Patent ID: 7925045
Filing Date: 2011-04-12
Classification: G06K,G06T,H04N

Abstract:
1. A method for authenticating a printed document, comprising: obtaining a digital document image; adding a watermark to the digital document image, the watermark including a pattern of dots, each dot being larger than or equal to one by one pixel as defined by a first spatial resolution, wherein the adding step includes, for each pixel within a dot of the watermark, adding a first watermark intensity value to or subtracting a second watermark intensity value from an original pixel value of the pixel in the digital document image depending on the original pixel value; printing the document including the watermark using halftone printing at the first spatial resolution; scanning the printed document at a second spatial resolution equal to or higher than the first spatial resolution to obtain a scanned image; and detecting any alteration in the scanned image by identifying areas within the scanned image containing low pixel intensity variation.