Apparatus and methods for improving detection of watermarks in content that has undergone a lossy transformation

摘要

Techniques for improving detection of watermarks in content that has undergone a lossy transformation. One of the techniques is used when the message that is contained in a watermark belonging to a digital representation that is derived from an original watermarked digital representation cannot be decoded. The technique obtains information about the watermark by comparing the watermark vector for the watermark that cannot be decoded with a replica of the watermark vector from the original watermarked digital representation. The replica is made using the message. Depending on the degree of similarity, the watermark's presence and some of its characteristics may be determined. Another technique improves the robustness of watermarks that are used for authentication by employing a short (even single-bit) watermark vector to make the watermark and using the message needed for the authentication to determine where the watermark is located in the digital representation. Authentication of a digital representation is done by determining whether the watermark is present in the digital representation. In another technique, detection of the presence of a watermark is used to determine what areas of a digital representation have been subject to alteration. Techniques for synchronizing digital representations for watermark detection and other purposes include adding marks whose locations can be automatically detected only with the help of information that is external to the digital representation, such as a key, and adding marks to a sequence of digital representations and detecting the marks by summing the sequence.