I used my iPhone to capture a painting. However, the angle was not perfect, so instead of getting a straight rectangle, I get a trapezium. 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 advise.
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.
paint.net will do this - you're looking for the 'layers', 'rotate/zoom' option. just move the ball to get it aligned