I have a simple cube and in edit mode , i stretched it in Z direction so now the cube become very tall and the faces on top and botom are still perfect square. I setup the edge seams already so it will unwarp the box perfectly. The proportion of uv will be correct if i use square image (texture). If my texture doesnt' have 1:1 aspect ratio, the uv will try to adjust itself to the image aspect ratio. How to ignore image/texture aspect ratio ?
If your texture is 800x600 do some simple math: 600/800=0.75, so scale the whole UV by 0.75 on X axis (A to select all UV, S X 0.75 Enter).
In 2.80 the active image on faces removed (because it is internal renderer property). So how to do you tell blender that the image, that you want to use, is active? Do this:
- Open Shader editor (object mode)
- Create a material in your object (if it doesn't exists)
- Put image into Shader editor (you can drag&drop it from file browser or from data-block selector) or add and setup image texture.
- Select this image node.
Now, when the node with image are selected, this image is going to be active, and UV-unwrapping will use it proportions.