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This disclosure describes methods for using embedded auxiliary signals in documents for copy detection and other applications. In on application, the auxiliary signal is formed as an array of elements selected from a set of print structures with properties that change differently in response to copy

This disclosure describes methods for using embedded auxiliary signals in documents for copy detection and other applications. In on application, the auxiliary signal is formed as an array of elements selected from a set of print structures with properties that change differently in response to copy operations. These changes in properties of the print structures that carry the embedded auxiliary signal are automatically detectable. For example, the changes make the embedded auxiliary signal more or less detectable. The extent to which the auxiliary data is detected forms a detection metric used in combination with one or more other metrics to differentiate copies from originals. Examples of sets of properties of the print structures that change differently in response to copy operations include sets of colors (including different types of inks), sets of screens or dot structures that have varying dot gain, sets of structures with different aliasing effects, etc. Robust and fragile watermarks are used in Image Replacement Documents for a variety of applications. Digital watermarks act as on-board mediators in authentication of a variety of printed documents. Finally, digital watermarks are used to help manage quality of the scanners used in imaging systems.

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1. A method, comprising: determining, using a computing device, whether a machine-readable auxiliary signal is embedded in an image of a printed object, wherein determining comprises detecting at least one of a divergence or convergence of at least one characteristic of a set of two or more print st

