things. I don't know how old that video is, but if in edit mode you just select all the faces of your mesh, and press 'U', you can unwrap your object how you want to again. Try it, it should work.

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    $\begingroup$ Not with a distorted (2*1) aspect ratio. At least not with the plane in the video. (In my instance) $\endgroup$ – Pro Effects MAX Jan 17 '20 at 18:20
  • $\begingroup$ Hey there :). Don't feel bad if you solution doesn't work here, Blender's behavior is pretty messed up. $\endgroup$ – Jachym Michal Jan 17 '20 at 22:24
  • $\begingroup$ Now that's something I can agree with. $\endgroup$ – Nate_Sycro27 Jan 17 '20 at 23:30

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