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针对基于倾斜摄影影像重建三维城市模型纹理映射过程中出现的遮挡和纹理碎片化问题,本文提出一种利用摄影光线与模型相交判断遮挡,然后利用图割算法优化纹理的方法。首先,连接摄影中心与三角形面片各顶点,判断连接线段与其他三角形面片的相交情况,从而确定是否遮挡,以选取当前三角形面片对应的未被遮挡影像。然后,根据对应关系,构建能量函数,通过图割算法最小化能量函数以确定最优纹理,使相邻三角形面片对应影像的选择趋于一致,从而解决纹理碎片化问题。通过实验证明,本文方法在大规模三维模型的纹理映射中,能够有效解决映射的遮挡重影现象,并能够极大地避免纹理碎片化问题。 相似文献
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关于地壳厚度计算方法的修正 总被引:2,自引:0,他引:2
利用前苏联学者(1958.)计算地壳厚度公式计算了兰州-天水-武都地区的布格重力异常.发现其结果与人工地震爆破得出的结果相差很大。造成差值大的原因可能是地壳界面边界效应的作用。为使计算结果尽量接近实际地壳厚度。所以要从计算地壳厚度公式中减去每一测点的效应值,依此法推算出任意点的地壳厚度。 相似文献
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毛乌素沙地东南部人工植被演替研究 总被引:7,自引:3,他引:7
毛乌素沙地东南部人工植被的演替过程大致可分为如下几个阶段:天然稀疏草本阶段人工灌木林阶段—人工灌木、天然半灌木、天然草本阶段—天然草本优势阶段。 相似文献
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为满足水库抗震设计的震要,合成了50年超越概率5%和100年超越概率2%的基岩加速度时程曲线,考虑到吉林吉台电站所处的特殊地震环境和场地条件,分析其地震危险性认为:50年超越概率5%的地震动为远震影响;100年超越概率2%的地震动为近震影响,目前使用的基岩加带度反应谱衰关于关系的来源资料多少丢失一些高频成分,可能低估高频成分的地震动对基岩场地的影响,为此根据大震,远震的特征周期Tg较大这一规律,把两个概率水平的加速度反应谱的特征周期Tg都定为0.3s,并增加基岩反应谱高频成分控制,从而把基岩地震加速度反应谱处理成相应的规准谱,在此基础上通过拟合规准谱合成的基岩地震动时程曲线,较好地满足坝址的设防要求。 相似文献
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A fundamental tool in seismic risk assessment of transportation systems is the fragility curve, which describes the probability that a structure will reach or exceed a certain damage state for a given ground motion intensity. Fragility curves are usually represented by two‐parameter (median and log‐standard deviation) cumulative lognormal distributions. In this paper, a numerical approach, in the spirit of the IDA, is applied for the development of fragility curves for highways and railways on embankments and in cuts due to seismic shaking. The response of the geo‐construction to increasing levels of seismic intensity is evaluated using a 2D nonlinear finite element model, with an elasto‐plastic criterion to simulate the soil behavior. A calibration procedure is followed in order to account for the dependency of both the stiffness and the damping to the soil strain level. The effect of soil conditions and ground motion characteristics on the response of the embankment and cut is taken into account considering different typical soil profiles and seismic input motions. This study will provide input for the assessment of the vulnerability of the road/railway network regarding the performance of the embankments and cuts; therefore, the level of damage is described in terms of the permanent ground displacement in these structures. The fragility curves are estimated based on the evolution of damage with increasing earthquake intensity, which is described by PGA. The proposed approach allows the evaluation of new fragility curves considering the distinctive features of the element's geometry, the input motion, and the soil properties as well as the associated uncertainties. A relationship between the computed permanent ground displacement on the surface of the embankment and the PGA in the free field is also suggested based on the results of the numerical analyses. Finally, the proposed fragility curves are compared with existing empirical data and the limitations of their applicability are outlined. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
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Automatic change detection and geo-database updating in the urban environment are difficult tasks. There has been much research on detecting changes with satellite and aerial images, but studies have rarely been performed at the street level, which is complex in its 3D geometry. Contemporary geo-databases include 3D street-level objects, which demand frequent data updating. Terrestrial images provides rich texture information for change detection, but the change detection with terrestrial images from different epochs sometimes faces problems with illumination changes, perspective distortions and unreliable 3D geometry caused by the lack of performance of automatic image matchers, while mobile laser scanning (MLS) data acquired from different epochs provides accurate 3D geometry for change detection, but is very expensive for periodical acquisition. This paper proposes a new method for change detection at street level by using combination of MLS point clouds and terrestrial images: the accurate but expensive MLS data acquired from an early epoch serves as the reference, and terrestrial images or photogrammetric images captured from an image-based mobile mapping system (MMS) at a later epoch are used to detect the geometrical changes between different epochs. The method will automatically mark the possible changes in each view, which provides a cost-efficient method for frequent data updating. The methodology is divided into several steps. In the first step, the point clouds are recorded by the MLS system and processed, with data cleaned and classified by semi-automatic means. In the second step, terrestrial images or mobile mapping images at a later epoch are taken and registered to the point cloud, and then point clouds are projected on each image by a weighted window based z-buffering method for view dependent 2D triangulation. In the next step, stereo pairs of the terrestrial images are rectified and re-projected between each other to check the geometrical consistency between point clouds and stereo images. Finally, an over-segmentation based graph cut optimization is carried out, taking into account the color, depth and class information to compute the changed area in the image space. The proposed method is invariant to light changes, robust to small co-registration errors between images and point clouds, and can be applied straightforwardly to 3D polyhedral models. This method can be used for 3D street data updating, city infrastructure management and damage monitoring in complex urban scenes. 相似文献
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图像分割是图像处理中的基础问题。研究利用GMM构造T链的可行性,并探索一种可以兼顾精度与速度的确定GMM中K值的方法,结合GMM和Graph Cuts理论完成了对不同图像的分割实验。实验表明,文中的分割方法准确、有效。 相似文献