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发布时间: 2017-06-16 |
图像处理和编码 |
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收稿日期: 2016-11-24; 修回日期: 2017-02-20
基金项目: 国家自然科学基金项目(61540056,61401185)
第一作者简介: 齐向明(1966-), 男, 副教授, 2008年于辽宁工程技术大学获管理科学与工程专业硕士学位, 主要研究方向为图形图像处理、数字水印。E-mail:qixiangming1223@163.com
中图法分类号: TP391;TP309.7
文献标识码: A
文章编号: 1006-8961(2017)06-0719-12
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摘要
目的 为协调水印算法不可见性与鲁棒性之间的矛盾,提高水印算法抵抗几何攻击的能力,提出一种图像块的不可见性与鲁棒性均衡水印算法。 方法 将宿主图像分成互不重叠的图像块,利用人类视觉系统的掩蔽特性对每个图像块的纹理特征和边缘特征进行分析,选择掩蔽性好的图像块作为嵌入子块。对嵌入子块作2级离散小波变换,将其低频子带进行奇异值分解,通过修改U矩阵第1列元素间的大小关系嵌入Arnold置乱后的水印信息。在水印提取前,对几何失真含水印图像利用图像尺度不变特征变换(SIFT)特征点的坐标关系和尺度特征进行几何校正,恢复水印的同步性。 结果 对标准灰度图像进行实验,含水印图像的峰值信噪比都可以达到44 dB以上。对含水印图像进行常规攻击和几何攻击,提取出的水印图像与原始水印图像的归一化互相关系数大部分都能达到0.99以上,说明该算法不仅具有良好的不可见性,对常见攻击和几何攻击都具有较强的鲁棒性。 结论 选择掩蔽性好的图像块作为水印嵌入位置能够充分保证水印算法的不可见性,特别是水印提取前利用SIFT特征点具有旋转、缩放和平移不变性对几何失真含水印图像实现有效校正,提高了含水印图像抵抗几何攻击的能力,较好地协调水印算法不可见性与鲁棒性之间的矛盾。
关键词
人类视觉系统(HVS); 奇异值分解(SVD); 尺度不变特征变换(SIFT); 不可见性; 鲁棒性; 几何校正
Abstract
Objective Embedding watermark information into the host image leads to a contradiction between invisibility and robustness. High watermark embedding strength means strong watermark robustness but poor invisibility. Low watermark embedding strength means good watermark invisibility but weak robustness. As an effective means of copyright protection, a watermarking algorithm must ensure good invisibility and effectively resist various attacks. Geometric attacks destroy the synchronization between the watermark and host image and thus leads to the failure of the watermarking algorithm. To address the contradiction between invisibility and algorithm robustness and improve the capability to resist geometric attacks, this study proposes an invisible and robust watermarking algorithm based on an image block. Method The host image is divided into non-overlapping image blocks, and the texture and edge features of each image block are analyzed by using the masking property of the human visual system to calculate the masking value of each image block. The masking values are arranged in a descending order, and an appropriate number of good masking image blocks are selected as embedded sub-blocks according to the size of the watermark information. Two-level discrete wavelet transform is performed on the sub-block, and its low-frequency sub-band is decomposed by singular value decomposition to obtain orthogonal matrices U and V and diagonal matrix S. The difference among the three sets of elements in the first column of the U orthogonal matrix is calculated according to the watermark bit information. If the difference is less than the threshold value, the Arnold scrambled watermark information is embedded into the U orthogonal matrix. Then, inverse singular value decomposition is applied on the selected image block, and the low-frequency sub-band and other middle-and high-frequency sub-bands of the image block are subjected to inverse wavelet transform. Afterward, all the image blocks are combined to obtain watermarked images. The scale-invariant feature transform (SIFT) feature points of the watermarked images are extracted, and the coordinate, scale, direction, and descriptor information are stored. In watermark extraction, the SIFT feature points of the watermarked image that may be attacked are extracted and matched with the feature points saved in the watermark embedding to determine if the watermarked image is subjected a geometric attack. If the image is subjected to geometric attacks, geometric correction of the watermarked image is realized by the coordinate relations and scales features of the SIFT feature points. Geometric correction restores the synchronization of the watermark. If no geometric attack occurs, two-level discrete wavelet transform