Maybe this is useful?
http://web.archive.org/web/20091027131421/http://geocities.com/evilsnack/matrix.htm
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I was using the Perspective tool (GIMP 2.6) on some photos to correct for some
minor rotation/skew issues, and realized how easy it is to throw the image's
aspect ratio out of proportion -- in my case, TOO easy. My photos were of some
quilt squares, and I was using the edges of these square