US Patent 8006092 Digital watermarks for checking authenticity of printed objects

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.