強健性雙域數位影像浮水印

Abstract

現今許多影像浮水印技術大多難以兼顧一般攻擊與幾何攻擊，所以本文的研究動機在於技術的兼併整合，讓浮水印能夠承受各式各樣的攻擊。實際的做法則是應用雙頻率域處理的方式來達到抵抗一般攻擊與幾何攻擊。
文中的雙域處理是採用Discrete Wavelet Transform(DWT)與Discrete Fourier Transform(DFT)。由於DWT具有多解析頻帶特性，因此我們將浮水印嵌入在最不易受到破壞的低頻區。另一方面，DFT在幾何特性中是具有不變性的，因此可用於抵抗幾何攻擊，我們的作法是在DFT的振幅頻譜中嵌入一個Trial template，作為抽取時判斷遭到幾何失真的可能性。
此外，為了提高浮水印的隱蔽性與安全性，我們應用展頻技術與渾沌理論，實驗結果發現，我們的雙域浮水印架構較許多獨立浮水印架構技術來得優越，PSNR(Peak Signal-to-Noise Ratio)可達到45dB，並且有效的抵抗一般攻擊（如：椒鹽雜訊、高斯雜訊、Jpeg壓縮…等）與幾何攻擊（如：旋轉、縮放、剪裁），證實是一種非常具有實用價值的強健性技術。As many current image watermarking technologies have difficulty to cope with general and geometric attacks at the same time, the motive of this thesis is thus to integrate a variety of technologies to allow the watermark to survive under various malicious attacks. In actual practice, a dualdomain watermark embedding scheme is developed for such a purpose.
In our study, the dual-domain watermarking employs the Discrete Wavelet Transform (DWT) and Discrete Fourier Transform (DFT). The DWT has multiband resolution properties so that the watermark can be embedded in the low-frequency band that is less susceptible to damages. On the other hand, the DFT has the invariant geometric properties that can withstand geometric attacks. Our scheme is to embed a trial template in the amplitude DFT for judging the possibility of geometric distortion during the process of watermark extraction.
In addition, in order to enhance the obscurity and security of the watermarks, we have used the spread spectrum technique along with chaos theory. The experiment results show that our dual-domain watermark frame is superior to many others. Not only its PSNR can reach 45 dB, but it can effectively shield against general attacks (e.g. salt and pepper noise, Gaussian noise, JPEG compression, etc.) as well as geometric attacks (e.g. rotating, zooming, cropping, etc.). This study will prove that our scheme constitutes a very effective and robust watermarking algorithm.