is performed on the selected image block, and its low-frequency sub-band is decomposed by singular value decomposition to obtain orthogonal matrix U. The watermark bit information is extracted according to the difference between the two elements in the first column of the U orthogonal matrix and then transformed into a binary image, which is subjected to inverse Arnold transformation to obtain the watermark image. Result Through experiments on standard gray-scale images, the watermark information is embedded into three images:Lena, Elaine, and Baboon. With the increase in the threshold value, image quality is reduced correspondingly, but the normalized correlation coefficient of the extracted watermark is improved. Hence, the threshold value of the experimental image is 0.04 considering invisibility and robustness. The peak signal-to-noise ratios (PSNRs) of the three watermarked images, Lena, Elaine, and Baboon, are 49.864 5, 46.304 6, and 44.683 2 dB, respectively. These values show that the algorithm possesses good invisibility. When no attack occurs, the normalized correlation coefficients between the original and extracted watermark images can reach 1, which shows the effectiveness of the algorithm. Various types of attacks, including JPEG compression, noise, and filter, are applied to the watermarked images. With the increase in the attack intensity, the normalized correlation coefficients of the extracted watermark are influenced but mostly exceed 0.99. In particular, the normalized correlation coefficients of the watermark extracted from the compression attack can reach 1. Rotating, scaling, cyclic shifting, and shearing attacks are then performed on the watermarked images. Afterward, geometric correction of the watermarked images is realized with the coordinate relations and scales features of the SIFT feature points. Given that the watermarked images are subjected to rotating attacks without changing the size of the images, some of the pixel information is lost during the rotation, such that the normalized correlation coefficients of the extracted watermark could not reach 1. The normalized correlation coefficient of the extracted watermark when a watermarked image is enlarged is larger than the normalized correlation coefficient of the extracted watermark when the watermarked image is reduced. All normalized correlation coefficients of the extracted watermarks under the cyclic shifting attack can reach 1. Shearing of the good masking region affects the anti-shearing attack capability, but the normalized correlation coefficients of the extracted watermarks under the shearing attack exceed 0.95. Experimental results on conventional and geometric attacks show that this algorithm exhibits strong robustness against both attacks. Conclusion The texture and edge information of the image can be calculated to obtain the masking value of each image block. The invisibility of the watermarking algorithm can be ensured by selecting the image block with good masking as the embedded sub-block. Selecting the pair of elements with the largest difference in the first column of the U orthogonal matrix as the embedded position minimizes the influence on the overall visual quality of the original image and improves the robustness of the watermarking algorithm. Given that SIFT feature points are a type of space-based image local feature description operators that are invariant to image rotation, scaling, translation, and so on, geometric correction of the watermarked image is realized by with the coordinate relations and scales features of the SIFT feature points to improve the ability of resisting geometric attacks. The above mentioned methods enable the watermarking algorithm to effectively address the contradiction between invisibility and robustness.
