Rotational correction and duplicate image identification by fourier transform correlation

A method of determining the degree of similarity between imaged pages, comprising the steps of:

• providing a reference and a target page, both in image form;
• rotationally aligning at least one of said target and reference pages by (a) identifying periodic components in said at least one page by performing a two dimensional Fourier transform of said page to obtain a power spectral density distribution (b) finding a page axis which corresponds in orientation with the periodic components in said at least one page, by performing a linear regression analysis of said power spectral density distribution to compute an angle, relative to a reference axis, of a page axis which best fits said power spectral density distribution of said page, and (c) rotating said at least one page through said angle to align said at least one page with said reference axis;
• cross-correlating in two dimensions said target and reference pages to produce a cross-correlation image;
• and
• analyzing said cross-correlation image to determine the presence of a correlation peak which indicates similarity between said reference and said target pages.