System and method for the generation of correlation-based digital watermarks using uniform-rosette color halftoning

1. An image forming method for digitally reproducing a moiré-free color: halftone image having an embedded correlation-based digital watermark using an enhanced halftone screen set consisting of a halftone screen for each of N colorants forming N color separations (where N≧3), the method comprising:

providing a color input image to be watermarked;

providing a watermark image to be embedded in the N-color image;

generating N different uniform rosette halftone screen configurations each meeting uniform rosette halftone screen conditions;

generating N three-dimensional threshold arrays each having a phase shift value as an input; and

halftoning the color input image by halftoning N different color separations using a different one of the halftone screen configurations and three-dimensional threshold arrays for each color separation to produce a moiré-free color output image having the watermark image embedded therein.

2. The method of claim 1 wherein the step of generating N different uniform rosette halftone screen configurations each meeting uniform rosette halftone screen conditions, the uniform rosette halftone screen conditions further comprising:

defining rosette fundamental frequency vectors V R1 , V R2 that satisfy a length and sum requirement to meet visual acceptability standards according to |V R1 |>f min , |V R2 |>f min , and |V R1 ±V R2 |>f min ;

defining N halftone screens for color separations i=1, N respectively (where N≧3), possessing first and second frequency vectors (V i1 , V i2 ), where no two screens possess identical fundamental frequency vector pairs; and

selecting fundamental frequency vectors for the N halftone screens according to (V i1 , V i2 )=(m i1 V R1 +m i2 V R2 , n i1 V R1 +n i2 V R2 ) for integer m's and n's, where for each color separation i, at least one fundamental frequency vector or its conjugate must also satisfy the following inequality: |V ik |>max [|V R1 |, |V R2 |, min[|V R1 +V R2 |, |V R1 −V R2 |]], k=1 or 2.

3. The method of claim 2 further comprising generating a key having a periodic structure matching the rosette patterns using the rosette fundamental frequency vectors V R1 , V R2 defined above.

4. The method of claim 3 further comprising retrieving the digital watermark by overlying the key on top of the color output image producing a color image of the output image having the watermark image visible therein.

5. The method defined in claim 1 wherein the three-dimensional threshold arrays are:

T ( x,y )=128−127·{cos [ k π( x·y a y·x a )+ s ]+cos [ k π( x·y b +y·x b )+ s ]}/2

where s is the phase shift in radians.

6. A system for digitally reproducing an N (where N≧3) color channel moiré-free color halftone output image having an embedded correlation-based digital watermark, comprising:

an input image source providing a color input image;

image memory for storing the input image to be watermarked;

watermark memory for storing the watermark image to be embedded in the color input image; and

an image processor for generating N (where N≧3) different uniform rosette halftone screen configurations each meeting uniform rosette halftone screen conditions, generating N three-dimensional threshold arrays each having a phase shift value as an input, and halftoning the color input image by halftoning N different color separations using a different one of the halftone screen configurations and three-dimensional threshold arrays for each color separation to produce a moiré-free color output image having the watermark image embedded therein.

7. The method defined in claim 6 wherein s=g·π/N g , where g is the gray level of the watermark and N g is the total number of gray levels contained in the watermark.

8. The system of claim 6 wherein the step of generating N different uniform rosette halftone screen configurations each meeting uniform rosette halftone screen conditions, the uniform rosette halftone screen conditions further comprising:

defining rosette fundamental frequency vectors V R1 , V R2 that satisfy a length and sum requirement to meet visual acceptability standards according to |V R1 |>f min , |V R2 |>f min , and |V R1 ±V R2 |>f min ;

defining N halftone screens for color separation i=1, N, respectively (where N≧3), possessing first and second frequency vectors (V i1 , V i2 ), where no two screens possess identical fundamental frequency vector pairs; and

selecting fundamental frequency vectors for the N halftone screens according to (V i1 , V i2 )=(m i1 V R1 +m i2 V R2 , n i1 V R1 +n i2 V R2 ) for integer m's and n's, where for each color separation i, at least one fundamental frequency vector or its conjugate must also satisfy the following inequality: |V ik |>max [|V R1 |, |V R2 |, min[|V R1 +V R2 |, |V R1 −V R2 |]], k=1 or 2.

9. The system of claim 8 further comprising generating a key having a periodic structure meeting the uniform rosette halftone screen conditions for retrieving the watermark from the N-color output image.

10. The system of claim 9 wherein the key is one of the N different uniform rosette halftone screen configurations.

11. The system of claim 9 wherein the key is not one of the N different uniform rosette halftone screen configurations.

12. The system of claim 9 further comprising retrieving the digital watermark by overlying the key on top of the color output image producing a color image of the output color image having the watermark image visible therein.

13. The system defined in claim 9 wherein the three-dimensional threshold arrays are:

T ( x,y )=128−127·{ cos [ k π( x·y a +y·x a )+ s ]+cos [ k π( x·y b +y·x b )+ s ]}/2

where s is the phase shift in radians.

14. The system defined in claim 13 wherein s=g·π/N g , where g is the gray level of the watermark and N g is the total number of gray levels contained in the watermark.

15. A method of retrieving a correlation-based digital watermark embedded in an N-color channel color image using N halftone screens each meeting uniform rosette halftone screen conditions comprising:

overlying a key having a periodic halftone structure meeting a uniform rosette halftone screen condition on top of the color image.

16. The method defined in claim 15 wherein the uniform rosette halftone screen conditions comprise:

defining rosette fundamental frequency vectors V R1 , V R2 that satisfy a length and sum requirement to meet visual acceptability standards according to |V R1 |>f min , |V R2 |>f min , and |V R1 ±V R2 |>f min ;

defining N halftone screens for color separations i=1, N, respectively (where N≧3), possessing first and second frequency vectors (V i1 , V i2 ), where no two screens possess identical fundamental frequency vector pairs; and

selecting fundamental frequency vectors for the N halftone screens according to (V i1 , V i2 )=(m i1 V R1 +m i2 V R2 , n i1 V R1 +n i2 V R2 ) for integer m's and n's, where for each color separation i, at least one fundamental frequency vector or its conjugate must also satisfy the following inequality: |V ik |>max [|V R1 |, |V R2 |, min[|V R1 +V R2 |, |V R1 −V R2 |]], k=1 or 2.