Key words
human visual system(HVS); singular value decomposition(SVD); scale invariant feature transform(SIFT); invisibility; robustness; geometric correction
0 引言
多媒体技术的发展和互联网的普及使数字产品的非法复制和篡改变得相对容易,数字产品的版权保护问题变得日益突出。数字水印技术能够有效解决图像版权和内容认证等问题,成为国内外专家和学者研究的热点。
嵌入式水印算法的关键是协调不可见性和鲁棒性之间的矛盾,水印的嵌入强度越大,则水印的鲁棒性越强,但不可见性越差;水印嵌入强度越小,则水印的不可见性越好,但鲁棒性就越弱。矩阵分解法如奇异值分解(SVD)、非负矩阵分解(NMF)和QR分解等是水印算法常用的方法,该方法使水印算法的不可见性和鲁棒性得到了显著提高。文献[1-3]对宿主图像变换域进行奇异值分解,将水印信息嵌入到奇异值(
以上算法都可以有效抵抗JPEG压缩、加噪、滤波等攻击,而对于旋转、缩放和平移等几何攻击鲁棒性较差。其主要原因是常规攻击对图像像素值影响较小,而几何攻击破坏了水印和图像空间坐标的同步性,无法在水印提取时明确水印信息的位置,使得水印算法失效。可见几何攻击对水印算法造成的影响是毁灭性的,因此提高水印算法的抗几何攻击能力变得尤为重要。文献[11-14]利用图像尺度不变特征变换(SIFT)特征点具有旋转、缩放、平移不变性,对几何攻击后的含水印图像进行几何校正,有效提高水印算法的抗几何攻击能力。与其他抗几何攻击的水印算法相比,该类算法使用的图像几何校正算法与水印算法和攻击方式无关,且其校正精度高,性能稳定,提高了水印算法抵抗几何攻击的能力。
借鉴以上思路,提出一种图像块的不可见性与鲁棒性均衡水印算法。先将宿主图像分成互不重叠的图像块,利用每个图像块的纹理信息和边缘信息计算该子块的掩蔽值,根据水印信息的大小选择合适数量掩蔽性好的图像块,并对其2级小波变换后的低频子带作奇异值分解,再根据水印位信息计算正交
1 算法理论
1.1 人类视觉掩蔽特性的分析
人类视觉系统具有纹理掩蔽、频率掩蔽、亮度掩蔽等特性,为保证水印算法的不可见性和鲁棒性,需要对载体图像的纹理和边缘信息进行分析计算,提取掩蔽性好的图像块作为水印嵌入子块。人眼对纹理变化复杂的区域敏感度低,对纹理变化平缓的区域敏感度高,因此在纹理复杂的区域嵌入水印具有较好的不可见性。纹理对应着图像灰度值的变化,灰度变化得越快,纹理强度越大,在频域中较高频带的能量也就越大(一个频带的能量主要是指这个频带所有像素灰度值的代数平均)。小波变换能将图像分解为从高到低的多个频带,其低频子带集中了大部分原始图像的能量,较高频带则包括图像的纹理和边缘等信息。将宿主图像平均分成
$\varepsilon ({m_1},{m_2}) = \frac{1}{{{n_1}{n_2}}}\sum\limits_{i = 1}^{{n_1}} {\sum\limits_{j = 1}^{{n_2}} {{{\left[ {{x_{{m_1},{m_2}}}\left( {i,j} \right)} \right]}^2}} } $ | (1) |
$\begin{array}{*{20}{c}} {\delta ({m_1},{m_2}) = }\\ {\frac{{{\varepsilon _{{\rm{HL1}}}}({m_1},{m_2}) + {\varepsilon _{{\rm{LH1}}}}({m_1},{m_2}) + {\varepsilon _{{\rm{HH1}}}}({m_1},{m_2})}}{{{\varepsilon _{{\rm{LL2}}}}({m_1},{m_2})}}} \end{array}$ | (2) |
式中,
人眼对图像的边缘较为敏感,即使纹理较为复杂,嵌入水印后人眼也能很容易察觉,因此提取图像的边缘位置信息很重要。Sobel算子利用像素点上下、左右邻点的灰度值加权算法,根据在边缘点处达到极致这一现象进行边缘检测[15],采用Sobel边缘检测器不但能产生较好的检测效果,而且对噪声也具有平滑作用,能够提供较为精确的图像边缘信息。对宿主图像使用Sobel边缘检测器提取出清晰的图像边缘,将所得边缘图像分成
