transform a trapezium into a rectangle

Question

I use my iPhone to capture a painting. However, the angle was not perfect, so instead of getting a straight rectangle, I get a trapezium. However, I want to transform this trapezium back into a rectangle (using some affine transformation). However, I cannot find a good way to do it. Please advice.

Answer 1

In GIMP, it is called the perspective tool. I think it is the same in Photoshop.

There is an icon for it in the menu on the left. It looks like a trapezoid with two arrows at the bottom corners.

Answer 2

paint.net will do this - you're looking for the 'layers', 'rotate/zoom' option. just move the ball to get it aligned

Answer 3

I know there already is an accepted answer, but Hugin that can also be used to fix perspectives in pictures, and is much better at it than a regular photo editing software. There is a tutorial on their website explaining how to do this.

Answer 4

ImageMagick gives you the -distort Perspective option. Like explained in Perspective Distortion:

convert image.jpg -distort Perspective '7,4 4,3 4,1 4,1 85,1 100,1 85,2 100,3' result.jpg