METHOD FOR COHERENT WATERMARK INSERTION AND DETECTION IN COLOR HALFTONE IMAGES

• US 20080151311A1
• Filed: 12/22/2006
• Published: 06/26/2008
• Est. Priority Date: 12/22/2006
• Status: Active Grant

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Wang, Shen-ge, Sharma, Gaurav
• Granted Patent: US 7,952,767 B2
• Time in Patent Office: Days
• US Class Current: 358/3.28
• CPC Class Codes: H04N 1/32144 embedded in the image data, i.e. enclosed or integrated in the image, e.g. watermark, super-imposed logo or stamp
• CPC Class Codes: H04N 1/32256 in halftone data
• CPC Class Codes: H04N 1/32309 in colour image data

Abstract

A method for generating an authenticable color image, the color image including a plurality of color separations, wherein an authentication image inserted in the multicolor image is not readily visually perceptible, includes halftoning image data corresponding to a first color separation using a single halftone screen, wherein the halftone screen includes means for generating an authentication image in a color image; and halftoning image data corresponding to a second color separation using the halftone screen and dot placement information for the image data corresponding to the first color separation to form a multicolor image; wherein halftoning of image data corresponding to the second color separation includes placing dots for the second color separation in thresholds of the halftone screen relative to those thresholds occupied by the first color separation in the halftone screen in accordance with a predetermined relationship.

Claims

1. A method for generating a color image carrying an embedded watermark image, the color image including a plurality of separations, wherein a watermark image inserted into the color image is not readily visually perceptible, comprising:

a. providing a single halftoning process for the plurality of separations, wherein the halftoning process includes a periodic tile, each tile offset from its neighbors by a fixed displacement, and where said periodic tile includes a first region and a second region and a first part of the first region is substantially identical to a corresponding part of the second region, and a second part of the first region is substantially conjugate to a corresponding part of the second region;

b. halftoning image data from the color image using the single halftoning process wherein a set of pixel locations associated with a first threshold value are filled by a first of the plurality of separations;

c. halftoning the image data using the single halftoning process wherein a second set of pixel locations associated with second of the plurality of separations, the second set of pixel locations are filled successively to the first set of pixel locations, such that when a first copy of the color image is spatially offset from a second copy of the color image by an integral multiple of the fixed displacement, at least the first part of the first region aligns with a second part of the second region and a second part of the first region aligns with a first part of the second region and contrast of the identical and conjugate first and second parts become visible to reveal the watermark image.

2. The method of claim 1, wherein the single halftoning process comprises comparing the image values to a stochastic screen of thresholds.

3. The method of claim 2, further comprising halftoning image data corresponding to a third of the plurality of separations using the single stochastic screen of thresholds, s wherein a third set of pixel locations associated with third of the plurality of separations, the third set of pixel locations are filled successively to the second set of pixel locations.

4. The method of claim 3, wherein the first color separation represents black, the second color separation represents magenta and the third color separation represents cyan.

5. The method of claim 2, further comprising:

determining a first separation halftone error as the difference between the first color separation image data value and first color separation halftone image data output value; and

adding the first separation halftone error to the second color separation image data prior to halftoning image data corresponding to the second color separation.

6. The method of claim 1 wherein the single halftoning method further comprises an error diffusion screen.

7. The method of claim 6 wherein the periodic tile comprises adding a tile of bias values to the threshold of the error diffusion screen, wherein at least one region of the tile raises the threshold, and at least one remaining region of the tile lowers the threshold.

8. The method of claim 6 wherein the error diffusion screen comprises

a. first computing an error diffused value based upon a sum of the values of all of the plurality of separations to determine how many of the separations are to be printed at each pixel location,

b. independently performing an error diffusion on each separation, and

c. printing only the number of separations that has been determined in step 8a.

9. The method of claim 8 wherein the separations to be printed are printed in order of increasing luminance impact, with the darkest separations printed first.

10. A system for generating an authenticable color image, the color image containing a plurality of separations, wherein an authenticable image inserted into the color image is not readily visible, comprising:

a circuit to receive color image data for the image to be authenticated;

a halftoning circuit coupled to the receiving circuit that halftones the color image data, generating a binary color image therefrom;

a tiling circuit coupled to the halftoning circuit that adds a tile to the halftoning circuit, the tile being periodically and regularly repeated throughout the halftoned image; and

a control circuit coupled to the receiving circuit and the halftoning circuit that controls the successive halftoning of each of the plurality of separations, wherein each successive separation fills pixels successive to the previously halftoned separation.

11. The system of claim 10, wherein the halftoning circuit comprises a stochastic screen.

12. The system of claim 11 wherein the tiling circuit comprises a stochastic mask with a first region and a second region, wherein a first part of the first region is substantially identical to a corresponding part of the second region, and wherein a second part of the first region is substantially conjugate to a corresponding part of the second region.

13. The system of claim 11 wherein the halftoning circuit further comprises means for determining a first separation halftone error between the first color separation image data and first color separation output halftone image data and adding the first separation halftone error to the second color separation image data prior to halftoning image data corresponding to the second color separation.

14. The system of claim 10 wherein the halftoning circuit comprises means for performing an error diffusion halftoning on the color image data.

15. The system of claim 14 wherein the tiling circuit includes a tile that adds a bias value to the threshold of the error diffusion screen, wherein some regions of the tile raise the threshold, and the remaining regions of the tile lower the threshold.

16. The system of claim 14 wherein the halftoning circuit includes means to perform a three step error diffusion halftoning on the color image.

17. The system of claim 16, where the halftoning circuit first computes a multi level error diffused value based the sum of the values of all of the plurality of separations to determine a number of the separations to be printed at each pixel location, and then computes an error diffused value for each separation, and finally outputs only the number of separations that has been determined in the first step.