Using Invariant Image Features for Synchronization in Spread Spectrum Image Watermarking
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Using Invariant Image Features for Synchronization in Spread Spectrum Image Watermarking Ebroul Izquierdo Multimedia and Vision Research Lab, Department of Electronic Engineering, Queen Mary, University of London, London, UK Email: [email protected] Received 10 August 2001 and in revised form 10 December 2001 A watermarking scheme is presented in which the characteristics of both spatial and frequency techniques are combined to achieve robustness against image processing and geometric transformations. The proposed approach consists of three basic steps: estimation of the just noticeable image distortion, watermark embedding by adaptive spreading of the watermark signal in the frequency domain, and extraction of relevant information relating to the spatial distribution of pixels in the original image. The just noticeable image distortion is used to insert a pseudo-random signal such that its amplitude is maintained below the distortion sensitivity of the pixel into which it is embedded. Embedding the watermark in the frequency domain guarantees robustness against compression and other common image processing transformations. In the spatial domain most salient image points are characterized using the set of Hilbert first-order differential invariants. This information is used to detect geometrical attacks in a frequency-domain watermarked image and to resynchronize the attacked image. The presented schema has been evaluated experimentally. The obtained results show that the technique is resilient to most common attacks including rotation, translation, and scaling. Keywords and phrases: watermarking, data hiding, image invariants.
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INTRODUCTION
Conventional analog media distribution systems have an inherent built-in defense against copying, alteration, and fraud. Each time a new copy is issued the quality of the duplicated content is degraded accordingly. In contrast to that, digital multimedia documents are completely susceptible to exact replication and alteration. This, together with the rapid proliferation of digital documents, multimedia processing tools and the world-wide availability of internet access have created an ideal medium for piracy, copyright fraud, and uncontrollable distribution of high quality but unregistered multimedia content. Since digital watermarking can be seen as a solution to this problem, both the number of activities in this area and the recognition of the difficulties and challenges involved in this new technology have increased in the last few years [1, 2]. Basically, the major challenge is to find a strategy that satisfies the conflicting objectives of performing image changes that are imperceptible for the human eye and being extremely robust against detection or removal either accidentally or intentionally. These two objectives are conflicting in nature because it is not possible to simultaneously maximize robustness and imperceptibility. Indeed, maximization of robustness leads to the introduction of large distortions in the image or video and consequently strong per
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