Methods and apparatus for watermarking digitally printed documents
View Patent ↗A form of stochastic random distribution is used to print an invisible watermark using a typical, e.g., 600 dpi, printer. The watermark contains such information as “Do Not Copy,” or other such information. Although the watermark is not visible on a first generation version of a document that is printed, various techniques are used to print the watermark in such a way that it becomes readily apparent when the first generation document is photocopied. The preferred embodiment of the invention is concerned with a technique for improving such techniques, for example so that gray levels are improved to address variations from printer to printer. Another preferred embodiment of the invention is also concerned with a technique for performing error diffusion watermarking without the need for performing a calibration step.
1. A method for printing a watermark in a document, the method comprising:
generating foreground data comprising the watermark;
generating background data;
applying the foreground data to a first screen to produce a first screened image;
applying the background data to a second screen to produce a second screened image wherein at least one of the first and second screens is a stochastic screen; and
combining the first screened image and the second screened image for printing as a single image, wherein the foreground data are not visible on a first generation version of a document that is printed, and wherein the foreground data are readily apparent when the first generation document is photocopied.
2. The method of claim 1 , the stochastic screens comprising any of a stochastic threshold screen and an error diffusion screen.
3. The method of claim 1 , wherein:
the stochastic screen comprises larger clustered dots with wider spacing between the dots; and
the other stochastic screen comprises smaller clustered dots with narrower spacing between the dots.
4. The method of claim 1 , wherein a discrepancy in reproducibility between the foreground image and the background image translates to a difference in their perceptual intensity, and renders a watermark easily visible on a photocopy.
5. Apparatus for printing a watermark in a document, the apparatus comprising:
foreground data comprising the watermark;
background data;
a first screen for producing a foreground screened image;
a second screen for producing a background screened image, wherein at least one of the first and second screens is a stochastic screen; and
means for combining the foreground screened image and the background screened image for printing as a single image, wherein the foreground screened image is not visible on a first generation version of a document that is printed, and wherein the foreground screened image is readily apparent when the first generation document is photocopied.
6. The apparatus of claim 5 , the stochastic screen comprising any of a stochastic threshold screen and an error diffusion screen.
7. The apparatus of claim 5 , wherein a stochastic screen comprises larger clustered dots with wider spacing between the dots, and the other stochastic screen comprises smaller clustered dots with narrower spacing between the dots.