Digital watermarking using stochastic screen patterns

• US 5,734,752 A
• Filed: 09/24/1996
• Issued: 03/31/1998
• Est. Priority Date: 09/24/1996
• Status: Expired due to Term

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Knox, Keith T.
• Primary Examiner(s): ROGERS, SCOTT A
• Application Number:
  US08/719,235
• Time in Patent Office: 553 Days
• Field of Search: 382/219, 382/212, 382/218, 382/278, 382/283, 382/294, 380/6, 380/9, 380/18, 380/23, 380/54, 380/59, 380/51, 283/901, 283/113
• US Class Current: 358/3.28
• CPC Class Codes: G06T 1/0028 Adaptive watermarking, e.g. Human Visual System [HVS]-based watermarking
• CPC Class Codes: G06T 2201/0051 Embedding of the watermark in the spatial domain
• 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 1/32352 Controlling detectability or arrangements to facilitate detection or retrieval of the embedded information, e.g. using markers for decoding, detection or retrieval operations H04N1/32149 and subgroups take precedence
• CPC Class Codes: H04N 2201/3233 of authentication information, e.g. digital signature, watermark
• CPC Class Codes: H04N 2201/327 which are undetectable to the naked eye, e.g. embedded codes

Abstract

A method for generating watermarks in a digitally reproducible document which are substantially invisible when viewed including the steps of: (1) producing a first stochastic screen pattern suitable for reproducing a gray image on a document; (2) deriving at least one stochastic screen description that is related to said first pattern; (3) producing a document containing the first stochastic screen; (4) producing a second document containing one or more of the stochastic screens in combination, whereby upon placing the first and second document in superposition relationship to allow viewing of both documents together, correlation between the first stochastic pattern on each document occurs everywhere within the documents where the first screen is used, and correlation does not occur where the area where the derived stochastic screens occur and the image placed therein using the derived stochastic screens becomes visible.

Claims

1. A process for generating a document to be authenticated, wherein an authentication image inserted therein is not readily visually perceptible, including the steps of:

generating a first stochastic halftone pattern, and generating a first user image using said first stochastic halftone pattern;

generating a second stochastic halftone pattern, related and distinct from the first;

generating a second user image using the first stochastic halftone pattern, and replacing portions of said second user image using the second stochastic pattern to form the authentication image therein;

whereby upon placing the first and second user images in superposition relationship to allow viewing of both user images together, correlation between the first stochastic halftone pattern in the first and second user images occurs everywhere within the first and second user images where the first stochastic halftone pattern is used, and visible non-correlation occurs where the area where the second stochastic pattern occurs and the authentication image placed therein using the second stochastic screen becomes visible to authenticate the document to be authenticated.

2. The process as described in claim 1, wherein said each of said first and second stochastic patterns are related as high spatial frequency patterns with relatively low power in any low spatial frequencies.

3. The process as described in claim 1, wherein said document to be authenticated is a color document comprising plural separations, and said first stochastic pattern is used in one of said separations in said first user image, and said first and second stochastic patterns are used together in one of said separations in said second user image.

4. The process as described in claim 1, wherein the document to be authenticated is a color document comprising plural separations, and said first stochastic pattern is used in one of said separations in said first user image, and said first and second stochastic patterns are used together in a distinct one of said separations in said second user image, which will not be printed or displayed with the same color as the separation which the first user image is printed or displayed.

5. A method of providing a document to be authenticated including the steps of:

generating a first user image from a first stochastic halftone pattern;

generating a second user image, using the first stochastic halftone pattern and a second, different stochastic halftone pattern, said second stochastic pattern used to form an authentication image in the second user image;

for a document to be authenticated and distributed, including therein one of said first and second user images;

providing the other of said first and second images to a user for authentication purposes;

providing a viewing arrangement to view said first and second user images in registered superposition, whereby correlation areas and non-correlation areas render said authentication image visible therein.

6. The method as described in claim 5, wherein said each of said first and second stochastic patterns are related as high spatial frequency patterns with relatively low power in any low spatial frequencies.

