Once unwrap and checked “constrain to image bound”. Is there are button or anyway to maximize the uv to the boundary and keep to proportion of the uv?
Unwrap then scale up with constrain enabled
Ensure you apply any scale before unwrapping
I just see “constrain to image bound”. But I also want to keep the shape of vertical and horizontal in same ratio when I try to scale up the uv to the bound of image.
Use the Pack Islands function in the UV editor it should scale up what you have selected to the image bounds automatically while maintaining the ratio of the UV map. The hotkey is Ctrl+P