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Consider this image:

enter image description here

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.

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Start the Perspective tool (backward). 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.

enter image description here

  • 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.
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  • Thank you for your answer. Do you know how to fix the barrel distortion? – Tomáš Zato - Reinstate Monica Nov 29 '17 at 14:38
  • You can try to eyeball something using Filters>Distorts>Lens distortion but this is more meant to add distortion than to fix an existing one. Otherwise there is a gimp-lensfun plugin but it depends if your camera+lens are supported or if you can create the necessary calibration data. – xenoid Nov 29 '17 at 15:33
  • How can I ensure it's exactly a rectangle? Dragging by eye only is wouldn't be that accurate for me. – AndrewHarvey Jun 1 '20 at 9:52
  • ^ found out from youtube.com/watch?v=3vvKu36Vg8A you can create guidelines by dragging down or across from the ruler giving a nice rectangle that I can ensure I transform into. – AndrewHarvey Jun 1 '20 at 9:55
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As @xenoid said, you need to correct the perspective, sometimes called quadrilateral reshape. As of GIMP 2.10.22, you can do this using the Handle Transform Tool (Shift+L) in the Unified Transform group (enter image description here).

Simple switch to the tool, drop handles at the 4 corners of your image and then drag them around until your perspective is what you want it to be.

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