【作者】 邹勤；

【导师】 李清泉；

【作者基本信息】 武汉大学 ， 摄影测量与遥感， 2012， 博士

【摘要】 裂缝是最常见的路面损坏,在路面裂缝演变成坑槽之前进行修补,可以大大节约路面维护的成本。传统的基于影像的路面裂缝检测方法通常假设路面裂缝具有较高的对比度和较好的连续性,但这种假设在实践中往往不成立。这是因为,1)路面影像常常含有路面阴影造成亮度不均匀,2)路面颗粒纹理带来大量的点状噪声,3)路面裂缝由于车轮载重碾压、风化等作用发生退化造成其对比度下降、连续性降低,以及4)裂缝的成像效果对光照方向的高敏感性造成裂缝不连续。以上原因,使得裂缝在路面影像中表现为低信噪比的线状目标,给裂缝的自动化识别带来了巨大挑战。本文针对低信噪比路面裂缝的增强与提取,开展了以下研究：(1)路面阴影不仅造成路面影像的亮度不均匀,而且破坏了路面裂缝的亮度一致性,极大地增加了路面裂缝识别的难度。准确界定并消除路面影像阴影,对路面裂缝识别非常关键。本文针对路面阴影由于半影区巨大而难以界定的难题,提出了基于亮度高程划分的阴影区域界定方法,实现对阴影区及半影区的准确界定。针对传统的加性亮度补偿不能均衡纹理细节的问题,提出了乘性亮度补偿方法,可以实现亮度补偿的同时,将阴影区的方差提高到非阴影区的水平,从而实现阴影区和非阴影区纹理细节的均衡。以上两者结合,得到了基于亮度高程模型的阴影消除算法(GSR),不仅能自动界定路面阴影区域,而且在保持裂缝的同时实现亮度和纹理细节的同步均衡。(2)由于路面材料的颗粒纹理特性,二值路面影像常含有大量的噪声面元,造成路面裂缝信号受到噪声的严重干扰,裂缝目标与路面背景之间的信噪比非常低。考虑到张量表达点状目标的优势,以及投票过程中融入的Gestalt性法则具有潜在的提取线性显著性的功能,本文研究了针对路面裂缝增强的张量投票算法,它首先通过球投票获取每一个目标点的方向,然后运用棒投票实现目标点之间的软连接,并通过矩阵的特征分解实现线状显著性的提取,进而依据线性显著性对裂缝进行增强。(3)针对路面裂缝由于受车轮载重碾压、自然风化等作用发生退化,导致裂缝与路面背景之间的对比度极低,甚至造成裂缝不连续的问题,研究了基于最小代价路径搜索的裂缝增强算法。在研究针对格状图最小代价路径搜索的F*算法的基础上,设计了具有较高运算效率的多尺度F*算法用于路面裂缝跟踪。在此基础上,提出了基于F*的种子生长算法——FoS,解决了F*路径跟踪起点和终点的自动选择的问题。接着,设计了自动获取裂缝种子点的算法流程,以及基于FoS的裂缝增强算法。最后,通过实验分析了种子生长半径对FoS算法效率的影响。(4)针对从含有强噪声的目标点集合中提取具有线状结构特征的点子集的问题,提出了目标点最小生成树算法(T-MST)。T-MST首先用图模型对目标点进行描述,然后根据Gestalt法则的邻近性,计算具有最小边权总和的最小生成树,接着根据Gestalt法则的连续性设计了树的修剪算法,得到线状目标。结合T-MST算法,提出了裂缝提取的FoSA方法和CrackTree方法。FoSA方法利用F*种子生长的方式获取裂缝目标点,CrackTree利用采样的方式获取裂缝目标点。设计了针对性实验,验证了FoSA方法从路面影像中提取对比度低、连续性差的复杂裂缝具有较高的效率和可靠性。同时,采用大量路面影像进行对比实验验证了CrackTree方法比分割后处理方法、边缘检测方法具有更高的裂缝提取精度。更多还原

