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基于计算机视觉的活立木三维重建方法
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摘要
中国木材需求量大,产量不足,发展人工林是解决这一问题的有效办法。合理抚育是提高人工林木材产量和质量的重要环节,智能型抚育装备是现代抚育的关键装备,智能型抚育装备包括地面修枝机器、间伐机器和爬树整枝机等,都配装备有计算机视觉系统。未来自动联合采伐装备也将配备计算机视觉系统,在林区自主行走和进行采伐作业。活立木三维重建方法是智能型抚育装备和自动联合采伐装备等林业装备的视觉系统中的关键技术,也是本文的核心内容。
     本文对针对智能型林业装备配备的单个相机或者两个相机从不同角度获取的同一活立木二维图像,利用双目立体视觉基本原理,研究活立木三维重建方法,即恢复活立木的三维信息。本文从活立木枝干提取、特征(角点)提取和匹配、射影重建、相机自标定和欧氏三维重建几个方面进行了详细的阐述。通过活立木枝干信息提取去除活立木的细小枝丫,得到活立木较大的枝干,为后续的三维重建服务;通过特征(角点)提取和匹配得到活立木不同角度图像中对应点对;在射影重建部分先求两幅图像之间的基本矩阵然后以第一幅图像拍摄位置的坐标为参考坐标进行射影重建;通过基于基本矩阵的相机自标定方法求解相机的内参数K,从本质矩阵估计相机的运动参数,即计算出旋转矩阵R和平移矢量t,最后根据相机内外参数和左右图象的匹配点坐标估计活立木枝干在欧氏空间下的三维坐标信息。本研究获得以下几点重要结论和创新点:
     1.提出了两种活立木图像分割方法,一种是基于粒子群算法的水平集活立木图像分割方法,另一种是基于数学形态学的树木图像分割方法。基于粒子群算法的水平集图像分割方法将C-V模型PDE求解问题视为一个最优化问题,利用粒子群最优化算法进行求解,实验证明该方法对不同背景的图像的两类分割问题很有效。基于数学形态学的树木图像分割方法先对灰度图像用分水岭算法进行分割,再用自动阈值法消除分水岭算法产生的过度分割问题,实验表明该方法的分割效果比Sobel算子等方法有效。这两种方法都能有效解决了复杂背景下活立木的图像分割问题。
     2.提出了一种基于SIFT角点检测和NCC匹配原则相结合的角点检测与匹配方法,该方法结合了SIFT角点检测尺度变换不变性的优点和NCC匹配速度快的优点,实验结果表明该方法检测角点的精度比Harris和SUSAN方法的高,运算速度比传统SIFT的快。
     3.提出了一种基于粒子群算法的基本矩阵估计算法,该方法解决了8点法在匹配点对较多时如何选择最佳8对匹配点对的问题,得到了在全局最优意义下的基本矩阵估计方法;但该方法也有一定的缺点就是算法的时间复杂度较高,需要进一步研究快速算法。
     4.提出了一种基于遗传算法的相机内参数估计方法,该方法将摄像机自标定转化为一个求解代价函数的最小值问题,用遗传算法进行求解。
     5.本文采用计算机视觉基本理论对活立木进行三维重建,恢复活立木枝干的三维信息的方法是行之有效的。实验结果表明该方法可以应用到智能型林业装备的视觉系统中,解决其在作业过程中估计立木枝干三维信息的问题。
For China, the demand for the wood is great, while the output of the wood is relatively less. Developping the plantation forest is a potent way of approaching this problem. For plantation forest, the correct cultivation can improve the quality of the wood and the output of the wood for every hektare plantation forest. The intelligent cultivating machines with the machine vision system including intelligent pruning machine on trees, intelligent pruning machine on ground and the selective cleanng machine are the key equipments for the cultivation. For the future, the forest harvesting machine will be equipped with the computer vision system. The method of the 3D reconstruction for the standing trees is an important technology for the computer vision system of the intelligent forestry machine.
     The main content of this thesis is to study the method of 3D reconstruction for the standing trees from the images got from different orientations by the camera on the intelligent forestry machine, that is to say to get the 3D information of the standing trees by using the binocular stereo vision. Getting the trunk and branches of the standing tree, extracting the corners and stereo matching, and projective reconstruction are discussed detailly in this thesis; also the camera self-calibration and 3D Euclidean reconstruction are presented in detail. Getting the trunk and branches, and eliminating the little branches and leaves are all done for the following 3D reconstruction. Extracting the corners and stereo matching are done to get the corresponding points on the different images. In the projective part, the fundamental matrix of two images can be computed firstly from the corresponding points of the two images, and then 3D reconstruction is made in the projective space according the cordinates of the position where the camera is to get the first image. In the camera self-calibration and 3D Euclidean reconstruction part, the intrinsic parameters K can be get from the fundamental matrix, and the rotation matrix R and transferation vector t can be got from the the essential matrix E computed from the the intrinsic parameters K and the fundamental matrix F. At last, the 3D information of the standing tree in Euclidean space can be computed from the intrinsic parameters and motion of the camera. Following main conclusions are drawn:
     1. Two methods on standing tree image segmentation are proposed. One is level set method in standing tree image segmentation based on particle swarm optimization, the other is tree image segmentation based on mathmatical morphology. For the level set method on standing tree image segmentation based on particle swarm optimization, the image segmentation based on C-V model is considered as an optimal problem that was traditionally considered as a PDE problem before, and then is approached by the particle swarm optimization. The experimental results show that it is effictive for the two class-image segmentation with differnet backgrounds. For the tree image segmentation based on mathmatical morphology, the tree image is segmentated by the watershed algrithm and then segmentated by the auto-threshold method to remove the over-segmentated problem. The experimental results show that the segmentation effct of the mathematical morphology method is better than those of the Sobel and other operation method. So the two methods are all useful for the standing tree image segemtation with tanglesome background.
     2. One method on corner detection and matching is proposed. The method combine the scale invariant strongpoint for corner detection of the SIFT and the merit for fastly computing of the NCC matching rule. The experimental results show that the method is more accurate than the Harris and has higher computing velocity than the traditional SIFT.
     3. One method on computing the fundamental matrix based on particle swarm optimization is proposed.The method can solve the problem how to select eight groups of the corresponding points in eight-point method. It is an optimal approaching to computing the fundamental matrix in the global space. But it is researched deeply that the fast computing algrithm of the method, because the method with higher time-complexity.
     4. One method on estimating the intrinsic parameters based on genetic algrithm is proposed. For the method, the camera self-calibration is considered as an optimal problem to get the minal value of the cost function, and it is solved by the genetic algrithm in this thesis.
     5. The method on 3D reconstruction or getting the 3D information of the standing tree by using the principle of computer vision is valid. The experimental results show that the method can be used in the vision system of the intelligent forestry machine to solving the problem of getting the 3D information of the standing trees.
引文
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