$f({m_1},{m_2}) = \delta ({m_1},{m_2}) - \frac{{e({m_1},{m_2})}}{{{n_3} \times {n_4}}}$ | (3) |
1.2 U矩阵水印嵌入位置的选择
奇异值分解(SVD)是一种将矩阵对角化的数值分析工具。一幅灰度图像从线性代数的角度来看,是一个具有非负值的矩阵。假设对大小为
$\mathit{\boldsymbol{A}} = \mathit{\boldsymbol{US}}{\mathit{\boldsymbol{V}}^{\rm{T}}}$ | (4) |
式中,
从矩阵分解的角度来看,
为进一步确定
表 1
Table 1
NC values between different elements in the first column of the
图像序列 | ||||||
Lena | 0.975 9 | 0.962 6 | 0.952 1 | 0.976 3 | 0.962 9 | 0.976 6 |
Baboon | 0.907 8 | 0.883 7 | 0.870 7 | 0.909 4 | 0.877 6 | 0.903 3 |
Plane | 0.961 1 | 0.941 6 | 0.922 0 | 0.964 8 | 0.938 9 | 0.960 7 |
Cell | 0.964 1 | 0.935 5 | 0.912 3 | 0.965 8 | 0.936 0 | 0.964 9 |
Boat | 0.958 5 | 0.928 9 | 0.908 8 | 0.958 9 | 0.928 6 | 0.958 6 |
Elaine | 0.968 5 | 0.954 6 | 0.943 0 | 0.970 0 | 0.954 5 | 0.968 9 |
Photography | 0.960 1 | 0.931 7 | 0.911 6 | 0.962 0 | 0.929 1 | 0.960 3 |
1.3 SIFT特征点匹配校正
尺度不变特征变换(SIFT)由Lowe[16]于1999年提出,在2004年加以完善,它是一种基于空间的对图像旋转、缩放、平移等保持不变性的图像局部特征描述算子。它的主要思想是在尺度空间检测极值点,然后对极值点进行筛选,找到稳定的特征点,最后在每个稳定的特征点周围提取图像的局部特性,形成局部描述符。此时已经去除了尺度变换、旋转等几何形变的影响,将得到的128维的特征描述符归一化,进一步去除光照变换的影响。通过计算两幅图像SIFT特征点描述符的欧氏距离实现两幅图像的匹配,得到匹配特征点的位置、尺度、方向以及128维的描述子,利用这些信息进行几何校正。
1.3.1 旋转校正
将两幅图像匹配后的特征点进行两两分组,假设原图中两个特征点的坐标为
求出每组的旋转角度
$\Delta \theta = {\rm{arctan}}\frac{{{y_j} - {y_i}}}{{{x_j} - {x_i}}} - {\rm{arctan}}\frac{{y{\prime _j} - y{\prime _i}}}{{x{\prime _j} - x{\prime _i}}}$ | (5) |
将所有分组求出的旋转角度进行
$agl = \frac{{\sum\limits_{i = 1}^{numb} {\left( {angl\left( i \right) \times m\left( i \right)} \right)} }}{{\sum\limits_{i = 1}^{numb} {m(i)} }}$ | (6) |
筛选出在
$\theta = \frac{1}{{num}}\sum\limits_{i = 1}^{num} {\Delta {\theta _i}} $ | (7) |
式中,
1.3.2 缩放校正
特征点的尺度因子能够随着图像尺度变化等比例地变化,可以利用特征点的尺度因子对缩放的图像进行校正。假设图像缩放前后匹配特征点的尺度因子分别为
$\varphi = \sqrt {\frac{1}{{num}}\sum\limits_{i = 1}^{num} {{{\left( {\frac{{{\gamma _1}\left( i \right)}}{{{\gamma _2}\left( i \right)}}} \right)}^2}} } $ | (8) |
1.3.3 循环平移校正
假设原图某一特征点的坐标为(
