Digital watermarking using stochastic screen patterns

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.

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.

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.

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.