| Summary: | 碩士 === 東海大學 === 資訊工程學系 === 98 === With the advent of the digital era, lots of files are stored in digital formats for quick and easy access. Documents stored as digital format have become the trend, and how to effectively prove the ownership of the digital document has developed into an important research issue. In this thesis, we propose a novel watermark scheme with unique features that consist of useful properties from the visible watermarking and invisible watermarking schemes. In this work, originally invisible patterns carrying metadata, or the watermark, can be revealed by the gamma correction method or by adjustment of saturation of the stego-image. One salient characteristic of the proposed watermark scheme is that we can obtain a stego-image similar to the cover image that is hard to tell them apart by the naked eye, however the unseen embedded watermarks can be revealed by adjusting the viewing angle. We embed unseen visible watermarks in the cover image by adjusting saturation values in the HSV model, a common cylindrical-coordinate representation of points in the RGB color model. The embedded watermark is revealed in the place where significant variances are found among neighboring pixels. Instead of mere binary image, the embedded watermark adapts its color according to the background color of the original cover image to have a better hidden effect. Also, based on ratios of R, G, and B values, we can adaptively adjust the strength of the watermark to retain the visual quality of the stego-image. As demonstrated by the experimental results, the PSNR values of the stego-image are more than 40 dB, sometimes even as high as 50 dB.
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