$\Delta x = \left\{ {\begin{array}{*{20}{l}} {x' - x + M}&{x' < x}\\ {x' - x}&{\rm{其他}} \end{array}} \right.$ | (9) |
$\Delta y = \left\{ {\begin{array}{*{20}{l}} {y' - y + N}&{y' < y}\\ {y' - y}&{\rm{其他}} \end{array}} \right.$ | (10) |
然后计算所有匹配点的循环平移参数的平均值就得到了原图的循环平移量
$\left\{ {\begin{array}{*{20}{l}} {\Delta {x_c} = \frac{1}{{num}}\sum\limits_{i = 1}^{num} {\Delta {x_i}} }\\ {\Delta {y_c} = \frac{1}{{num}}\sum\limits_{i = 1}^{num} {\Delta {y_i}} } \end{array}} \right.$ | (11) |
式中,
2 算法实现
2.1 水印图像的预处理
为提高水印的安全性和鲁棒性,需要对水印图像进行加密处理,本文采用Arnold置乱作为水印图像的预处理方法。Arnold置乱能够变换像素空间位置,使图像被随机而均匀地置乱,消除像素空间的相关性。
Arnold变换具有周期性,即对图像反复使用Arnold变换,可以在某一时刻恢复原始图像,其周期
2.2 水印的嵌入
设原始载体图像
1) 首先将置乱后的二值水印图像
2) 将宿主图像
3) 将选定的图像块
4) 修改
当嵌入的水印信息位为0时,计算
${{U'}_{c,1}} = \mu + T/2$ | (12) |
${U_{c + 1,1}} = \mu - T/2$ | (13) |
当嵌入水印信息位为1时,计算
${{U'}_{c,1}} = \mu - T/2$ | (14) |
${{U'}_{c + 1,1}} = \mu + T/2$ | (15) |
5) 对选定图像块进行逆SVD变换,将该块低频子带和其他中高频方向子带作逆小波变换,并对所有子块进行合成运算,得到含水印图像
6) 对含水印图像
2.3 水印的提取
水印提取的具体流程如下:
1) 对可能受到攻击的含水印图像
2) 将宿主图像
3) 对每个子块
4) 根据密钥
$L' = \left\{ {\begin{array}{*{20}{l}} 0&{{d_c} \ge 0}\\ 1&{{d_c} < 0} \end{array}} \right.$ | (16) |
式中,
5) 将1维水印序列
3 仿真实验与分析
已通过大量实验证明本文算法的有效性和可行性,限于篇幅,在此仅给出代表性的实验结果。仿真实验采用matlab R2014b作为实验平台,选取Lena、Elaine和Baboon 3幅标准灰度图像作为宿主图像,其大小为1 024×1 024。选取二值图像“辽宁工大”作为水印图像,其大小为32×32。Arnold置乱中的迭代次数
3.1 不可见性测试
原始载体图像图 4(a)—(c)分别嵌入水印信息得到含水印图像,表 2列出本算法在不同阈值
表 2
不同阈值时的含水印图像PSNR值
Table 2
PSNR values of the watermarked image with different threshold values
/dB | |||
阈值 | Lena | Elaine | Baboon |
0.012 | 55.423 0 | 53.577 8 | 55.133 3 |
0.02 | 53.796 2 | 51.205 2 | 51.551 6 |
0.04 | 49.864 5 | 46.304 6 | 44.683 2 |
0.06 | 46.728 7 | 42.792 9 | 40.664 8 |
表 3
不同阈值时Lena图像受到攻击后提取的水印NC值
Table 3
NC values of the extracted watermarks from attacked "Lena": image with different threshold values
攻击方式 | ||||
压缩攻击(30) | 0.943 2 | 0.967 2 | 0.998 9 | 0.999 5 |
压缩攻击(50) | 0.973 2 | 0.989 1 | 1.000 0 | 1.000 0 |
