| Summary: | 碩士 === 國立成功大學 === 電機工程學系 === 88 === In this Thesis, a new watermark system that is based on the discrete cosine transformation is proposed. Traditional watermark systems lose its hidden copyright information easily after serious attacks. The proposed algorithm provides a new method that embeds multiple copyright information into the middle band as invisible watermark. During watermark detection, the majority voting is adopted to extract the embedded information. To accommodate the characteristics of different images, the proposed algorithm uses adaptive watermark strength factor to strike a good balance between robustness and perceptual invisibility. Furthermore, the proposed algorithm can prevent the infirmity that traditional watermark systems suffer seriously: the security would weaken because of the publication of watermark infrastructure. The experimental results show that the proposed technique can preserve a higher image quality and withstand various common image operation attacks. Especially, it can withstand JPEG lossy compression under 1:18 or higher compression ratio. Most of the copyright information may survive under cropping attack and some basic affine transformations.
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