Digital Watermarks for Checking Authenticity of Printed Objects

• US 20080016360A1
• Filed: 07/17/2007
• Published: 01/17/2008
• Est. Priority Date: 01/24/2001
• Status: Active Grant

• Current Assignee: Digimarc Corporation
• Original Assignee: Digimarc Corporation
• Inventors: Haynes, Mark, Brunk, Hugh, Rhoads, Geoffrey, Rodriguez, Tony
• Granted Patent: US 8,006,092 B2
• Time in Patent Office: Days
• US Class Current: 713/176
• CPC Class Codes: G06T 1/0021 Image watermarking
• CPC Class Codes: G06T 2201/0051 Embedding of the watermark in the spatial domain
• CPC Class Codes: G06T 2201/0065 Extraction of an embedded watermark; Reliable detection
• CPC Class Codes: H04N 17/004 for digital television systems
• CPC Class Codes: H04N 21/4425 Monitoring of client processing errors or hardware failure
• CPC Class Codes: H04N 21/8358 involving watermark protecting executable software by watermarking G06F21/16; image watermarking in general G06T1/0021; watermarks inserted in still images for transmission purposes H04N1/32144; inserting watermarks during video coding H04N19/467

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 a halftoning process. If copies are made of the printed image, the image characteristics change due a change in the halftoning process used to create the copies. These changes are detected to determine whether a suspect document is authentic.

Claims

1. A method of embedding a digital watermark in a digital image to be printed on an object comprising:

generating an image to be printed based on the digital image, including converting the digital image to a halftone image; and

embedding an auxiliary signal in the digital image so that the auxiliary signal is substantially imperceptible, yet machine readable; and

encoding a message in the auxiliary signal that includes halftone information, the halftone information being automatically readable from a printed version of the image to enable checking whether measured attributes of the printed version of the image correspond to the halftone information.

2. The method of claim 1 including spread spectrum modulating a carrier signal with message to form the auxiliary signal.

3. The method of claim 2 wherein the message includes information used to determine the type of halftone conversion used to create the halftone image.

4. A computer readable medium on which is stored software for performing the method of claim 1.

5. The method of claim 1 wherein the auxiliary signal is embedded in the digital image prior to converting the digital image to a halftone image.

6. A method of authenticating a printed object comprising:

receiving a digital image scanned of the printed object;

detecting a digital watermark in the digital image;

extracting halftone information from the digital watermark; and

evaluating the digital image for attributes that correspond to the halftone information from the digital watermark to determine authenticity of the printed object.

7. The method of claim 6 including:

using the digital watermark to align the digital image geometrically before evaluating the digital image for attributes that correspond to the halftone information.

8. The method of claim 6 wherein the evaluating includes evaluating the digital image for attributes corresponding to a halftone screen type identified in the halftone information.

9. The method of claim 6 wherein the evaluating includes evaluating the digital image for signal peaks corresponding to a halftone type identified in the halftone information.

10. A computer readable medium on which is stored software for performing the method of claim 6.

11. A method of authenticating a printed object comprising:

detecting a first digital watermark from a substrate of the printed object;

detecting a second digital watermark from an image scanned from the printed object; and

using the relationship between the first and second digital watermarks to determine authenticity of the printed object.