Consider this image:
I know the aspect ratio. I would like to turn this skewed rectangle to rectangle with correct aspect ratio, as big as possible.
I can use perspective transformation in GIMP, but that doesn’t allow me to control the aspect ratio of the result. Also, there are no guidelines for horizontal and vertical lines, so it’s hard to get it right.
Answer
Start the Perspective tool 
. Set the tool to “Corrective (Backward) directiion” mode in the Tool options dialog. Drag the markers in the four corners of the image to the four corners of the painting, hit “Transform” in the side dialog.
- See the “Clipping” options for what to do with the surroundings of the image.
- You may have to scale the image (on X or Y only) to achieve the original aspect ratio
- In your picture you have a very visible “barrel” lens distortion that is better fixed before your fix the perspective.
Attribution
Source : Link , Question Author : Tomáš Zato – Reinstate Monica , Answer Author : xenoid