一种P2P环境下高效Topk资源搜索技术

随着网格的广泛应用,在网格下查询最符合用户需求的k个资源成为资源搜索研究的重点之一.特别是资源在地域上广泛分布,使得这种Topk搜索的效率成为影响系统性能的关键因素之一.提出了一种P2P环境下的Topk搜索算法,它根据资源属性,将网格资源看做是m维空间中的点,而Topk搜索就转换为在m维空间中搜索距离查询点最近的k个点.该算法根据Agrawal发现的资源密集现象,在m维空间中确定搜索区间大小,并利用P2P领域的多区间搜索算法,迭代地在多个区间中搜索资源,使得算法同时保持高效和低负载的特点.证明了该算法的正确性并分析了它的性能,分析和实验表明,该算法在高维资源属性空间中具有较好的查询效率和较低的网络负载.     

一种基于映射法的散乱点云Delaunay三角剖分算法

针对直接在三维空间构建海量点云的Delaunay三角网格效率低下,提出一种新的基于映射法的Delau-nay三角网格构建算法.首先提出一种基于区域增长法的点云分片方法,能够保证对分片后的点云数据进行映射而不产生重叠;然后保持空间点云之间的距离特性,将三维点云映射到二维平面;在二维平面内进行Delaunay三角剖分,再将结果返回到三维空间内.实验结果表明,算法能够构建质量较好的三角网格.由于该算法将点云的三角剖分转换到低维空间,通过实验结果对比本算法与其他算法效果,证明该方法能够更快地完成重构.     

一种采用Z曲线高维空间范围查询算法

低维空间中线性扫描算法及基于R树、VA文件和NB树的空间范围查询算法的效率较高,高维空间中它们的效率出现恶化现象.Z曲线将空间分割成大小相等网格并依次穿过它们,将网格中的点映射到线性空间中,从而能够使用B+树作为点集的索引结构.利用Z曲线聚类和降维特性,本文给出网格划分方法、搜索区域分解过程,提出一种高维空间范围查询算法.实验结果表明在高维空间中算法的效率优于上述算法.     

基于n维空间模型的计算网格资源查找方法

提出了描述计算网格资源的空间模型,按网格资源的性质将网格资源组织在n维空间中,并按资源之间的相似程度对资源进行关联,该模型不需建立资源的全局视图,系统复杂度不会随着资源数量的增加而增大。基于资源空间模型的资源查找算法能够保证资源查找沿着正确的方向进行,从而提高资源搜索效率。同时还给出了该算法的时间复杂和空间复杂度分析。     

非封闭曲面的海量空间数据点四边形网格生成算法

从数据模型的任意一点开始选择一个初始的四边形网格单元,采用动态边界边扩展的方法在三维空间直接进行四边形网格划分;在网格划分过程中实现了边界冲突检测、网格顶点优化处理、网格边界处理和网格综合优化.最后给出了网格生成实例.实验结果表明:该算法生成的网格质量较好,运行速度较快.     

曲面等值分割中的界点跟踪方法EI北大核心CSCD

面向任意输入条件的曲面等值分割方法具有通用性,但已有方法未对其界点跟踪环节进行深入探讨.为此,提出曲面等值分割中的界点跟踪算法.首先,通过循环选择当前网格单元的出入界点,构建后继网格单元,并设为当前网格单元来跟踪界点;其次,对算法中的关键技术——网格单元动态构建方法以及基于界边生长趋势预测的网格单元出入界点选择方法进行介绍;最后,针对坐标、可加工性、法向量、高斯曲率和平均曲率等面点属性设计5个分割条件集,并进行实例测试.当网格单元均仅有2个界点时,该方法与已有方法的分割精度相同;当存在网格单元有2个以上界点时,该方法的平均界点误差比已有方法降低91.78%.实验结果表明,该方法适用于更复杂的分割问题,且具有较高的分割精度和鲁棒性.     

