基于虚拟格网的建筑物点云轮廓线自动提取

在分析现有轮廓线提取方法不足的基础上，提出基于虚拟格网的建筑物轮廓线自动提取方法。该方法利用建筑物点云生成虚拟格网并进行二值填充；采用邻域分析方法进行边界格网的标记与追踪；为了避免边界追踪错误，设计了基于方向的单边缘格网抑制方法及基于距离的连接关系调整方法以改善提取结果质量；根据格网追踪结果，从原始建筑物点云中提取真实轮廓点以保持原始建筑物轮廓形态；采用随机抽样一致性估计及最小二乘拟合方法进行轮廓线规则化处理，实现建筑物轮廓线的自动提取。实验结果表明，该方法能快速从建筑物点云中提取轮廓线，可为建筑物轮廓线的自动提取提供一种可行的解决方案。     

基于UG_OPEN的边界面法CAE模型边界条件可视化算法

针对边界面法分析模型前处理过程中边界条件的加载,提出了一种基于UG/OPEN的可视化算法。算法首先利用UG系统中几何模型的边界表征数据,在参数空间生成背景网格以及计算显示点的坐标,然后再映射到三维空间进行图形显示,映射后能够保证几何信息的准确性。该算法不仅实现了与UG系统的无缝连接、载荷和位移约束条件的自动显示,而且有利于实现CAD和CAE模型的一体化。     

Differential quadrature method (DQM) for a class of singular two-point boundary value problems

Differential quadrature method is applied in this work to solve singular two-point boundary value problems with a linear or non-linear nature. It is demonstrated through numerical examples that accurate results for the problem with different types of boundary conditions can be obtained using a considerably small number of grid points. The relative, root mean square and maximum absolute errors in computed solutions are given to show the performance of the method.     

Partial semi-coarsening multigrid method based on the HOC scheme on nonuniform grids for the convection–diffusion problems

A partial semi-coarsening multigrid method based on the high-order compact (HOC) difference scheme on nonuniform grids is developed to solve the 2D convection–diffusion problems with boundary or internal layers. The significance of this study is that the multigrid method allows different number of grid points along different coordinate directions on nonuniform grids. Numerical experiments on some convection–diffusion problems with boundary or internal layers are conducted. They demonstrate that the partial semi-coarsening multigrid method combined with the HOC scheme on nonuniform grids, without losing the high-order accuracy, is very efficient and effective to decrease the computational cost by reducing the number of grid points along the direction which does not contain boundary or internal layers.     

二维单域弹性问题虚边界无网格伽辽金法分析

使用加权残值法中的伽辽金法,并利用无网格法中的径向基函数法插值虚边界上的虚拟源函数,形成了虚边界无网格伽辽金法,具有边界元法和无网格法的共同优点。且该方法的加权值为位移或面力的偏导数,数值意义明确,形成的方程具有对称性。详细推导了单域虚边界无网格伽辽金法的具体数值离散格式,便于其他学者编程、研究。数值实例为文献中的三峡水利枢纽中的双排列输水孔实例,计算虚、实边界不同距离,单元上不同高斯点数,虚拟源函数的不同虚节点径向基函数插值的影响,结果均说明单域虚边界无网格伽辽金法的可行性与精确性。     

一种基于网格核密度的自适应边界点检测算法

为了快速有效的检测聚类的边界点,提出基于网格核密度的自适应边界点检测算法ADAPT(An Adaptive Grid Kernel-Density-Based BoundaryPoints Detecting Algorithm for Spatial Database with Noise),使用网格核密度更精确地拟合网格在其邻域内的密度,采用自适应选取网格近邻策略更好地反应对象的空间分布特征.实验结果表明:该算法可以在含有任意形状、不同大小和不同密度的数据集上快速有效地检测出聚类的边界点.     

Improving Initial Flattening of Convex-Shaped Free-Form Mesh Surface Patches Using a Dynamic Virtual Boundary

This study proposes an efficient algorithm for improving flattening result of triangular mesh surface patches having a convex shape. The proposed approach, based on barycentric mapping technique, incorporates a dynamic virtual boundary, which considerably improves initial mapping result. The dynamic virtual boundary approach is utilized to reduce the distortions for the triangles near the boundary caused by the nature of convex combination technique. Mapping results of the proposed algorithm and the base technique are compared by area and shape accuracy metrics measured for several sample surfaces. The results prove the success of the proposed approach with respect to the base method.     