【Abstract】 Cracks are the most common distresses on the road pavement. Fixing a crack beforeits deterioration can greatly reduce the cost of pavement maintenance. Most traditionalimage-based approaches for pavement crack detection implicitly assume that pavementcracks in images are with high contrast and good continuity. However, this assumptiondoes not hold in practice due to1) uneven illuminance caused by pavement shadows,2)speckle noise brought by grain-like texture of pavement,3) low contrast between cracksand the surrounding pavement and intensity inhomogeneity along the cracks caused bycrack degradation and4) bad continuity of cracks incurred by the high sensitivity ofcrack-imaging results to the direction of illumination. In the above conditions, pavementcracks show their linear structures in a low-SNR manner, which brings great challengesto the automatic detection of pavement cracks. To address these problems, this paperconducts the following research.(1) Pavement shadows not only create uneven illuminance for pavement images, butalso undermine the intensity homogeneity of pavement cracks, which greatly increasethe difculty of identifcation of pavement cracks. To locate and remove pavement shad-ows is critical to the detection of pavement cracks. Considering that pavement shadowstypically hold a big penumbra area, we propose an intensity-based geodesic model tolocate the shadow area, as well as its penumbra area. Moreover, since traditional ad-ditive illuminance-compensation algorithm can not balance the texture detail betweenthe shadow area and the non-shadow area, we propose a multiplicative illuminance-compensation algorithm, which can improve the contrast of the shadow area to the levelof the non-shadow area by adjusting the variance. Then, a novel shadow removal al-gorihtm, i.e., GSR, is formed by integration of the above two components. GSR canautomatically locate the shadow and balance both the intensity and texture betweenthe shadow area and non-shadow area, and meanwhile preserve the cracks.(2) Due to the particle texture of pavement materials, the binary pavement imagesoften contain a lot of speckle noise, which results in low SNR of pavement cracks againstthe pavement background. Note that the tensor is ft for describing the point target,and the Gestalt laws embedded into the voting process can potentially infer out the linear saliency. We exploit a tensor-voting-based method for crack enhancement, whichfrst uses a ball voting to form the orientation at each token, and then applies a stickvoting to softly connect the neighboring tokens, and at last, conduct an eigen-featureanalysis to extract the linear saliency.(3) Since pavement cracks constantly sufer from the rolling of loaded wheels andthe weathering, crack degradation often exists and hence makes a low contrast betweenthe cracks and the pavement background, a bad continuity of the cracks as well. Toenhance these cracks, we exploit a method based on minimum-cost-path searching. First,we present a multi-scale F*algorithm for crack tracking in pavement images, which ismuch efcient than the original F*algorithm. Based on that, we propose an F*seed-growing algorithm, i.e., FoS, which achieves automatic selection of the tracking startand the tracking end. We develop an algorithm to automatically collect the crack seedsand subsequently apply a FoS process to enhance the cracks. We also experimentallystudy how the radius of seed growing impacts the enhancement results.(4) To extract linear structures from a set of spatial points, we propose target-point minimum spanning tree, i.e., T-MST. T-MST inputs the target points into agraph model, and then compute the minimum spanning tree by considering the Gestaltlaw of proximity. Since a crack is a linear structure in a macro perspective, T-MSTembeds the Gestalt law of continuity into a priming algorithm and extract the fnal linecurves. Based on T-MST, we proposed the FoSA approach and CrackTree approachfor pavement crack extraction. The former acquires the target crack points by an F*seed-growing algorithm, while the later adopts a sampling strategy to collect the targetpoints. A range of experiments demonstrate that FoSA is efective and efcient inextracting complex cracks featured with low contrast and bad continuity. Meanwhile,experiments on large-scale dataset show that the proposed CrackTree achieves a muchbetter performance than several existing methods.更多还原