A DCT-domain system for robust image watermarking
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Digital watermarking for multimedia protection
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With the coming and the expansion of the World Wide Web an increased amount of digital information, become available to a large number of people. Since the digital network is often used to offer digital media for profit, there is a strong need for copyright protection and a considerable interest in methods for inserting in a multimedia document an invisible mark to identify the owner. Such 驴digital signature驴, also called 驴watermark驴, can be used to identify the document owner, to discourage unauthorized document copying and distribution, and, possibly, to earn royalties.A wavelet-based image-watermarking scheme is proposed in this work, based on the insertion of pseudo-random codes in the frequency domain. The watermark, consisting of a sequence of pseudo-random numbers normally distributed, is embedded into most perceptually significant coefficients of the frequency domain. The scheme is private, that is the original image is required for the watermark detection.In order to optimize the tradeoff between robustness and imperceptibility of the watermark, the 驴strength驴 of the signature to be embedded depends on the image to be marked, thus fitting its perceptual capacity. The insertion procedure scales the watermark differently from one image to another: that means to add more watermark energy inside detailed images, where slight modifications do not reduce image fidelity, and vice versa to decrease the watermark energy in homogeneous images, characterized by large regions with few color variations. The experimental results prove that the method is more tamper resistant and less perceptible than other well-known private methods in the frequency domains. Moreover, the proposed method is able to cast watermarks in any kind of image, included cartoon images, which are characterized by a very low perceptual capacity, and thus allow only a little information to be hidden.