A robust watermarking scheme using phase shift keying with the combination of amplitude boost and low amplitude block selection
Introduction
Because digital information is easy to transmit and duplicate unauthorized reproduction becomes a serious problem. While copyright and authentication gradually lose its security, the need for protecting intellectual property becomes more and more important. Recently, digital watermarking has been proposed as one solution to the problem of protecting the intellectual property [1], [2], [3], [4], [5], [6], [7], [8], [9]. Unlike the traditional visible watermark found on papers, a digital watermark does not change the perceived quality of the image content. It is a potential method to discourage unauthorized copying or attesting origin of the image. Generally, a digital watermark must fulfill the following requirements: imperceptible, robust and secure.
Watermarking techniques can be classified into two categories, one is processed in the spatial domain and the other is accomplished in the transform domain. In the spatial domain [1], [2], the visual modes derived from data compression are very suitable for digital watermarking. Many excellent hiding methods published are based on just noticeable distortion (JND) [9]. Vector quantization is one of the widely used schemes to embedding watermarks. Lu and Sun [10] used codeword indices to carry the watermark information. However, the watermarks embedded in the spatial domain are not as robust as most of those embedded in the transform domain.
In the transform domain many approaches [3], [4], [5], [6], [7] are based on the Discrete Cosine Transform (DCT). Hsu and Wu [4] proposed a scheme by block-based image-dependent permutation of the watermarks in the middle band of the DCT coefficients and obtained good performance. Wu and Hsieh [6] used zerotree structure to embed watermark by rearranging the DCT coefficients in a way similar to the multi-resolution analysis (MRA) of wavelet transforms. Moreover, Langelaar and Lagendijk [7] proposed a scheme that employs three parameters to guarantee the performance. That is, first use a large number of the DCT blocks to embed single information bit; next, use small JPEG quality factor to enhance the watermark robustness against re-encoding attacks; third, adopt the so-called minimal cutoff index in the zigzag scanned fashion of the DCT coefficients that can be removed from the watermarked image.
The discrete wavelet transform (DWT) is another effective transform-domain method for concealing watermarks. Tsia et al. [8] utilizes the wavelet multi-resolutional structure to decompose the image and scatter the two-dimensional watermark to select the location during the secret data embedding. Wei et al. [9] controlled the wavelet coefficients so that the watermark noises do not exceed the just-noticeable difference (JND) of wavelet coefficients during watermark insertion. More papers based on the DWT for watermarking can be found in Refs. [11], [12].
Another method in the transform domain is to hide watermarks in the discrete Fourier transform (DFT) coefficients of the host image. Ruanaidh et al. [13] presented a phase-based method in the DFT domain and used an optimal detector for watermark recovery. Based on the Fourier–Mellin Transform Ruanaidh and Pun [14] presented a watermarking scheme that achieves rotation, scale and translation (RST) invariant. The scheme achieves robustness while sustains the RST attacks. Premaratne and Ko [15] proposed a new concept for embedding and detecting the watermark in the Discrete Fourier Transform. Since the embedding is independent of the image content, speedy embedding highly suitable for video streams can be achieved. Solachidis and Pitas [16] proposed a watermarking scheme which embeds a circularly symmetric watermark on a ring in the 2D DFT domain. The circularly symmetric watermark was used to solve the rotation invariance problem in the watermark detection in which a correlation operation was used.
In a previous paper [17], we proposed a DCT domain watermarking scheme using the frequency shift keying (FSK). In the scheme, high-variance block selection (HVBS) is employed to enhance robustness. In this paper, we proposed a DFT domain watermarking scheme using the phase shift keying (PSK). In our proposed scheme, the watermark bits are first expanded by spread spectrum and then concealed by PSK modulation in the DFT coefficients of the host image. The PSK embedding is employed due to its superior noise immunity. In the PSK embedding, the watermark information is embedded in the phase part of the host image. Thus, the threshold effect in which the quality of the recovered watermark plunges when the amplitudes of the DFT coefficients used for embedding the secret bits are below a threshold value may occur [18]. In this paper, a novel idea combining amplitude boost (AB) and low-amplitude block selection (LABS) is proposed to curb the threshold effect raised by PSK. We demonstrated that by properly combining AB and LABS robustness can be enhanced without sacrificing imperceptibility. Meanwhile, in our scheme neither the original host image nor the original watermark is required during the watermark detection process.
The remainder of this paper is organized as follows. In Section 2, the proposed concealing algorithm is presented. The watermark extracting process is presented in Section 3. Empirical results are presented in Section 4. Finally, Section 5 concludes this paper.
Section snippets
Concealing algorithm
A robust watermarking scheme must survive all kinds of attacks, and at the same time sustain the virtual quality of the host image when the watermark is concealed. Besides, security is also an important factor required by a watermarking scheme. In order to construct a superior watermarking scheme, several skills are used in this paper to achieve the goal. The overall concealing process of our proposed scheme is shown in Fig. 1. The watermark is first transformed to by toral automorphism
Watermark extraction algorithm
Robustness, imperceptibility and security are three issues concerned in our watermarking scheme. In the embedding process, the DFT, the PSK modulation and the SS are used to enhance the robustness. The LABS is used to increase the imperceptibility. The SS, the TA and the set of bit sequences (, Key) are hired for the security reason. In the extracting process, the secret data must be extracted from the same positions where they are embedded. Therefore, the same set of bit sequences (
Experimental results
Imperceptibility is an important factor for watermarking. In this paper we employ the PSNR to indicate the degree of transparency. The PSNR of is given bywhere and , are the gray values at of the host image and the watermarked image of size , respectively. The watermark similarity measurement is dependent on factors such as the knowledge of the experts, the experimental conditions, etc. Therefore a quantitative
Conclusions
Digital watermarking is a potential method to discourage unauthorized copying or attest origin of digital data that includes audio, video and images. In this paper we present a robust watermarking scheme for still images using PSK with amplitude boost and low amplitude block selection. The amplitude boost strategy is used to enhance the robustness and the low amplitude block selection strategy is used to reduce the degradation of the host image caused by watermark concealing. Empirical results
About the Author—WEN-YUAN CHEN was born in Taichung, Taiwan, in 1957. He received the B.S. and M.S. degrees in Electronic Engineering from National Taiwan University of Science and Technology in 1982 and 1984, respectively, and the Ph.D. degree in Electrical Engineering from National Cheng Kung University at Tainan Taiwan, in 2003. Since 2003, he has been an associate professor with the department of Electronic Engineering at National Chin-Yi Institute of Technology. His research interests
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About the Author—WEN-YUAN CHEN was born in Taichung, Taiwan, in 1957. He received the B.S. and M.S. degrees in Electronic Engineering from National Taiwan University of Science and Technology in 1982 and 1984, respectively, and the Ph.D. degree in Electrical Engineering from National Cheng Kung University at Tainan Taiwan, in 2003. Since 2003, he has been an associate professor with the department of Electronic Engineering at National Chin-Yi Institute of Technology. His research interests include digital signal processing, image compression, pattern recognition and watermarking.
About the Author—CHIN-HSING CHEN received the B.S. degree in electrical engineering from National Taiwan University, Taiwan, in 1980, and the M.S. and Ph.D. degrees in electrical and computer engineering from the University of California at Santa Barbara, in 1983 and 1987, respectively. Since 1988, he has been with the Department of Electrical Engineering at National Cheng Kung University in Taiwan where he is now a professor. His current research interests include pattern recognition, image processing and VLSI array design. He has published over 160 papers and given more than 80 technical presentations in public in more than 15 countries.