1. A method implemented in a data processing apparatus for detecting alterations between an original image and a target image, the original and target images being binary bitmap images, the method comprising: (a) extracting connected image components from the original and target images; (b) calculating a centroid for each of the extracted connected image components in the original and target images; (c) comparing all centroids of the original image and all centroids of the target image to determine whether each centroid of the original image has a matching centroid of the target image, and whether each centroid of the target image has a matching centroid of the original image; (d) for each centroid of the original image that does not have a matching centroid in the target image, identifying the corresponding connected image component of the original image as a deleted connected image component; (e) for each centroid of the target image that does not have a matching centroid in the original image, identifying the corresponding connected image component of the target image as an added connected image component; (f) for each pair of matching centroids of the original and target images, comparing a pair of sub-images of the original and target images containing connected image components corresponding to the centroids to detect alterations; and (g) generating an alteration map which indicates any deleted or added connected image components identified in steps (d) and (e) and any alterations detected in step (f).