压缩攻击(70) | 0.991 6 | 0.999 7 | 1.000 0 | 1.000 0 |
中值滤波(3×3) | 0.948 2 | 0.988 6 | 0.993 7 | 1.000 0 |
中值滤波(5×5) | 0.928 4 | 0.967 2 | 0.989 4 | 0.991 9 |
高斯滤波(3×3) | 0.984 8 | 0.996 2 | 1.000 0 | 1.000 0 |
高斯滤波(5×5) | 0.989 8 | 0.993 6 | 1.000 0 | 1.000 0 |
高斯噪声(0.001) | 0.956 8 | 0.987 5 | 0.991 2 | 0.995 4 |
椒盐噪声(0.001) | 0.963 4 | 0.982 3 | 0.991 8 | 0.994 6 |
图 4(b)为阈值
3.2 几何失真校正实验和分析
将3.1节得到的含水印图像进行旋转、缩放和循环平移攻击,利用1.3节中提到的原理提取并匹配攻击前后两幅图像的特征点,得到每个特征点的坐标和尺度信息,再利用这些信息进行几何失真校正,得出校正参数。
3.2.1 旋转校正实验
旋转校正实验先对含水印图像进行不改变图像大小的逆时针旋转攻击,也就是说图像的部分边缘信息会在旋转的过程中丢失。提取并匹配旋转攻击前后两幅图像SIFT特征点,利用匹配特征点的位置信息进行旋转校正。表 4列出了旋转校正实验得到的数据,从表 4可以看出,3幅图像的实际校正值和理论校正值非常接近,说明利用本算法得到的实际校正值具有较高的准确性。
表 4
旋转校正实验结果
Table 4
The experimental results of Rotation correction
旋转度数 | 校正参数 | Lena | Elaine | Baboon |
1° | -1 | -1.016 3 | -0.986 3 | -1.024 5 |
5° | -5 | -4.997 2 | -4.985 1 | -5.020 7 |
10° | -10 | -10.010 0 | -9.998 9 | -9.971 0 |
20° | -20 | -19.987 1 | -19.973 0 | -19.988 8 |
30° | -30 | -30.012 6 | -30.035 4 | -29.985 7 |
50° | -50 | -50.102 7 | -50.095 3 | -49.9793 |
3.2.2 缩放校正实验
缩放校正实验先对含水印图像进行缩放,当缩放倍数小于1时,该图像将被缩小,反之图像将被放大。提取并匹配缩放攻击前后两幅图像SIFT特征点,利用匹配特征点的尺度信息进行缩放校正。表 5列出了缩放校正实验得到的数据,通过3幅图像理论校正参数与实际校正值的对比。可以看出,利用SIFT特征点的尺度不变性进行缩放校正具有较高的准确性。由于在图像缩放的抽样过程中丢失了部分图像信息,使得图像缩放时的校正准确率比放大图像低一些。
表 5
缩放校正实验结果
Table 5
The experimental results of Scaling correction
缩放倍数 | 校正参数 | Lena | Elaine | Baboon |
0.125 | 8.000 0 | 8.017 0 | 7.982 0 | 8.037 0 |
0.25 | 4.000 0 | 4.003 8 | 4.006 5 | 3.994 2 |
0.5 | 2.000 0 | 2.000 9 | 2.002 0 | 2.004 9 |
0.75 | 1.333 3 | 1.335 0 | 1.330 9 | 1.349 5 |
1.5 | 0.666 7 | 0.666 6 | 0.666 0 | 0.668 1 |
3.2.3 循环平移校正实验
循环平移实验先对含水印图像进行循环平移,提取并匹配循环平移前后两幅图像SIFT特征点,利用匹配特征点的位置信息进行缩放校正。表 6列出了循环平移校正实验得到的数据,由于图像的平移是以像素为单位,实际校正值需要对计算出的平移校正值进行取整,取整后的实际校正值和理论校正参数一致,说明利用SIFT特征点的平移不变性可以有效地进行循环平移校正。
表 6
循环平移校正实验结果
Table 6
The experimental results of cyclic translation correction
