Improved Robust Watermarking Based on Rational Dither Modulation Zairan Wang 1,2 , Jing Dong 1 , Wei Wang 1 , and Tieniu Tan 1,2 1 Center for Research on Intelligent Perception and Computing, National Laboratory of Pattern Recognition, Institute of Automation, Chinese Academy of Sciences 2 College of Engineering and Information Technology, University of Chinese Academy of Sciences {zairan.wang,jdong,wwang,tnt}@nlpr.ia.ac.cn Abstract. Rational dither modulation (RDM) watermarking was presented to re- sist amplitude scaling attack. This property is achieved by quantizing the ratio of consecutive samples instead of samples themselves. In this paper, we improve the performance of basic RDM watermarking to resist more types of watermark- ing attacks. We improve the robustness of our modified RDM watermarking by the following three aspects: 1) The quantization step size is increased by modi- fying two coefficients instead of only one coefficient in the basic RDM method, 2) Several modification rules are defined to reduce embedding distortion, and 3) The coefficients with larger magnitudes in the lowest sub-band in DWT domain are selected to embed watermark. A variety of attacks are implemented to eval- uate the performance of our method. Experimental results demonstrate that our method outperforms the basic RDM method and two state-of-the-art watermark- ing methods over a wide range of attacks and it also has good imperceptibility. Keywords: Watermarking, RDM, Amplitude scaling attack. 1 Introduction Digital watermarking has always drawn extensive attention for digital copyright pro- tection since it was born. So far, many watermarking schemes have been proposed in the literature. One of the most popular algorithms is quantization based watermarking scheme [1]. The main idea of quantization based watermarking is that the host data is quantized into different quantization intervals according to different watermark in- formation. Chen and Wornell [1] proposed a quantization based watermarking scheme which they called quantization index modulation (QIM). Chen [2] quantized the mean of a set of wavelet coefficients to embed watermark. Lin [3] embedded watermark by quantizing the local maximum coefficients in mid-frequency wavelet sub-band. Chen and Horng [4] embedded watermark by modulating the wavelet coefficients. The main weakness of QIM based watermarking is that it is very sensitive to am- plitude scaling attack. Therefore, many watermarking schemes have been proposed to deal with this problem in recent years. Shterev [5] proposed a maximum likelihood technique to estimate the amplitude scale in the watermark extraction process. Some researchers made use of amplitude-scale invariant codes to combat amplitude scaling T. Tan et al. (Eds.): IGTA 2014, CCIS 437, pp. 305–314, 2014. c  Springer-Verlag Berlin Heidelberg 2014