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. While 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 said watermark;
generating background data;
applying said foreground data to a line screen to produce a foreground screened image having alternating or unequal line spacing;
applying said background data to a line screen to produce a background screened image having equal line spacing; and
combining said foreground screened image and said background screened image for printing as a single image;
wherein said foreground screened image is not visible on a first generation version of a document that is printed; and
wherein said foreground screened image is readily apparent when said first generation document is photocopied.
2. The method of claim 1 , wherein said foreground line screen spaces at least some of the lines to be printed near to each other, wherein a photocopier is unable to resolve said lines into individual lines, and wherein said lines have greater perceptual intensity when photocopied than when printed.
3. The method of claim 1 , wherein both said foreground data and said background data are printed using line screens at a same angle.
4. The method of claim 1 , wherein said foreground and background screened lines are one pixel wide and have different spacing therebetween.
5. The method of claim 4 , wherein in said line-screened background image, all of said lines are equally spaced substantially at 100 lines per inch or below; and wherein said printer puts down one horizontal line of ink for every six rows of pixels, with a constant spacing of five pixels, achieving 100 lines per inch, which can be accurately reproduced on a photocopy machine.
6. The method of claim 4 , wherein in said line-screened foreground image, said line spacing alternates between two values, one very small and another larger, such that an average spacing in said foreground image is the same as in said background image, and therefore both have a same average intensity.
7. The method of claim 6 , wherein said line-screened foreground image comprises spacing of two and eight pixels; and wherein any two lines that are spaced two pixels apart are not reproducible on a typical photocopier.
8. The method of claim 1 , wherein a discrepancy in reproducibility between said foreground image and said background image translates to a difference in their perceptual intensity, and renders a watermark easily visible on a photocopy.
9. A method for printing a watermark in a document, the method comprising:
generating foreground data comprising said watermark;
generating background data;
using constant and photocopy reproducible line spacing in connection with said background data: and
using variable and photocopy irreproducible line spacing in connection with said foreground data;
wherein there is a variance between a foreground image and a background image in perceptual intensity when a printed document comprising both said foreground image and said background image is photocopied.
10. Apparatus for printing a watermark in a document, the apparatus comprising:
means for processing foreground data with a foreground line screen, said foreground line screen comprising variable and photocopy irreproducible line spacing;
means for processing background data with a background line screen, said background line screen comprising constant and photocopy reproducible line spacing; and
means for compositing said line-screened foreground and background data to produce a combined image for printing on a printer, wherein there is a variance between a foreground image and a background image in perceptual intensity when a printed document comprising both said foreground image and said background image is photocopied.
11. Apparatus for printing a watermark in a document, the apparatus comprising:
foreground data comprising said watermark;
background data;
a foreground line screen for producing a foreground screened image having alternating or unequal line spacing;
a background line screen for producing a background screened image having equal line spacing; and
means for combining said foreground screened image and said background screened image for printing as a single image, wherein said foreground screened image is not visible on a first generation version of a document that is printed, and wherein said foreground screened image is readily apparent when said first generation document is photocopied.
12. The apparatus of claim 11 , wherein said foreground line screen spaces at least some of the lines to be printed near to each other, wherein a photocopier is unable to resolve said lines into individual lines, and wherein said lines have greater perceptual intensity when photocopied than when printed.
13. The apparatus of claim 11 , wherein both said foreground data and said background data are printed using line screens at a same angle.
14. The apparatus of claim 11 , wherein said foreground and background screened lines are one pixel wide and have different spacing therebetween.
15. The apparatus of claim 14 , wherein in said line-screened background image, all of said lines are equally spaced substantially at 100 lines per inch or below; and wherein said printer puts down one horizontal line of ink for every six rows of pixels, with a constant spacing of five pixels, achieving 100 lines per inch, which can be accurately reproduced on a photocopy machine.
16. The apparatus of claim 14 , wherein in said line-screened foreground image, said line spacing alternates between two values, one very small and another larger, such that an average spacing in said foreground image is the same as in said background image, and therefore both have a same average intensity.
17. The apparatus of claim 16 , wherein said line-screened foreground image comprises spacing of two and eight pixels; and wherein any two lines are spaced two pixels apart and are not reproducible on a typical photocopier.
18. The apparatus of claim 11 , wherein a discrepancy in reproducibility between said foreground image and said background image translates to a difference in their perceptual intensity, and renders a watermark easily visible on a photocopy.
19. Apparatus for printing a watermark in a document that comprises foreground data comprising said watermark, and background data, the apparatus comprising:
a first line screen for providing constant and photocopy reproducible line spacing in connection with said background data; and
a second line screen for providing variable and photocopy irreproducible line spacing in connection with said foreground data, wherein there is a variance between a foreground image and a background image in perceptual intensity when a printed document comprising both said foreground image and said background image is photocopied.
20. A method for printing a watermark in a document, the method comprising: processing foreground data with a foreground line screen, said foreground line screen comprising variable and photocopy irreproducible line spacing;
processing background data with a background line screen, said background line screen comprising constant and photocopy reproducible line spacing; and
compositing said line-screened foreground and background data to produce a combined image for printing on a printer, wherein there is a variance between a foreground image and a background image in perceptual intensity when a printed document comprising both said foreground image and said background image is photocopied.