1. A method, comprising: determining, using a computing device, whether a machine-readable auxiliary signal is embedded in an image of a printed object, wherein determining comprises detecting at least one of a divergence or convergence of at least one characteristic of a set of two or more print structures embedded in the printed object such that the machine-readable auxiliary signal becomes one of more or less detectable, wherein the at least one characteristic changes a detection zone of the auxiliary signal in the image based on the at least one characteristic having a gray level value that varies as a function of location in the printed object; andevaluating the machine-readable auxiliary signal, using the computing device, to determine whether the printed object is an original version of the printed object. 2. The method of claim 1, wherein the set of two or more print structures comprises a first color and a second color that change differently in response to a copy operation. 3. The method of claim 2, wherein at least one of the colors corresponds to an ink color that is out of gamut of at least one of a printer or scanner. 4. The method of claim 2, wherein a difference in luminance of the colors changes in response to the copy operation. 5. The method of claim 1, wherein the set of two or more print structures comprises a first print structure having a first dot gain characteristic and a second print structure having a second dot gain characteristic, wherein the first print structure is more susceptible to dot gain than the second print structure in response to a copy operation. 6. The method of claim 1, wherein a difference in luminance of the set of two or more print structures changes in response to a copy operation due to a difference in susceptibility to dot gain of the set of two or more print structures. 7. The method of claim 1, wherein the set of two or more print structures comprises a first print structure having a first aliasing characteristic and a second print structure having a second aliasing characteristic, wherein the first print structure aliases differently than the second print structure. 8. The method of claim 1, wherein the machine-readable auxiliary signal is embedded by varying continuity of line structures. 9. The method of claim 1, wherein one print structure comprises a line segment in a first color, and another print structure comprises a line segment in a second color, wherein the first color is different than the second color. 10. The method of claim 9, wherein the line segments of the different colors are generated by varying between the first color and the second color along a printed line. 11. The method of claim 1, wherein the evaluating comprises evaluating a frequency domain metric to detect changes in the print structures. 12. The method of claim 11, wherein the frequency domain metric is a radial frequency domain metric. 13. The method of claim 11, wherein evaluating the frequency domain metric comprises evaluating changes in color of a print structure. 14. A non-transitory computer-readable medium having instructions stored thereon, the instructions comprising: instructions to determine whether a machine-readable auxiliary signal is embedded in an image of a printed object, wherein determining comprises detecting at least one of a divergence or convergence of at least one characteristic of a set of two or more print structures embedded in the printed object such that the machine-readable auxiliary signal becomes one of more or less detectable, wherein the at least one characteristic of the print structures changes a detection zone of the auxiliary signal in the image based on the at least one characteristic having a gray level value that varies as a function of location in the printed object; andinstructions to evaluate the machine-readable auxiliary signal to determine whether the printed object is an original version. 15. The non-transitory computer-readable medium of claim 14, wherein the set of two or more print structures comprises a first color and a second color that change differently in response to a copy operation. 16. A method comprising: embedding, using a computing device, a machine-readable auxiliary signal in an image at embedding locations using at least one print structure wherein embedding comprises configuring the at least one print structure to change a detection zone of the auxiliary signal in the image, wherein the change causes at least one of a divergence or convergence of at least one characteristic of the at least one print structure such that the machine-readable auxiliary signal becomes one of more or less detectable, and wherein the change is based on the at least one characteristic having a gray level value that varies as a function of location in the image; andcreating a metric based at least in part on the machine-readable auxiliary signal embedded in the image, using a computing device, to facilitate detection of the convergence or divergence from a scanned image of a suspect printed object to determine whether the suspect printed object is a copy of a printed object. 17. A non-transitory computer-readable medium having instructions stored thereon, the instructions comprising: instructions to embed a machine-readable auxiliary signal in an image at embedding locations using at least one print structure wherein embedding comprises configuring the at least one print structure to change a detection zone of the auxiliary signal in the image, wherein the change causes at least one of a divergence or convergence of at least one characteristic of the at least one print structure such that the machine-readable auxiliary signal becomes one of more or less detectable, and wherein the change is based on the at least one characteristic having a gray level value that varies as a function of location in the image; andinstructions to create a metric, based in part on the machine-readable auxiliary signal embedded in the image, to facilitate detection of the convergence or divergence from a scanned image of a suspect printed object to determine whether the suspect printed object is a copy. 18. A method, comprising: determining, using a computing device, whether a machine-readable auxiliary signal is embedded in an image of a printed object, wherein determining comprises detecting at least one of a divergence or convergence of at least one characteristic of at least one print structure embedded in the printed object such that the machine-readable auxiliary signal becomes one of more or less detectable, wherein the at least one characteristic of the at least one print structure changes a detection zone of the auxiliary signal in the image based on the at least one characteristic having a gray level value that varies as a function of location in the printed object, and wherein the at least one print structure comprises a color; anddetermining, based on evaluating the machine-readable auxiliary signal, using a computing device, whether the printed object is an original version of the printed object. 19. The method of claim 18, wherein the machine-readable auxiliary signal is embedded by varying continuity of line structures. 20. A non-transitory computer-readable medium having instructions stored thereon, the instructions comprising: instructions to determine whether a machine-readable auxiliary signal is embedded in an image of a printed object, wherein determining comprises detecting at least one of a divergence or convergence of at least one characteristic of the at least one print structure embedded in the printed object such that the machine-readable auxiliary signal becomes one of more or less detectable, wherein the at least one characteristic of the at least one print structure changes a detection zone of the auxiliary signal in the image based on the at least one characteristic having a gray level value that varies as a function of location in the printed object, and wherein the at least one print structure comprises a color; andinstructions to determine, based on evaluating the machine-readable auxiliary signal, whether the printed object is an original version of the printed object. 21. The non-transitory computer-readable medium of claim 20, wherein the machine-readable auxiliary signal is embedded by varying continuity of line structures. 22. A device comprising: a memory configured to store an image of a printed object; anda processor operatively coupled to the memory and configured to: determine whether a machine-readable auxiliary signal is embedded in the image of the printed object, wherein determining comprises detecting at least one of a divergence or convergence of at least one characteristic of a set of two or more print structures embedded in the printed object such that the machine-readable auxiliary signal becomes one of more or less detectable, wherein the at least one characteristic of the print structures changes a detection zone of the auxiliary signal in the image based on the at least one characteristic having a gray level value that varies as a function of location in the printed object; andevaluate the machine-readable auxiliary signal to determine whether the printed object is an original version of the printed object. 23. A device comprising: a memory configured to store an image; anda processor operatively coupled to the memory and configured to: embed a machine-readable auxiliary signal in an image at embedding locations using at least one print structure, and wherein embedding comprises configuring the at least one print structure to change a detection zone of the auxiliary signal in the image, wherein the change causes at least one of a divergence or convergence of at least one characteristic of the at least one print structure such that the machine-readable auxiliary signal becomes one of more or less detectable, and wherein the change is based on the at least one characteristic having a gray level value that varies as a function of location in the image; andcreate a metric, based at least in part on the machine-readable auxiliary signal embedded in the image, to facilitate detection of the convergence or divergence from a scanned image of a suspect printed object to determine whether the suspect printed object is a copy. 24. A device comprising: a memory configured to store an image of a printed object; anda processor operatively coupled to the memory and configured to:determine whether a machine-readable auxiliary signal is embedded in the image, wherein determining comprises detecting at least one of a divergence or convergence of at least one characteristic of at least one print structure embedded in the printed object such that the machine-readable auxiliary signal becomes one of more or less detectable, wherein the at least one characteristic of the at least one print structure changes a detection zone of the auxiliary signal in the image based on the at least one characteristic having a gray level value that varies as a function of location in the image, and wherein the at least one print structure comprises a color; anddetermine, based on evaluating the machine-readable auxiliary signal, whether the printed object is an original version of the printed object.