循环平移参数 | 理论校正参数 | Lena | Elaine | Baboon |
(10,10) | (-10,-10) | (-10.008 0,-9.995 0) | (-10.070 0,-9.980 3) | (-9.952 4,-10.061 0) |
(30,30) | (-30,-30) | (-30.017 5,-30.035 0) | (-30.225 0,-30.005 0) | (-29.956 9,-30.120 0) |
(50,50) | (-50,-50) | (-49.930 0,-50.014 0) | (-50.173 6,-50.018 3) | (-49.970 0,-50.005 0) |
(100,0) | (-100,0) | (-100.010 0,-0.017 3) | (-100.117 2,-0.005 8) | (-100.203 0,-0.000 0) |
(0,100) | (0,-100) | (-0.020 8,-100.205 0) | (-0.020 0,-100.270 0) | (0.101 9,-99.985 0) |
(128,128) | (-128,-128) | (-128.001 4,-128.000 0) | (-128.051 6,-128.023 0) | (-128.0035,-128.0257) |
3.3 鲁棒性测试
为了检测本算法具有较好的鲁棒性,对含水印图像分别进行常规攻击和几何攻击实验。并且在水印提取前,对含水印图像进行几何校正。
3.3.1 常规攻击
对含水印图像分别进行JPEG压缩攻击、滤波攻击和噪声攻击,表 7列出了3幅图像在受到不同攻击后所提取水印的NC值。从表 7中可以看出,随着攻击强度的增大,对提取出的水印NC值有一定的影响,但仍能提取出具有较大NC值的水印,特别是受到压缩攻击时,提取出来的水印NC值大部分都能达到1。图 5展示了各种常规攻击下的含水印图像和其所提取的水印图像,从视觉效果来看,所提取出的水印清晰可辨,说明本算法可以有效抵抗各种常规攻击,具有较强的鲁棒性。
表 7
含水印图像常规攻击后提取出的水印NC值
Table 7
NC values of the extracted watermarks from the watermarked image under conventional attacks
攻击方式 | 参数 | Lena | Elaine | Baboon |
压缩攻击 | 20 | 0.989 1 | 0.998 7 | 0.996 9 |
30 | 0.998 9 | 1.000 0 | 1.000 0 | |
50 | 1.000 0 | 1.000 0 | 1.000 0 | |
70 | 1.000 0 | 1.000 0 | 1.000 0 | |
100 | 1.000 0 | 1.000 0 | 1.000 0 | |
中值滤波 | 3×3 | 0.993 7 | 0.996 2 | 0.999 0 |
5×5 | 0.989 4 | 0.986 1 | 0.996 2 | |
高斯滤波 | 3×3 | 1.000 0 | 1.000 0 | 1.000 0 |
5×5 | 1.000 0 | 0.998 9 | 1.000 0 | |
高斯噪声 | 0.001 | 0.991 2 | 1.000 0 | 0.998 7 |
椒盐噪声 | 0.001 | 0.991 8 | 1.000 0 | 1.000 0 |
3.3.2 几何攻击
含水印图像分别进行旋转、缩放、循环平移和剪切攻击,并利用3.2节得到的校正参数对旋转、缩放、循环平移攻击后的图像进行校正。
表 8列出了3幅图像受到几何攻击后所提取出的水印NC值。图像受到旋转攻击后提取出的水印NC值都没有达到1,这是因为其受到的是不改变图像大小的旋转攻击,在旋转过程中丢失了部分像素信息,校正时通过逆向旋转可以恢复水印的同步性,但丢失的像素信息导致其不能提取出完整的水印图像。对缩放攻击后提取的水印NC值进行分析,发现缩小图像比放大图像提取的水印NC值要低一些,其主要是因为缩小图像时部分像素信息丢失,导致图像缩放时的校正准确率低于放大图像时的校正准确率。循环平移攻击并没有丢失像素信息,但图像像素点的坐标位置发生变化,同步性被破坏,校正后水印信息得到同步,提取出的水印NC值都能达到1。通过以上分析得出,利用SIFT特征点可以进行有效校正,提高了水印算法抵抗旋转、缩放、循环平移攻击的能力。剪切攻击会剪切掉图像的部分像素信息,虽然没有破坏水印信息的同步性,但其直接导致剪切掉的部分水印信息丢失,在嵌入前对水印图像进行Arnold置乱,进一步分散水印信息,使其能提取出具有较大NC值的水印。
