There is provided a method of measuring the similarity of parts of digital image files (IF1, IF2,IF2-z)), which comprising the steps of calculating a first change value in a similarity between pixel values in a first segment (BIF1a) of a first digital image file (IF1) and in a second segment (BIF1b) of this first file (IF1), said first and second segments of said first file being spatially separated by a first translation vector (td1), calculating a second change value (v) in the similarity between pixel values in a first segment (BIF2a) of the second digital image file (IF2, IF2-z) and in a second segment (BIF2b) of the second file (IF2, IF2-z), said first and second segment of said second file being spatially separated a second translation vector (td2), wherein the first segment of the first file corresponds to substantially same part of the image as the first segment of the second file and wherein the second segment of the first file corresponds to substantially the same parts of the image as the second segment of the second file, and calculating a structure evolution value indicative of the similarity between the first and second changes.