【作者】 李琳琳；

【导师】 颉耀文；

【作者基本信息】 兰州大学 ， 地图学与地理信息系统， 2012， 硕士

【摘要】 在遥感图像的应用中,多数情况下人们往往只对图像中的某些部分感兴趣。为了识别和分析目标,就需要将这些部分从整幅图像中分割出来,并尽量避免背景的干扰及影响。不同的阈值设定方法直接影响到目标提取的精度和图像特征信息的保留程度。传统的通过人机交互确定阈值或者使用经验值的方法主观影响比较大,不能实现自动化提取,并且不能完全保证普遍适用。因此实现遥感图像分割中阈值的自动选取并研究其适用性是一个值得研究的重要内容。对已有的研究成果分析表明,目前尚没有一种适合于所有图像的通用的阈值自动选取算法。但是,对于具体的图像、具体的需求,的确可以找到效果相当好的自动化阈值选取方法。本文分别对简单图像和复杂图像中阂值的自动选取方法进行了介绍并进行实验。由于后者基于前者,所以本文重点讨论简单图像中的阈值选取方法。简单遥感图像采用阈值分割时,可使用基于像元灰度的全局阈值法。在保持图像的均匀照度下,对于直方图双峰明显,谷底较深的图像,可采用迭代法；对目标大小适合的图像,可采用最大类间方差法寻找最佳阈值进行图像分割；实时性和稳定性要求高时可选择最小误差法；当图像中目标和背景灰度对比度较低,直方图为单峰,或者目标与背景比例不均衡,目标地物比较小时,可采用最大熵法,但其对噪声点比较敏感；当注重原始图像和分割后图像之间的信息量差异最小时,可采用最小交叉熵阈值法,它在双峰图像和单峰图像中的适用性都很高,但它对目标的大小较敏感。简单遥感图像中含有噪声,背景灰度不均匀时,可使用基于像元邻域属性的阈值法。本文将一维Otsu算法和一维最大熵算法拓展到二维空间,提出了二维阈值算法；同时还提出一种过渡区加权的算法。这三种方法都能有效地克服噪声的干扰,分割效果总体上都优于一维阈值算法。但从运算时间来看,由于运算量成倍增加,二维算法的计算时间也高于一维算法。复杂遥感图像中目标灰度层次比较丰富、边界模糊,物体和背景的对比度在图像中各处不一样,可使用局部自适应阈值法。Mean法和Median法对每个像素确定一个以它为中心的窗口,然后求取窗口内的灰度均值和中值作为此像素的阈值,最后移动窗口得到每个像素的阈值。这种方法大大简化了传统阈值差值的算法,并且能兼顾考虑图像各处的具体情况,保证分割效果。更多还原

【Abstract】 In the application of remote sensing images, people are often interested in specific information of the image. When extracting the specific target information, it is necessary to separate the target from the whole original image and try to avoid the disturbances from the background. How to rationally and effectively obtain the threshold which is used to differentiate the object from the background is the key process. However, thresholds based on different segmentation algorithms would affect the precision of the result and the detail level of the characteristic information. The traditional methods, such as human-computer interaction or using experienced values, are often influenced by subjective factors. They can neither determine the threshold automatically nor be applied to all situations. Therefore, studying on the automatic selection threshold methods is an important subject for image segmentation and worth further research.By now, there is no certain threshold algorithm suitable for all types of images. Certainly for specific images and specific requirements, we can find the optimum threshold methods with quite good effect. This paper studies and discusses several automatic selection threshold methods for simple and complex images separately. Since the latter is based on the former, this paper is focus on global threshold in simple images.Global threshold method based on pixel gray scale can be used in a simple remote sensing image. While iterative method can be used in the uniform illumination images which have double-peak and deep-valley histogram. When there are higher real-time and stability requirements, the minimum error method is a better choice. Maximum entropy method can not only be applied to the images with low gray contrast and single-peak histogram, but also be applied to small targets extraction; however, it is sensitivity to noise. For the images that with the suitable size of target, the maximum between-class variance method can be used to find the optimal threshold for image segmentation. Minimum cross-entropy threshold method focuses on minimizing the discrepancies of information between the original image and segmented image, it has high applicability in images with single-peak histogram as well as double-peak histogram; but, it is sensitivity to the size of the target.For the remote sensing images with much noise and non-uniform background characteristics, the threshold method based on attributes of pixel neighborhood is more appropriate. This article develops one-dimensional Otsu algorithm and one-dimensional maximum entropy method to two-dimensional space; and also proposes an algorithm based on edge weighting. All of them can effectively overcome the noise interference, and the segmentation results are better than the one-dimensional threshold algorithm. However, the computation time of the two-dimensional algorithm is much longer than the one-dimensional algorithm due to the multiplied computation.As for complex remote sensing images which have relatively rich gray levels, fuzzy boundaries, complex structure and different contrast, adaptive threshold method based on local properties can be used to segment the image. Mean method and Median method take the mean and median value of each pixel in an operator window as the threshold, which can greatly simplify the algorithm of traditional interpolation method. It takes the different characteristics of each part of the image into account, and can be success in segmentation.更多还原