Dig water

The process of digital watermarking involves the modification of the original multimedia data to embed a watermark containing key information such as authentication or copyright codes. The embedding method must leave the original data perceptually un-changed, yet should impose modifications which can be detected by using an appropriate extraction algorithm. Common types of signals to watermark are images, music clips and digital video. The application of digital watermarking to still images is concentrated here. The major technical challenge is to design a highly robust digital watermarking technique, which discourages copyright infringement by making the process of watermarking removal tedious and costly.In general, the digital watermarking is classified into two classes by depending on the domain of watermarking embedding: the spatial domain watermarking and the frequency domain watermarking. The spatial domain watermarking algorithms usually embed the watermark to the least significant bits (LSBs) of the image pixels. The watermark is embedded on randomly selected pixels using neighbours dependant function. In the detection process, the idea is to verify the surrounding pixels to decide if one has truly been modified. The detection of changes in small details of the image is based on mathematical morphology. Furthermore, it is able to reject small distortion introduced by high quality image compression. However, the technique suffers from the major drawbackof spatial domain watermarking: frequency localization of modifications is difficult. Because the marks are on certain particular pixels, it is often impossible to detect frequency alterations applied to the entire image. . But, these algorithms have relatively low-bit capacity and are not resistant enough against the lossy image compression and other image processing. For example, JPEG operation may eliminate the watermark but, the frequency domain watermarking algorithms can embed more bits of watermark and are more robust against attack.