Using invisible watermarks to protect visibly watermarked images
Physical visible watermarks have been widely used for centuries. Now digital visible watermarks such as electronic logos find their applications in digital library, video broadcasting, and other multimedia services. Several visible watermarking techniques have been proposed in the literature, and meanwhile, some problems with visible watermarks are also under investigation. Among these problems, watermark removal and unauthorized insertion are two major concerns. We propose using an invisible watermark in visibly watermarked images to overcome these problems. When a visibly watermarked image is in question, the invisible watermark can provide appropriate ownership information. We first investigate what kind of invisible watermark is needed, and then, focus on the details of the invisible watermarking technique. The experiments have shown that the proposed algorithm can provide a very effective protection for visibly watermarked images.