Position based hardcopy watermarking for document authentication

This paper investigates hardcopy watermarking based on the embedding of transparent logos into documents. Grayscale logos are embedded into specific locations of the text document aiming communication over the print-and-scan (PS) channel. The authenticating message is encoded into the document by selecting the relative position of the logos. At the detector, the logo positions are estimated by correlation and an optimal detection threshold is derived considering a print-scan channel model. An error-rate analysis is provided for the method. By employing character segmentation, the technique can be also combined with other popular text hardcopy watermarking techniques. The method is verified to be robust for the PS channel with high transparency. Results illustrate the validity of the analyses and the applicability of the method to practical documents.