一种基于网格延展法的等值线生成算法

已有等值线生成算法研究存在算法复杂、不健壮,实现难度大,或与应用系统集成难度大,应用不灵活等问题,以雨量等值线生成算法为例,介绍一种基于网格延展法的等值线生成算法。基于网格延展法的等值线生成算法,通过网格延展法实现等值面的提取,并使用一种简单的方法实现等值线平滑计算,整体等值线生成算法简单、易于实现;基于规则网格进行空间插值计算,对研究区域数据点分布具有很强的适应性,算法表现出较好的健壮性;无须进行等值线边界裁剪计算,也无须考虑等值线追踪时出现的分叉、连通域等问题,可大大减少等值线生成的计算量,降低等值线算法的实现难度;同时计算过程多数是对二维数组的操作,可以使用多种计算机语言实现,具有较好的生产应用价值。     

Improved MS_CMAC Neural Networks by Integrating a Simplified UFN Model

Macro_Structure_CMAC (MS_CMAC) is a variational CMAC neural network that is designed for modeling smooth functional mappings. The MS_CMAC learning strategy involves constructing virtual grid-distributed data points from random-distributed training data points, and then using the virtual data points to train a tree structure network that is composed of one-dimensional CMAC nodes. A disadvantage of the MS_CMAC is that the prediction errors near the boundary area might sometimes be unexpectedly large. Another disadvantage of the MS_CMAC is that generating virtual grid-distributed data points generally takes a long computational time. Therefore, this study develops an improved model by integrating an unsupervised fuzzy neural network (UFN) into the MS_CMAC to initialize systematically the virtual grid-distributed data points. Additionally, a new error feedback ratio function is adopted to speed up the MS_CMAC training. Several numerical problems are considered to test the improved MS_CMAC. The computed results indicate that a simplified UFN model can produce good initial values of the virtual grid-distributed data points to aggrandize MS_CMAC training. The MS_CMAC prediction is also improved by using the initialized virtual grid-distributed data points.     

一种基于Delaunay背景图改进的网格变形方法

针对Delaunay网格变形方法中因计算网格点在背景网格中映射不够精细导致的大变形失效问题,提出了一种改进后的网格变形方法,旨在进一步提高大变形情形时变形后的网格质量。该方法将原始的Delaunay网格变形方法中的背景网格远场边界进行加密,增加了映射背景网格单元的数量,改善了计算网格在背景网格中的映射精细程度,从而提升了变形后网格的质量。通过一正方形网格变形基础算例和30P30N三段翼型流场网格变形算例分别进行了测试与验证,结果表明该改进方法可以在保证计算效率的前提下,显著提升大变形时变形后的网格质量。与原始的Delaunay网格变形方法相比较,改进后的网格变形方法变形能力较强,所生成网格质量较高。     

Speed and Accuracy Tradeoff for LiDAR Data Based Road Boundary Detection

Road boundary detection is essential for autonomous vehicle localization and decision-making, especially under GPS signal loss and lane discontinuities. For road boundary detection in structural environments, obstacle occlusions and large road curvature are two significant challenges. However, an effective and fast solution for these problems has remained elusive. To solve these problems, a speed and accuracy tradeoff method for LiDAR-based road boundary detection in structured environments is proposed. The proposed method consists of three main stages: 1) a multi-feature based method is applied to extract feature points; 2) a road-segmentation-line-based method is proposed for classifying left and right feature points; 3) an iterative Gaussian Process Regression (GPR) is employed for filtering out false points and extracting boundary points. To demonstrate the effectiveness of the proposed method, KITTI datasets is used for comprehensive experiments, and the performance of our approach is tested under different road conditions. Comprehensive experiments show the road-segmentation-line-based method can classify left, and right feature points on structured curved roads, and the proposed iterative Gaussian Process Regression can extract road boundary points on varied road shapes and traffic conditions. Meanwhile, the proposed road boundary detection method can achieve real-time performance with an average of 70.5 ms per frame.      

SPH 流固耦合模拟自适应采样固壁虚粒子边界处理方法

固壁虚粒子边界处理方法是流体模拟中一种主要边界处理方法,但其不能确保流 体粒子不穿透固体边界,并且计算量较大。为防止流体粒子穿透边界,在边界附近设置一个阻 尼区,阻尼区内的流体粒子被边界施加一个弹性力和一个和流体粒子运动速度方向相反的阻尼 力,使得边界附近流体粒子更加稳定。为减少计算量,提出两种边界粒子自适应采样法：一种 是依据边界周围粒子数目的不同,边界粒子自适应地采样质量不同的大小粒子;另一种是依据 边界周围粒子数目的不同,边界粒子自适应的采样不同层数的相同质量粒子。与传统的固体边 界粒子采样方法相比,该方法减少了边界粒子数目,加快了模拟速度,节省了计算机内存,基 于GPU 加速技术实现的三维流体模拟,能够进行实时交互。     

Adaptive Techniques for Spline Collocation

We integrate optimal quadratic and cubic spline collocation methods for second-order two-point boundary value problems with adaptive grid techniques, and grid size and error estimators. Some adaptive grid techniques are based on the construction of a mapping function that maps uniform to non-uniform points, placed appropriately to minimize a certain norm of the error. One adaptive grid technique for cubic spline collocation is mapping-free and resembles the technique used in COLSYS (COLNEW) [2], [4]. Numerical results on a variety of problems, including problems with boundary or interior layers, and singular perturbation problems indicate that, for most problems, the cubic spline collocation method requires less computational effort for the same error tolerance, and has equally reliable error estimators, when compared to Hermite piecewise cubic collocation. Comparison results with quadratic spline collocation are also presented.     

