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

• US 7,889,390 B2
• Filed: 02/25/2008
• Issued: 02/15/2011
• Est. Priority Date: 02/25/2008
• Status: Expired due to Fees

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Wang, Shen-Ge
• Primary Examiner(s): Rogers; Scott A
• Application Number:
  US12/036,481
• Publication Number:
  US 20090213430A1
• Time in Patent Office: 1,086 Days
• Field of Search: 358/1.9, 358/3.06, 358/3.09, 358/3.11, 358/3.2, 358/3.28, 358533-536, 347/107, 283/113
• US Class Current: 358/1.9
• CPC Class Codes: H04N 1/32203 Spatial or amplitude domain methods H04N1/32309 takes precedence
• CPC Class Codes: H04N 1/32256 in halftone data
• CPC Class Codes: H04N 1/32309 in colour image data
• CPC Class Codes: H04N 1/52 Circuits or arrangements for halftone screening

Abstract

Systems and method for digitally reproducing a moiré-free color halftone output image having an embedded correlation-based digital watermark are provided using an enhanced halftone screen set consisting of a halftone screen for each of N colorants forming N color separations (where N≧3). The N different uniform rosette halftone screen configurations are generated such that each meets uniform rosette halftone screen conditions. A corresponding three-dimensional threshold array is generated for each halftone screen configuration having a phase shift value as an input. The color input image is halftoned, halftoning each one of the N different color separations using a corresponding halftone screen configuration and three-dimensional threshold array to form a moiré-free color output image having the correlation-based digital watermark embedded therein. A single key can be used to produce a full color image of the output image having the watermark image visible therein.

Claims

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_i1V_R1+m_i2V_R2, n_i1V_R1+n_i2V_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 [kπ

(x·

y_ay·

x_a)+s]+cos [kπ

(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_i1V_R1+m_i2V_R2, n_i1V_R1+n_i2V_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 [kπ

(x·

y_a+y·

x_a)+s]+cos [kπ

(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_i1V_R1+m_i2V_R2, n_i1V_R1+n_i2V_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.