基于边折叠和质点-弹簧模型的网格简化优化算法

通过边折叠实现网格曲面简化,提出了保持曲面特征的边折叠基本规则,引入边折叠顺序控制因子λ,给出了折叠点坐标获取方法,简化过程中网格边长度趋于均匀．在曲面简化基础上,利用质点-弹簧模型优化网格形状．将网格顶点邻域参数化到二维域上,在质点-弹簧模型中引入约束弹簧,约束调整网格顶点,并逆映射到三维原始曲面上,局部优化网格顶点的相邻网格;调整曲面上所有网格顶点,在全局上优化网格形状．在曲面简化优化过程中,建立原始模型曲面和简化优化后曲面之间的双向映射关系;曲面的网格顶点始终在原始模型表面上滑动,并以双向Hausdorff距离衡量、控制曲面间的形状误差．应用实例表明：文中算法稳定、高效,适合于任意复杂的二维流形网格．     

平面离散点集的边界搜索算法

进行有限元仿真首先要建立有限元网格模型。使用不含有任何拓扑信息的离散点集直接进行网格划分可以快速、精确地建立网格模型。使用铺路法进行网格剖分是从边界开始向内生成网格单元。该文提出一种使用搜索盒的搜索平面离散点集边界的算法。该方法将离散点分配到搜索盒中，遍历位于边界的搜索盒，将其中的点连接成边界点链表。该算法能正确地搜索包含有凹点、孔洞特征的离散点集的边界，具有较强的通用性。文中介绍了算法的基本思想，并给出算例。     

四边形网格间接生成方法

研究了基于背景三角网格的四边形网格间接生成算法,并针对三角形合并过程中容易残留三角形的缺陷提出了确定侧边的详细算法,该算法主要是依据背景三角网格中边的位置和前沿边的情形,通过背景三角网格中已存在的边、边交换或边分割确定侧边,以避免在三角形合并过程中残留三角形单元。最后给出实例验证了算法的有效性。     

一种快速的可逆累进网格算法

在分析已有累进网格生成算法的基础上,构造了一种新的网格简化信息记录表示法,并提出一种基于“边折叠”网格简化方法的累进网格生成算法。此算法不仅消除了累进网格技术中的二义性,而且能够较大地提高累进网格的运算速度。     

Fun sheet matching: towards automatic block decomposition for hexahedral meshes

Depending upon the numerical approximation method that may be implemented, hexahedral meshes are frequently preferred to tetrahedral meshes. Because of the layered structure of hexahedral meshes, the automatic generation of hexahedral meshes for arbitrary geometries is still an open problem. This layered structure usually requires topological modifications to propagate globally, thus preventing the general development of meshing algorithms such as Delaunay??s algorithm for tetrahedral meshes or the advancing-front algorithm based on local decisions. To automatically produce an acceptable hexahedral mesh, we claim that both global geometric and global topological information must be taken into account in the mesh generation process. In this work, we propose a theoretical classification of the layers or sheets participating in the geometry capture procedure. These sheets are called fundamental, or fun-sheets for short, and make the connection between the global layered structure of hexahedral meshes and the geometric surfaces that are captured during the meshing process. Moreover, we propose a first generation algorithm based on fun-sheets to deal with 3D geometries having 3- and 4-valent vertices.     

任意三角形网格的基于二元四次箱样条分片C1曲面

提出一种以任意三角剖分为控制网格的二元箱样条曲面算法.二元三方向剖分是方向最少的三角剖分,建立在其上的二元三向四次箱样条在CAGD等领域有着广泛的应用.其规范的箱样条曲面计算仅适用于控制点的价数均为6的网格.从规范的算法出发,提出了一种任意价数控制网格的曲面计算算法,并对算法的连续性等进行了详细的分析.生成的曲面具有保凸性,且是分片C¹连续的.该算法可进行3D离散点全局或局部插值,并可应用于3D曲面重构等领域.     

任意三角形网格的基于二元四次箱样条分片C1曲面

提出一种以任意三角剖分为控制网格的二元箱样条曲面算法.二元三方向剖分是方向最少的三角剖分,建立在其上的二元三向四次箱样条在CAGD等领域有着广泛的应用.其规范的箱样条曲面计算仅适用于控制点的价数均为6的网格.从规范的算法出发,提出了一种任意价数控制网格的曲面计算算法,并对算法的连续性等进行了详细的分析.生成的曲面具有保凸性,且是分片C1连续的.该算法可进行3D离散点全局或局部插值,并可应用于3D曲面重构等领域.     

