【作者】 邓成；

【导师】 高新波；

【作者基本信息】 西安电子科技大学 ， 信息与通信工程， 2009， 博士

【摘要】 在以信息数字化和传播网络化为主要特征的比特时代,信息传播的深度和广度得到了前所未有的扩展,信息表达的效率和准确性也得到了显著的提高。但随之而来的还有一些负面影响,比如这些数字产品有可能在没有得到作品所有者许可的情况下被随意拷贝、篡改和传播,这将给数字产品的作者和生产者带来巨大的经济损失。因此如何在当前网络环境中实施对数字产品的有效版权保护、确保信息安全已经成为一个亟待解决的现实问题,也成为企业界和学术界共同关注的热点问题之一。为此,一门新兴的交叉学科——数字水印应运而生,作为解决数字产品版权保护的一种重要技术,如今正得到广泛的研究与应用。虽然经历了十多年的发展,但是数字水印远未成熟,在理论和实际应用中还有许多开放的问题。本文在第二代数字水印的框架下,以静止图像作为研究对象,通过构建几何不变的局部特征区域并寻求有效的水印嵌入和检测策略,为抗几何攻击图像水印技术的理论发展和面向版权保护的实际应用提供有效的解决方案。论文的主要创新点概括如下:(1)考虑到在基于图像特征的数字水印方案中局部特征区域的稳定性及分布直接影响水印系统的鲁棒性,提出了一种新的水印嵌入基准点选择策略。针对尺度空间特征点的多类属性,将图论聚类算法引入其中,有效地解决了特征区域稳定性和分布均匀性之间的矛盾。实验结果表明该选择策略不仅适用于不同的尺度空间特征检测算子和不同类型的图像,而且具有较好的稳定性。(2)鉴于目前第二代图像水印方案不能抵抗诸如局部随机扭曲、裁剪,纵横比改变等几何攻击的问题,提出了一种基于仿射协变区域的抗几何攻击水印算法。利用Harris-Affine特征检测算子提取仿射协变区域,这些区域具有很好的空间局部性、方向选择性、多样性以及协变性。将二维圆形水印仿射变换为特征区域的相应形状后,通过空域叠加的方式实现水印嵌入。实验结果表明该算法能有效抵御包括全局几何攻击和局部几何攻击在内的多类攻击。(3)为了进一步增强图像水印系统对几何攻击的抵抗能力,提出了一种基于归一化仿射协变区域的水印算法。在深入分析仿射变换前后同一协变区域之间关系的基础上,利用局部区域归一化将协变区域之间的仿射变换关系约简为旋转变换关系;利用方向归一化进一步消除协变区域之间存在的角度差异,从而得到了具有几何不变特性的局部特征区域。与基于仿射协变区域的水印算法相比,新算法的鲁棒性得到了显著提高。(4)针对现有基于图像特征的数字水印方案对常规图像处理和几何攻击不能同时获得较高的水印检出率这一问题,提出了一种基于局部不变性的变换域鲁棒水印算法。文中利用SIFT算子提取局部不变特征区域,水印的嵌入和检测是在局部区域的DFT变换域中进行。大量的实验结果表明,将局部不变特征提取技术与变换域水印嵌入和检测策略有机结合不仅增强了该算法的整体鲁棒性,而且提高了水印的检测精度。(5)为了更好地解决由常规图像处理和几何攻击产生的移位以及插值误差等问题,提出了一种基于局部Tchebichef矩的鲁棒水印算法。利用Tchebichef矩提取局部不变区域的全局特征,使之相对独立于图像像素的位置偏移和数值计算误差,从而实现水印系统抵抗几何攻击的目的;此外,Tchebichef矩对噪声和压缩的良好抑制力又进一步增强了水印系统对常规图像处理的抵抗力。与同类算法相比,该算法嵌入容量大,不可见性好,且抵御常规图像处理和几何攻击的能力强。总之,本文在有效解决局部特征区域稳定性和分布均匀性之间矛盾的基础上,将构建几何不变的局部特征区域与设计水印嵌入、检测策略有机结合,设计的水印方案能充分抵御常规图像处理、几何攻击以及组合攻击,为抗几何攻击图像水印技术的理论研究和应用推广提供了新思路。更多还原