Convergence of Summation-by-Parts Finite Difference Methods for the Wave Equation

When using a finite difference method to solve a time dependent partial differential equation, the truncation error is often larger at a few grid points near a boundary or grid interface than in the interior. In computations, the observed convergence rate is often higher than the order of the large truncation error. In this paper, we develop techniques for analyzing this phenomenon, and particularly consider the second order wave equation. The equation is discretized by a finite difference operator satisfying a summation by parts property, and the boundary and grid interface conditions are imposed weakly by the simultaneous approximation term method. It is well-known that if the semi-discretized wave equation satisfies the determinant condition, that is the boundary system in Laplace space is nonsingular for all Re \((s)\ge 0\), two orders are gained from the large truncation error localized at a few grid points. By performing a normal mode analysis, we show that many common discretizations do not satisfy the determinant condition at \(s=0\). We then carefully analyze the error equation to determine the gain in the convergence rate. The result shows that stability does not automatically imply a gain of two orders in the convergence rate. The precise gain can be lower than, equal to or higher than two orders, depending on the boundary condition and numerical boundary treatment. The accuracy analysis is verified by numerical experiments, and very good agreement is obtained.     

Grid VFS中代理管理客户端磁盘缓存的设计与实现

设计并实现了在网格虚拟文件系统Grid VFS客户端采用代理管理磁盘缓存来隐藏网络延迟，提高网格数据利用率。此方法扩展了虚拟化分布式文件系统，提高了对网格资源有效完整的数据访问。在Grid VFS中，客户端代理动态地产生和管理磁盘缓存，每一个未修改的应用程序和OS都能应用，支持write-back策略、文件系统之间缓存的共享以及磁盘缓存的多级化，通过用户级VFS可以完整的集成到网格应用程序和资源中，从而提高了网格数据的访问性能。     

基于医学体数据生成四面体网格的方法

为了从医学体数据直接构造四面体网格,提出一种基于栅格的网格生成算法.该算法的主要思想是从背景栅格中提取并填充代表区域边界的等值面.首先,对医学体数据进行预处理与采样,构建一个背景栅格.其次,用对偶方法从栅格提取三角表面网格,用于分段线性逼近等值面.然后,对栅格中所有位于等值面之内或与等值面相交的立方体,用预定义的模板分解成四面体单元.最后,用Laplacian平滑技术优化四面体网格.在均匀网格的基础上,研究了自适应网格生成算法,在保持网格几何精度的同时精简单元数量,以提高有限元计算效率.给出了从CT数据生成人体股骨远端四面体网格的实例,该网格模型被用于虚拟膝关节镜手术.     

存在运动目标时的图像镶嵌方法研究*

针对在图像镶嵌过程中,存在运动目标时所导致的虚影现象及明显拼接缝的问题,提出了一种图像无缝镶嵌方法。该方法在结合支持向量机处理运动目标的基础上,通过尺度不变特征点的提取以及变换模型参数的更新,得到更加精确的变换矩阵。另外,在图像融合过程中,最优拼接缝的寻找综合了颜色、纹理及结构等多方面的信息。实验表明,该方法能够很好地消除虚影现象和拼接缝隙,得到高质量图像,从而获得无缝镶嵌的效果。     

Semi-analytical solution of Laplace’s equation in non-equilibrating unbounded problems

Some two-dimensional problems of elastostatics are governed by Laplace’s equation. Using the terminology of elastostatics, if the face loads and body loads are not self-equilibrating, even when the displacement at infinity is restricted to zero, displacements in the near field will be infinite. However, the stress field within the domain is well behaved, and is of practical interest. In this paper the semi-analytical scaled boundary finite-element method is extended to permit the analysis of such problems. The solutions in the primary variable so obtained include an infinite component, but the difference in value between any two points in the domain can be computed accurately. The method is also extended to solve the non-homogeneous form of Laplace’s equation.     

三维非定常扩散方程的虚边界元法

根据位势问题虚边界元法的基本思想,结合扩散方程与时间有关的基本解,提出了针对单层热势的三维非定常扩散方程虚边界元-配点法的一个具体实施方案.该方法既保留了边界元法的优点,也避免了传统边界元法中时间和空间上的奇异积分计算,采用较少的边界单元即可达到较高的精度.算例表明此方法的有效性和可行性,不过虚实边界比例选取范围比虚边界元方法应用于椭圆型问题时狭窄很多,对此本文进行了探讨,但还应继续从理论上加以论证.