7. A method of providing a document to be authenticated including a continuous tone image, said continuous tone image defined pixels each having an intensity defined by an N-bit pixels, including:

generating a first user image from a first stochastic halftone pattern comprising binary values representing said first user image;

generating a second user image, using the first stochastic halftone pattern and a second, different stochastic halftone pattern, comprising binary values representing said second user image, said second stochastic pattern used to form an authentication image in the second user image;

receiving said document containing said continuous tone image defined by pixels, the intensity of which are defined by N-bit values;

for an area of the document containing said continuous tone image, replacing a first less significant bit value, with binary values from said first user image.

8. A method of authenticating a document processed in accordance with claim 7, including:

for an area of the document containing said continuous tone image, replacing a second less significant bit value, with binary values from said second user image;

detecting areas of correlation and non-correlation between the first and second less significant bits, which together will render the authentication image visible.

9. The method as described in claim 7 wherein said each of said first and second stochastic patterns are related as high spatial frequency patterns with relatively low power in any low spatial frequencies.

10. A method of providing a document to be authenticated including a continuous tone image, said continuous tone image defined pixels each having an intensity defined by an N-bit pixels, including:

receiving said document containing said continuous tone image defined by pixels, the intensity of which are defined by N-bit values, as a first user image;

generating a second continuous tone user image defined by pixels, the intensity of which are defined by N bit values, including a set of binary values at a less significant bit thereof, which are derived from a less significant bit of the first user image;

inserting into the second user image, an image pattern varying from said second user image, to form an authentication image in the second user image.

11. A method of authenticating a document processed in accordance with claim 10, including:

detecting areas of correlation and non-correlation between the first and second less significant bits, which together will render the authentication image visible.

12. A document authentication set, said document set including a first document to be authenticated, including an authentication pattern thereon;

a second document, prepared on a transparent substrate, to authenticate the first document, including an authentication pattern key thereon;

said authentication pattern including;

a first user image generated from a first stochastic pattern and a second user image using the first stochastic halftone pattern, and replacing portions of said second user image using a second stochastic pattern to form the authentication image therein;

whereby, when the first and second documents are superposed, correlation between the first stochastic halftone pattern in the first and second user images occurs everywhere within the first and second user images where the first stochastic halftone pattern is used, and non-correlation occurs where the area where the second stochastic pattern occurs and the authentication image placed therein using the second stochastic screen becomes detectable to authenticate the document to be authenticated.

13. A method for generating watermarks in a digitally reproducible document which are substantially invisible when viewed including the steps of:

(1) producing a first stochastic screen pattern suitable for reproducing a gray image on a document;

(2) deriving at least one stochastic screen description that is related to said first pattern;

(3) producing a document containing the first stochastic screen;

(4) producing a second document containing one or more of the stochastic screens in combination, whereby correlation between the first stochastic pattern on each document occurs everywhere within the documents where the first screen is used, and correlation does not occur where the area where the derived stochastic screens occur and the image placed therein using the derived stochastic screens becomes detectable.

14. A method of providing a document to be authenticated including a continuous tone image described by at least one separations, each separation of said continuous tone image defined by pixels each having an intensity defined by N-bits, including:

generating an authentication image from a first pattern comprising binary values in a less significant bit in one of said separations;

generating an authentication key image, using the first pattern and a second, different pattern, in a different less significant bit in one of said separations;

whereby upon placing the authentication image and key in superposition relationship to allow viewing of both user images together, correlation between the first stochastic halftone pattern in the first and second user images occurs everywhere within the first and second user images where the first stochastic halftone pattern is used, and visible non-correlation occurs where the area where the second stochastic screen occurs and the authentication image placed therein using the second stochastic pattern becomes visible to authenticate the document to be authenticated.

15. The method as provided in claim 14, wherein said first and second patterns are stochastic halftone patterns.

16. The method as provided in claim 14, wherein said first and second patterns are derived from the continuous tone image.