Digital watermarking using conjugate halftone screens

A process for generating an authenticable document, wherein an authentication image inserted therein is not readily visually perceptible, including the steps of:

• generating a first halftone screen;
• generating a first image using the first halftone screen;
• generating a second halftone screen, related and distinct from the first halftone screen to form the authentication image wherein a region of the second halftone screen is substantially a conjugate of a corresponding region of the first halftone screen and the remaining region of the second halftone screen is identical to the remaining region of the first halftone screen;
• generating a second user image using the second stochastic halftone screen;
• superposing the first and second images to enable the detection of a conjugate correlation between the halftone screens in the first and second user images everywhere within the corresponding regions where the second halftone screen is a conjugate of the first halftone screen and auto-correlation between the halftone screens in the remaining regions where the second halftone screen is identical to the first halftone screen, the authentication image placed therein becoming detectable in contrast between the conjugate correlation and the auto-correlation to allow authentication of the authenticable document.

A method of providing an authenticable document which is self-referencing, including the steps of:

• generating a first stochastic screen;
• generating a second stochastic screen, related and distinct from the first stochastic screen to form the authentication image therein, wherein a region of the second stochastic screen is substantially conjugate of a corresponding region of the first screen and a remaining region of the second stochastic screen is identical to a remaining region of the first halftone screen;
• generating a user image by alternatively using the first stochastic halftone screen and the second stochastic halftone screen;
• for a document to be authenticated and distributed, including the generated user image, selecting a first portion generated by the first stochastic screen from the user image and a second portion generated by the second stochastic halftone screen from the same user image and superposing the two portions, whereby the authentication image placed therein using the second stochastic halftone screen is detectable as a result of contrast between the conjugate correlation of the corresponding regions and the auto-correlation of the remaining regions to authenticate the authenticable document.

A method for authenticating a document including a digital watermark, including:

• concurrently designing a first stochastic screen and a second stochastic screen simultaneously in accordance with an optimization metric, said designing step further including the constraint that a corresponding authentication region of both screens be comprised of values which are conjugates of one another;
• rendering an image using the first stochastic screen and the second stochastic screen to produce a first image, wherein the image is rendered by alternating between the two stochastic screens so as to produce a self-referencing watermark;
• reproducing at least a portion of the image to produce a second image;
• and
• detecting the authentication region by superposing a representation of the first image and a representation of the second image in an offset relationship so as to optimize the contrast of the authentication region resulting from the conjugate relationship between the first stochastic screen and the second stochastic screen.