【Abstract】 With the rapid development of digital technologies and Internet, the scope and depth of information dissemination have been greatly expanded. Moreover, the efficiency and accuracy of information expression have been significantly improved. On the other hand, digital media can be easily copied, manipulated and redistributed without any permission from the copyright owner, which is potentially capable of incurring considerable financial loss to the media producers and content providers. Therefore, how to protect the copyright of digital media in open networks has been a challenging issue and also has become a hot topic in both academic community and business circles. Under this circumstance, the concept of digital watermarking came up while trying to solve the problems related to the management of intellectual property of digital media.After decade development, digital watermarking is still in its infancy and remains a great deal of open problems demanding prompt solution in theory and application. Under the framework of the second generation watermarking, this paper takes still images as main research object and presents several geometrically resistant watermarking algorithms by incorporating the advantages of the local invariant feature regions and the embedding/detection strategies seamlessly. The main achievements of this paper are summarized as follows.(1) In feature-based image watermarking, the stability and distribution of local feature regions will direct affect the robustness of the watermarking system. Taking this situation into account, this paper proposes a novel selection criterion for watermarking references. After choosing the feature points in the middle-scale band, this criterion utilizes graph theory clustering to mine the latent distant information betweeen feature points and groups these feature points according the distance constraint. With regarding to the same cluster, the points whose strength is the largest are used to form the feature regions. Experimental results show that this criterion is not only suitable for different types of feature point detector and image, but also stable under various watermark attacks.(2) Nowadays, the existing second generation watermarking methods are still vulnerable to some particular geometric distortions, such as local random bending, shearing, and aspect ratio changes. With regarding to this, a geometrically robust image watermarking approach is developed via affine covariant regions extracted by Harris-Affine detector. These regions have good spatial localization, orientation selection, distinctiveness, and more importantly, their covariance records not only the geometric 2D transformations but also nonrigid deformations. From the circular watermark, the elliptical watermark pattern can be obtained by the affine transform- ation according to the shape of the feature regions. The elliptical watermark is embedded additively in the spatial domain. The experimental results illustrate that the proposed scheme can resist kinds of geometric attacks.(3) To further improve the robustness against geometric attacks, an image watermarking method is proposed based on normalized affine covariant regions. By analyzing the relationship of the identical regions in both the original image and the affine-transformed version, local normalization technique is first used to reduce the affine ambiguity to a rotational one. Then, orientation alignment is applied to remove the rotation effect. In this way, geometrically invariant local feature regions can be achieved. Compared with the original method based on affine covariant regions, this new approach evidently improves the performance in terms of robustness.(4) The previous feature-based image watermarking methods are difficult to obtain the relatively higher watermark detection ratio under common image processing as well as geometric attacks. To this end, a robust image watermarking by using local invariant features is proposed. Local invariant feature regions are first constructed with Scale Invariant Feature Transform (SIFT), and then watermarking embedding and detection are conducted in DFT domain of these invariant regions. Extensive experimental results confirm that this algorithm enhances not only the overall robustness bust also the detection accuracy.(5) To deal with the shift problems and interpolation error problems caused by common image processing and geometric attacks, a local Tchebichef moments based robust image watermarking is developed. The Tchebichef moments are employed to describe the global characteristics of the local invariant regions which are relatively independent to the location deviation and the numerical error in pixel values. Here, Tchebichef moments have not only insensitivity to noise but also better feature representation capability and reconstruction accuracy. Experiments using a subset of USC-SIPI image database demonstrate the newly developed algorithm outperforms some representative methods consistently in terms of watermark capacity, impercepti- bility, and robustness.On the basis of solving the conflict between the stability and the spatial distribution of local feature regions, the construction of geometrically invariant local feature regions is combined with the design of watermark embedding/detection strategy. The proposed algorithms can effectively resist common image processing, geometric attacks, and even combined attacks. In generally, these research results enrich the theories and applications of image watermarking robust against geometric attacks.更多还原