A Watermarking Algorithm by Modifying AC Coefficies in DCT Domain

A blind watermarking algorithm based on DCT for digital image is proposed, in which a 2-bit image embeds in a 8-bit gray image. Preprocess of image watermark, watermark embedding and extraction are the main parts of the algorithm. In the preprocess of image watermark Arnold scrambling is used to reduce the relevance of the watermark and also to improve the robustness of the image embedded watermark by scattering the bits of watermark which have been damaged by some attacks such as shearing. In the process of watermark embedding the original image is divided into some 8 times 8 blocks and each block is transformed into DCT coefficies which are sorted by means of Zig-Zag.An AC coefficie is selected in each block and the average of all of them is calculated. And then the AC coefficie is modified into a positive or negative value around the average according to the coefficie and the current value of the image watermark.In the process of extraction the bit of watermark is set to 1 or 0 according to positive and negative judgment of the AC coefficie.The experimental results show that the algorithm is robust and can resist some digital image attacks, such as JPEG compression, filtering, noise and shearing.