To permit scaling or rotation of an image to be discernedeven if the original image is not availablecertain marker signals are steganographically encoded in the image. In one particular implementation, encoding is effected by adding an overlay signal to the image. The overlay signalwhen represented in...http://www.google.com/patents/US6567533?utm_source=gb-gplus-sharePatent US6567533 - Method and apparatus for discerning image distortion by reference to encoded marker signals
