Digital watermarking technology is often used to embed ID data in analog content (such as an image on printed material), and recently, many algorithms have been proposed for using a camera-equipped cellular phone to detect watermarks in a captured image. One of these, based on information embedding in a frequency space has the advantage of robustness against a partial distortion of the captured image, but it is hampered by a low limit of embedded information quantity. A watermarking method for improving the embedded information quantity by using multiple-valued area of signals is proposed. Multiple-value conversion in the signal area greatly increases the amount of embedded information compared with the conventional binarization algorithm. Experimentation with actual natural images demonstrated the feasibility of the proposed algorithm.
