Image Watermarking for Quality Control Based on Modified Key-Dependent DCT Basis Functions

The vast majority of the available watermarking schemes, operate in the frequency domain. Use of key-dependent transforms has been proposed as an extra level of security for these schemes while providing at the same time the ability to distribute different levels of quality to different customers. The major drawback of such schemes, is the computational complexity, both in the generation of such transforms and making them orthogonal. The proposed scheme, creates a modified DCT basis based on a secret key, which is approximately orthogonalized by means of a fast and simple algorithm. The method is very useful in cases of JPEG input images, since it can be applied directly into the compressed domain. The main focus here is both on speed issues as well as on the fact that only authorized users can enjoy the best quality from the marked medium.