An Improved Image Watermarking Method in Frequency Domain

Abstract

Digital watermarking is a technique for resolving copyright law in E-Commerce. In this article a combined watermarking method based on Discrete Wavelet Transform (DWT) and Discrete Cosine Transform (DCT) is proposed. For embedding watermark, a cover image is decomposed by a 2-level DWT, and the HL2 subband coefficients are divided into 4 × 4 blocks, then the DCT is performed on each of these blocks. The watermark bits are embedded by predefined pattern_0 or pattern_1 on the middle band coefficients of DCT. After inserting watermark, inverse DCT is applied to each 4 × 4 blocks of HL2 subband coefficients, and inverse DWT is applied to obtain the watermarked image. For watermark extraction, the watermarked image, which may be attacked by various image attacks, is decomposed with 2-level DWT and DCT similarly as watermark embedding process, then correlation between middle band coefficients of block DCT and the predefined pattern (pattern_0 and pattern_1) is calculated to decide whether a 0 bit or a 1 bit is embedded. Genetic algorithm is used to optimize the performance of embedding and extracting parameters. Simulation results show this technique is robust against JPEG attacks and many other strong attacks.