T.-h. Kim et al. (Eds.): FGCN 2011, Part I, CCIS 265, pp. 236–243, 2011. © Springer-Verlag Berlin Heidelberg 2011 Unseen Visible Watermarking for Gray Level Images Based on Gamma Correction Chu-Hsing Lin 1 , Chen-Yu Lee 2 , Shu-Yuan Lu 1 , and Shih-Pei Chien 1 1 Department of Computer Science, Tunghai University, Taichung 407, Taiwan chlin@go.thu.edu.tw, g99350011@thu.edu.tw 2 Department of Computer Science, National Chiao-Tung University 1001 Ta-Hsueh Road, HsinChu, 30050, Taiwan chenyu@cs.nctu.edu.tw Abstract. In this paper, we proposed a novel watermarking technology with the properties that from visible watermarking and invisible watermarking. In the common situation our approach can be used to transmit the multimedia files, according to the gamma correction, the watermark can be revealed. Our proposed method has a unique feature that we can obtain a watermarked-image similar to the original image after embedding watermarks. The naked eye hardly realized the watermarks, but after adjusting the viewing angle, it then demonstrated the embedded watermarks. We embed the unseen visible watermarks by adjusting degree of saturation and value of HSV model in the color space. The experiment result shows that the PSNR values of the watermarked images are more than 45 dB, sometimes even as high as 55 dB. Keywords: Unseen visible watermark, Gamma Correction. 1 Introduction In the digital era, any digital data is possible to be easily copied or modified, so the digital data protection is an extremely important area of research in the information security. Digital watermarking [7] is used to verify an image is a falsification of an effective way. The most of current commercial signs or notations are color images, of which the complexity is more than the binary image used as watermark traditionally. If a colorful signs or notations are transformed to a binary image, it would be hard to recognize by human eyes. Therefore, a gray image converted from a colorful sign or a notation could display its outline of original image by the dark degree. As a result, the gray-scale watermark has become the main watermark technology recently. Watermark technology is divided into visible and invisible watermark [2]. Visible watermark is visible by naked eye easily, and its main purpose is to declare the copyright or ownership. The disadvantage is easy to destroy the beauty of original image and is easy removed additionally. On the other hand, the embedded invisible watermark is not easily identified by naked eyes, but the watermark needs special methods to be retrieved [3] [6]. Although this method increases the security of the watermark [4], but the identification is not as convenient as the visible watermark. In