Patent ID: 7561308

Claim:
A method for decoding an encoded image produced from a primary image and at least one secondary image using at least one encoding parameter, the encoded image being formed so that when the encoded image is printed, the secondary image is not discernible to a viewer without an optical decoding device having characteristics corresponding to the at least one encoding parameter, the method comprising: obtaining a digital version of the encoded image formed so that when the encoded image is printed, the secondary image is not discernible to a viewer without an optical decoding device having characteristics corresponding to the at least one encoding parameter; determining the at least one encoding parameter; and constructing a decoded image from the digital encoded image using the at least one encoding parameter, wherein the action of constructing a decoded image includes rasterizing a control image to produce a rasterized control image having a rasterization frequency and orientation derived from the at least one encoding parameter, comparing each pixel of the encoded image to a corresponding pixel in the rasterized control image to identify those encoded image pixels whose content is different from the content of the corresponding rasterized control image pixel, constructing a difference image using the encoded image pixels whose content is different from the content of the corresponding rasterized control image pixel, and forming the decoded image from the difference image.