Determining document authenticity in a closed-loop process
A document authentication method uses a watermark added in a printed document to detection possible alterations made to the document after it was printed. First, a visible watermark in the form of a dot pattern is overlapped with an original digital image. The watermarked image is printed out as a halftone image at a first resolution. The watermark in the printed document appears as a light gray shade. Later, the printed document is scanned back using a grayscale scan at a resolution higher than the first resolution. In the scanned image, altered areas would appear flat (lacking intensity variation) whereas unaltered areas will have relatively large density variations due to the watermark dots and the fact that the image was halftone printed at a lower resolution. Alternations are detected by identifying flat areas within the image using a combination of flat block detection and a multiple thresholds method.
1. A method for generating 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; and
printing the document including the watermark using halftone printing at the first spatial resolution.
2. A computer program product comprising a computer usable medium having a computer readable program code embedded therein for controlling a data processing apparatus, the computer readable program code configured to cause the data processing apparatus to execute a process for generating a printed document, the process 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; and
printing the document including the watermark using halftone printing at the first spatial resolution.
3. A data processing system, comprising:
a processing section for processing the a digital document image, including 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; and
a printing section for printing the document including the watermark using halftone printing at the first spatial resolution.