# An Adaptive and Secure Holographic Image Watermarking Scheme
## 1 Introduction  
Owing to the rapid growth of internet and multimedia technologies, the acquisition, transmission and exchange of digital multimedia data including images, audio and videos has become a simple task. On the other hand, digital images can be manipulated or reproduced easily by the use of powerful image processing tools. How to effectively protect the copyright of the digital products has become a significant topic. A great many techniques have been proposed for protecting the digital rights of image content recently. Among these methods, digital watermarking is viewed as the most promising solution for digital copyright protection. Digital watermarking is a process that hides a piece of secret information (watermark) in the original digital multimedia data for the purpose of copyright protection and its verification [1]. For the requirements of actual application, a watermarking system has some an essential characteristic, more specifically for its imperceptibility, robustness, security, and so on [2]. In recent years, a large number of image watermarking methods have been reported [2; 3; 4; 5; 6], which can be categorized into two groups: transform-domain and spatial-domain. In spatial domain schemes, the watermark is embedded directly into the original image by manipulating the pixel intensity values [3]. In contrast, the transform domain method inserts the watermark by changing the frequency coefficients of the original image in a transform domain. There are numerous transform domain watermarkingtechniques such as discrete wavelet transform (DWT) [4], discrete cosine transform (DCT) [2], fractional Fourier transform [5], gyrator transform [6] and singular value decomposition (SVD) [2], etc. As a powerful matrix decomposition technique, SVD has been widely applied in image watermarking [2]. To improve robustness, some hybrid SVD-based watermarking methods have been designed [2; 4; 7; 8]. In comparison to spatial domain methods, these techniques are more robust against different image attacks. In general, robustness and imperceptibility, which are the most two important properties, are adopted to evaluate the performance of a watermarking system [9]. But these two characteristics oppose each other [9]. To achieve a tradeoff between transparency and robustness, adaptive watermarking algorithms [7; 9] have been studied extensively recently. In recent years, entropy has been used to select adaptively the embedding positions which determine the performance of the watermarking scheme [2; 4; 10]. In [10], an image watermarking approach is proposed to insert the watermark into the DWT sub-band with the highest entropy. The major drawback of this method is that the quality of the watermarked image is seriously degraded. Since the above method is non-blind, the original image is needed in the watermark extraction process. Based on SVD and DCT, [PERSON] designed a watermarking method where the watermark is superimposed on the blocks selected by entropy and edge entropy [2]. But this method cannot resist filtering, JPEG compression, blurring and rescaling attacks. To achieve a level of higher imperceptibility and robustness, [PERSON] et al. developed a block-based watermarking scheme based on SVD and DWT [4]. However, it is weak with respect to filtering, JPEG compression and blurring attacks too. Because of the encryption characteristic and the strong anti-interference property of a hologram, a new technique wherein digital holograms are employed as watermarks has been explored to increase the watermarking system's performance [11; 12; 13] in recent years. In [11], based on the improved fuzzy c-means clustering and the iterative algorithm for embedding processes, an adaptive watermarking scheme that inserts the mark hologram into the DWT-transformed original image is proposed. The drawback of this method is high computation cost. Reference [12] reported that a hologram watermark was performed in the DWT domain by using an encrypted kinoform as watermark.