表 8
含水印图像几何攻击后提取出的水印NC值
Table 8
NC value of the extracted watermarks from the watermarked image under geometric attacks
攻击方式 | 参数 | Lena | Elaine | Baboon |
旋转攻击 | 1 | 0.996 2 | 0.993 7 | 0.989 4 |
10 | 0.936 6 | 0.926 8 | 0.904 8 | |
30 | 0.864 6 | 0.863 4 | 0.827 4 | |
缩放攻击 | 0.25 | 0.951 8 | 0.947 0 | 0.921 6 |
0.5 | 0.998 8 | 0.996 2 | 0.980 2 | |
1.5 | 1.000 0 | 0.993 7 | 0.992 3 | |
循环平移 | (50,50) | 1.000 0 | 1.000 0 | 1.000 0 |
(100,0) | 1.000 0 | 1.000 0 | 1.000 0 | |
(0,100) | 1.000 0 | 1.000 0 | 1.000 0 | |
(128,128) | 1.000 0 | 1.000 0 | 1.000 0 | |
剪切攻击 | 左上角剪切1/16 | 0.952 0 | 0.963 4 | 0.998 1 |
中心剪切1/16 | 0.970 9 | 0.951 9 | 0.968 2 |
图 6展示了各种几何攻击下的含水印图像及其所提取出的水印图像,图中的水印图像清晰可辨,说明本算法可以有效抵抗几何攻击,具有较强的鲁棒性。
3.4 对比实验
为了验证本算法具有较强的鲁棒性,选择Lena图像为宿主图像的实验结果与文献[7, 14]算法进行对比,其NC值对比实验结果如表 9所示。从表 9可以看出,与文献[7]相比,本算法在受到旋转、缩放和循环平移攻击后所提取出的水印NC值得到了显著提升,说明其在对抗几何攻击方面的优势更为突出,鲁棒性能明显优于文献[7]算法。由于文献[14]也在水印提取前引进SIFT特征点进行几何校正,使得两种算法都可以有效对抗几何攻击,但本算法提取出的水印NC值更高,鲁棒性能稍好于文献[14]。
攻击方式 | 参数 | 文献[7] | 文献[14] | 本文算法 |
压缩攻击 | 30 | 0.994 9 | 0.997 4 | 0.998 9 |
70 | 1.000 0 | 1.000 0 | 1.000 0 | |
中值滤波 | 3×3 | 0.998 6 | 0.991 2 | 0.993 7 |
高斯滤波 | 3×3 | 0.993 2 | 1.000 0 | 1.000 0 |
椒盐噪声 | 0.01 | 0.949 3 | 0.957 8 | 0.962 3 |
旋转攻击 | 10 | 0.639 1 | 0.911 8 | 0.936 6 |
30 | 0.503 5 | 0.863 2 | 0.864 6 | |
缩放攻击 | 0.5 | 0.899 0 | 0.969 3 | 0.998 8 |
2 | 0.577 3 | 1.000 0 | 1.000 0 | |
循环平移 | (10,10) | 0.329 4 | 1.000 0 | 1.000 0 |
(100,0) | 0.498 7 | 1.000 0 | 1.000 0 | |
剪切攻击 | 中心剪切1/16 | 0.971 3 | 0.969 8 | 0.970 9 |
在水印算法方面,文献[7]计算宿主图像各子块的信息熵和边缘熵,对选中子块小波域低频子带进行SVD分解,然后在各子块
4 结论
本文分析和讨论了当前水印算法存在的问题,提出一种图像块的不可见性与鲁棒性均衡水印算法。根据人类视觉掩蔽特性确定图像块的掩蔽值,选择掩蔽性好的图像块能够充分保证水印算法的不可见性。在嵌入水印过程中,根据水印位信息选择正交
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