Digital watermarks for checking authenticity of printed objects
First Claim
1. A method of embedding a digital watermark in a digital image to be printed on an object comprising:
- embedding an auxiliary signal in the digital image so that the auxiliary signal is substantially imperceptible, yet machine readable;
converting the image to a halftone image using an inherently unstable halftone screen structure that is likely to cause ink flow errors when reproduced, wherein errors introduced by reproducing the unstable halftone screen structure incorrectly are automatically detectable by reading the auxiliary signal.
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Abstract
The invention provides methods for embedding digital watermarks for authentication of printed objects, and corresponding methods for authenticating these objects. One aspect of the invention is a method of embedding a digital watermark in a digital image to be printed on an object. The method embeds an auxiliary signal in the digital image so that the auxiliary signal is substantially imperceptible, yet machine readable. It converts the image to a halftone image using an inherently unstable halftone screen structure that is likely to cause ink flow errors when reproduced. The errors introduced by reproducing the unstable halftone screen structure incorrectly are automatically detectable by reading the auxiliary signal.
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Citations
16 Claims
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1. A method of embedding a digital watermark in a digital image to be printed on an object comprising:
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embedding an auxiliary signal in the digital image so that the auxiliary signal is substantially imperceptible, yet machine readable; converting the image to a halftone image using an inherently unstable halftone screen structure that is likely to cause ink flow errors when reproduced, wherein errors introduced by reproducing the unstable halftone screen structure incorrectly are automatically detectable by reading the auxiliary signal. - View Dependent Claims (2, 3, 4, 5)
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6. A method of authenticating a printed object comprising:
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receiving a digital image scanned of the printed object; detecting a digital watermark in the digital image; using a measurement of strength of the digital watermark to detect reproduction errors due to inaccurate reproduction of unstable halftone screen structures in the printed object. - View Dependent Claims (7, 8, 9, 10, 11)
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12. A method of authenticating a printed object comprising:
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detecting a first digital watermark from image data captured from a substrate of the printed object, the substrate having a varying structure that causes optical variations that convey the first digital watermark in the image data captured from the substrate; detecting a second digital watermark from the image data captured from the printed object; and
using the relationship between the first and second digital watermarks to determine authenticity of the printed object;
wherein the first digital watermark is embedded in the surface micro-topology of the printed object by varying the surface micro-topology according to the first digital watermark signal.
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13. A method of authenticating a printed object comprising:
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detecting a first digital watermark from image data captured from a substrate of the printed object, the substrate having a varying structure that causes optical variations that convey the first digital watermark in the image data captured from the substrate; detecting a second digital watermark from the image data captured from the printed object; and
using the relationship between the first and second digital watermarks to determine authenticity of the printed object;
wherein the first digital watermark is embedded in an invisible ink on the printed object.
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14. A method of authenticating a printed object comprising:
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detecting a first digital watermark from image data captured from a substrate of the printed object, the substrate having a varying structure that causes optical variations that convey the first digital watermark in the image data captured from the substrate; detecting a second digital watermark from the image data captured from the printed object; and
using the relationship between the first and second digital watermarks to determine authenticity of the printed object;
wherein the first digital watermark is used to geometrically align image data scanned from the printed object, and the second digital watermark is detected from the aligned image data. - View Dependent Claims (15)
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16. A method of authenticating a printed object comprising:
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detecting a visible fiducial in an image scanned from the printed object; detecting a location of a digital watermark hidden in the printed object; determining authenticity of the printed object by comparing the location of the digital watermark to the visible fiducial.
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Specification