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基于Directionlet变换的图像去噪和融合
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摘要
图像去噪和融合是图像处理的两个重要组成部分。图像在获取和传输过程中,常常受到各种噪声的干扰,这将对后续的图像处理,如分割、压缩等产生不利影响。图像去噪的目的是尽可能恢复图像的原貌,改善图像的质量。图像融合是将多个图像数据所含的优势信息进行互相补充,并有机结合起来产生新的、信息含量更大的图像,来提高图像的信息可用程度,弥补单一信息源的不足。
     近年来,小波变换在图像处理中得到广泛应用。小波分析对含点状奇异性的目标函数具有最优的非线性逼近性能,但在高维情况下,小波并不是最优的或最稀疏的表示方法,小波系数不再稀疏,因此不能很好地挖掘图像中的方向信息。新提出的多尺度几何分析工具,致力于构建最优逼近意义下高维函数的表示方法,在图像去噪和融合中有着良好的应用前景。
     本文主要研究了多尺度几何分析工具中的Directionlet变换,并将其用于图像的去噪和融合中。本文的主要工作如下:
     (1)提出了一种改进的多尺度Directionlet变换的方法。当Directionlet基函数的方向与图像中各向异性目标匹配时,对图像的逼近效果较好,不匹配时则退化为小波,逼近效果很差。本文通过自适应地寻找图像的主要方向,构造Directionlet变换的采样矩阵,使Directionlet的变换方向和队列方向尽量与图像的主要方向一致,能够自适应地捕捉图像中的各向异性特征。
     (2)提出了基于改进Directionlet变换的图像去噪方法。针对图像改进Directionlet系数的高尖峰、重拖尾特点,用广义高斯分布模型对图像Directionlet变换系数的各高频子带系数进行建模。在去噪时,结合广义高斯分布形状参数的大小,将形状参数分为两个区间,采用不同的无噪系数估计方法,利用局部标准差估计,对图像进行去噪处理。
     (3)将改进的Directionlet变换用于图像融合中。改进的Directionlet变换能自适应地捕捉图像沿不同方向的奇异信息,因而具有更好的方向性和各向异性。在融合时,采用基于区域特性量测的融合规则,改善了融合图像的主观视觉效果和客观评价指标。
Image denoising and fusion are two important components of image processing. During acquisition and transmission, images are often corrupted by diversified noises, which could have a negative impact on the following process such as image segmentation and compression. Denoising is to obtain clearer object and increase the recognition rate through filtering noises in the images. Fusion is to combine advantageous information from multiple images of the same scene to acquire more exact and comprehensive description of the image.
     Wavelet transform has found wide applications in image processing in recent years. Wavelet is the optimal bases for functions with point singularity. But in the case of high dimensional, wavelet analysis can not take advantages of the data geometrical features. Then it is not the optimal or the sparsest representation of the functions, so it can not make good use of direction information in images. To solve this problem, a series of new multiscale geometric analysis emerges to building the optimal representation of high dimensional functions, which have a wonderful prospect in the research of image denoising and fusion.
     This paper studies one of the multiscale geometric analysis tools, i.e. Directionlet, and its application on image denoising and fusion. The main innovative points are as follows:
     (1) We proposed an improved multiscale Directionlet transformation method. When the direction of the Directionlet bases matches that of the anisotropic object in images, Directionlet can represent images well, otherwise bases of Directionlet will degenerate into wavelets and have poor approximation power. In this paper, we find main directions of an image and construct the sample matrix adaptively, which can adaptively catch the anisotropic features in images.
     (2) An image denoising algorithm based on improved Directionlet is presented. For the high peaks and heavy tail character of Directionlet coefficients, we model each subband with a generalized Gaussian distribution, then denoise images combining the values of shape parameter and local variance estimate.
     (3) We also study the application of the improved Directionlet transform. As the improved Directionlet can catch image information from different directions, so it has better directionality and anisotropy. Image fusion with improved Directionlet transform and regional measures outperforms wavelet fusion method.
     This research is supported by National Nature Science Foundation (No.60672126, 60702062,60971128), National High Technology Research and Development Program (863) (No.2007AA12Z136), Major State Basic Research Development Program of China (973) (No.2006CB705707).
引文
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