| The development of the Internet has greatly contributed to the expanded use of digital media, for example, images, audio, video, VRML objects. These media can be duplicated unlimitedly without any quality loss and can also be easily distributed and tempered. This presents problems of digital media security, such as copyright protection and authentication, which creates a pressing need for digital media protection schemes. A new emerging technology, digital watermarking, protects digital media by embedding a robust signal directly into the media, thus providing a promising way to protect the digital media from illicit copying and manipulation.; In this thesis, we mainly consider digital image watermarking for ownership verification. Though lots of such schemes have been proposed in literature, there are still some problems that have not been addressed or have not been adequately addressed such as how to detect a watermark progressively while at the same time be robust to geometrical transformations, how to embed a watermark in the fractal domain and how to embed a watermark into an object in an image instead of into the whole image. Several new watermarking algorithms are proposed here to address these problems respectively and experiments show satisfactory results.; The next problem that has been ignored by the watermarking community is the joint ownership verification. If an image is created collaboratively by multiple owners, all current watermarking schemes will fail to protect the image from potential dishonest owners, since they are only designed to protect an image from dishonest users. To address this problem, a novel framework of the watermarking system that makes use of a secret sharing scheme is proposed. Based on this framework, several new watermarking algorithms are proposed where the secret watermark embedding key is split into shares and each owner holds one share. Multiple watermarks contributed by each owner are embedded into spectrum domain of the image and multiple thresholds are set so that the watermark can verify joint ownership as well as partial ownership. To the best knowledge of the author, the problem of joint ownership has not been addressed so far and the watermarking schemes proposed are the first ones to address them. |
