Application of invisible image watermarks to previously half-toned images

The ability to detect the presence of invisible watermarks in a printed copy is generally useful to help establish ownership and authenticity. and to establish the origin of an unauthorized disclosure. Heretofore watermarking methods have been concerned with inserting a watermark into a digitized image of an entire composed page prior to its being halftoned for printing. However, this may not be feasible if elements of the page being composed are available only as blocks of text and sub-images, where each unmarked sub-image has previously been halftoned, perhaps even using different halftoning methods. Earlier, we presented a highly robust invisible watermarking method having a payload of one bit - indicating the presence or absence of the watermark. Using that method, it will be shown that it is a straightforward process to place one or more watermarks into a printed page composed of unmarked and previously halftoned sub-images, and to successfully detect the inserted watermark from a scan of the printed page. The method presented applies to a variety of invisible watermarking methods, not just the one used in the example.