Full Spectrum coding is a method of providing a picture with a machine readable code image. The transparency and robustness of the code can be influenced by the embedding parameters, according to the intended application (e.g. robust or fragile marking). The code image contains hidden information, identifying the document in some way. In principle, it can be any binary pattern or grey-scale picture. Alternatively, it may be desirable to encode (binary) data, e.g. ID numbers or (biometric) templates. Such data should be transformed into a code image suitable for the proposed kind of embedding. In the presented implementation, the code image contains a binary block structure, similar to a 2-dimensional (bar) code. Even in the presence of noise, this code can be extracted from the magnitude spectrum of the captured image by iterated down-sampling. Full Spectrum is well-suited for application to printed documents. It survives graphical processes (halftone screening, printing, digitizing) and the reconstructed code image is invariant to shifting and robust to cropping. A technique is presented to perform registration for rotation and scaling of the captured image with respect to the original. The approach is to embed a marking signal based on perfect correlation sequences in the radial-polar representation of the code image. In this domain, the rotation angle and (under certain conditions) the scale factor, can be determined by linear cross-correlation. Additionally, techniques are proposed to obtain resistance to more general distortions.
A method of providing a digital image with a unique, machine readable, code image is presented. It is called "Full Spectrum," because the method uses Fourier transform techniques to embed the code in a wide range of spatial frequencies. The changes made to the original image by the encoding process are meaningless to the observer, and, by proper choice of the embedding parameters and the resolution (reproduction size) of the marked image, they can be made totally imperceptible. Full Spectrum has some intrinsic advantages for application to printed security structures in a document authentication/-identification environment. Using its mathematical properties, the method is shown to be invariant to shifting and cropping, which enables the code image to be reconstructed from a recording of the document with arbitrary position and size. Techniques to deal with possible rotation and scaling of the recorded image with respect to the (printed) original are elaborated. Finally, a general method is developed to match the reconstructed code image to the reference image (expected code image). Experiments show that the code image can survive various graphical transfer processes, such as halftone screening, printing and digitising. An actual document containing a printed Full Spectrum structure is demonstrated, the Security ID.