An improved method and computer program product for detecting an amount of
rotation or magnification in a modified image, of the type including the
steps of: a) embedding a marker image having a pair of identical features
separated by a distance d and oriented at an angle .alpha. in an original
image to produce a marked image, the marked image having been rotated
and/or magnified to produce the modified image; b) performing an
autocorrelation on the modified image to produce a pair of autocorrelation
peaks corresponding to the location of the features of the marker image in
the modified image; and c) comparing the separation d and orientation
.alpha. of the autocorrelation peaks with the separation d and orientation
.alpha. of the features in the marker image to determine the amount of
rotation and magnification in the modified image, wherein the improvement
comprises the step of: d) preprocessing the modified image to have a
constant standard deviation over all local regions in the image.
