Abstract: In this paper an improved SDQ watermarking scheme was developed based on "Significant Difference of Wavelet coefficient Quantization" (SDQ) by Lin et al. Firstly, two blocks of the middle frequency band in DWT were divided into a group of 8 wavelet coefficients in a pseudorandom manner using a key. Then,1 bit watermark information was embedded in a group.The important coefficients difference was enhanced through slight quantization of the maximum coefficient, and the robustness was improved. Watermark was extracted by a secret key generated by coefficients difference and watermark. hhe experimental results show that the proposed scheme is quite effective against JPEG compression, low-pass filtering, Craussian noise and slight geometric attacks, and the PSNR of watermarked image is about 55dB.

Key words: Wavelet coefficient, Important coefficients, Significant difference, Digital watermarking