Digital Watermark Technology

With the wide application of network technology and digitalization of information media, digitalized information can be conveniently and quickly transferred to every part of the world with the assistance of computers, digital scanners, numeral cameras, etc. Furthermore, it is highly possible for fully duplication and widespread transferring of digital images, audio and visual digital products through the network. However, when digital files or products are being transferred on the Internet, some malicious personnel or groups may try to duplicate and disseminate copyright contents without authorization. Therefore, it has become an urgent issue as to how to provide a limited and effective protection for digital products and also how to maintain a secure communication system. Meanwhile, conventional data encryption techniques seem unable to keep an effective checking and tracking for some data's legality and their property rights when a normal employment of these data is required.With the development of modern data hiding technique, the protection of various information based on similar nature-color-protection principle will greatly reduce the danger of hidden data. Firstly, nobody ,except the author, knows that there have hidden data . Consequently, the others will not be spurred to decrypt the data. Secondly, the hidden data would not bedisseminated even if the carrier data were destroyed.Digital watermark is one of the hiding techniques. The technique embeds some data into a carrier data, and these data can be checked any time with some methods. Therefore, the embedded information ,as required, should not interfere the normal use of the carrier data, and this is the hiding requirement of watermark technique. Also, the carrier data might be compressed or be processed so as to suit various requirements. Even under these situations, the watermark should still be checked, which is called as robustness. After many researchers' hard work for several years, it have been proved that watermark techniques must be noninvertible, otherwise, the correctness and uniqueness of the embedded information can not be guaranteed, and thus it's authenticity would be doubtful.Since transparency and excellect effect are the two contradictory sides of watermark technique, to find the balance point between the two is the key to the problem. Usually, to enhance transparency, greater watermark energy should be added to the high frequency part of uncorrupted image; meanwhile, to satisfy excellect effect, watermark loss which results from ordinary signal processing, wearing compression and the likes should be avoided. To solve the problem, many scholars recently put forward various watermark algorithms.According to the researches on Human Visual System, human eyes are relatively sensitive to low frequency area noise. Therefore, to reduce the notability, watermarks should be embedded into the higher frequency area. However, the image energy mainly lies in low frequency area, and consequently watermarks embedded into high frequency area are susceptible to the loss caused by various processing methods. The current paper, in order to solve the problem, studied the technique of embedding the watermarks into the mid-frequency area of uncorrupted image, and thus satisfied both the transparency and excellect effect to some extent.Although digital watermark technique has only been studied domestically for several years, the corresponding software industry has been welldeveloped. Being a new thing in data security field, it is highly technical and vigorous. At present, it is widely applied to property right protection, digital broadcast, multi-media authentication, etc. Further development in watermark technology and the establishment of basic theory frame will enhance the application of it to a larger scale and various fields.Current watermark algorithm are still resistant to signal processing, geometric distortion of data and malicious attack, and is also resistant to wave or noise attack. However, all of this are based on cu...