A reliable triangular mesh intersection algorithm and its application in geological modelling

We introduce a reliable intersection algorithm for manifold surface meshes. The proposed algorithm builds conforming surface meshes from a set of intersecting triangulated surfaces. This algorithm effectively handles all degenerate triangle–triangle intersection cases. The key idea of the algorithm is based on an extensive set of triangle–edge intersection cases, combined with an intersection curve tracking method. The intersection operations do not rely on global spatial search operations and no remeshing steps are needed. The intersection curves are introduced into each surface mesh using a unique curve imprinting algorithm. The imprinting algorithm naturally handles degenerate intersection cases of many surfaces at an edge or at a point. The algorithm produces a consistent mesh data structure for subsequent mesh optimization operations. The mesh intersection algorithm is used within a general framework for modelling and meshing of geological formations, which are essential for reliable mathematical modelling of oil reservoirs.     

STL rapid prototyping bio-CAD model for CT medical image segmentation

This paper presents a simple process to construct 3D rapid prototyping (RP) physical models for computer tomography (CT) medical images segmentation. The use of stereolithography (STL) triangular meshes as a basis for RP construction facilitates the simplification of the process of converting CT images to an RP model. This is achieved by constructing the STL triangular meshes directly from data points without having to draw the curve model first. The grey prediction algorithm is used to sort contour point data in each layer of the medical image. The contour difference detection operation is used to sequence the points for each layer. The 3D STL meshes are then constructed by this proposed layer-by-layer sequence meshes algorithm to build the STL file. Once this STL file is saved, a 3D physical model of the medical image can be fabricated by RP manufacturing, and its virtual reality model can also be presented for visualization. CT images of a human skull and femur bone were used as the case studies for the construction of the 3D solid model with medical images. The STL models generated using this new methodology were compared to commercial computer-aided design (CAD) models. The results of this comparative analysis show that this new methodology is statistically comparable to that of the CAD software. The results of this research are therefore clinically reliable in reconstructing 3D bio-CAD models for CT medical images.     

基于八叉树空间分割的三维点云模型密写

针对三维点云模型的信息隐藏,提出一种基于八叉树空间分割的空域密写算法。对经过主成分分析后的三维点云模型建立包围盒,利用八叉树空间分割得到小体元并记录分割过程,通过顶点位移将信息嵌入到小体元内的不同空间位置。实验结果表明,该算法在提取信息时不需要原始模型数据,具有嵌入量高、失真度低的特点,能够抵抗旋转、平移、均匀缩放和顶点重排序攻击,适合于任意网格的三维模型信息隐藏。     

Towards locally and globally shape-aware reverse 3D modeling

The process of re-creating CAD models from actual physical parts, formally known as digital shape reconstruction (DSR) is an integral part of product development, especially in re-design. While, the majority of current methods used in DSR are surface-based, our overarching goal is to obtain direct parameterization of 3D meshes, by avoiding the actual segmentation of the mesh into different surfaces. As a first step towards reverse modeling physical parts, we extract (1) locally prominent cross-sections (PCS) from triangular meshes, and (2) organize and cluster them into sweep components, which form the basic building blocks of the re-created CAD model. In this paper, we introduce two new algorithms derived from Locally Linear Embedding (LLE) (Roweis and Sauk, 2000 [3]) and Affinity Propagation (AP) (Frey and Dueck, 2007 [4]) for organizing and clustering PCS. The LLE algorithm analyzes the cross-sections (PCS) using their geometric properties to build a global manifold in an embedded space. The AP algorithm, then clusters the local cross sections by propagating affinities among them in the embedded space to form different sweep components. We demonstrate the robustness and efficiency of the algorithms through many examples including actual laser-scanned (point cloud) mechanical parts.     

A fast algorithm for point-location in a finite element mesh

An algorithm for the point-location problem in 2D finite element meshes as a special case of plane straight-line graphs (PSLG) is presented. The element containing a given point P is determined combining a quadtree data structure to generate a quaternary search tree and a local search wave using adjacency information. The preprocessing construction of the search tree has a complexity ofO(n·log(n)) and requires only pointer swap operations. The query time to locate a start element for local search isO(log(n)) and the final point search by ‘point-in-polygon’ tests is independent of the total number of elements in the mesh and thus determined in constant time. Although the theoretical efficiency estimates are only given for quasi-uniform meshes, it is shown in numerical examples, that the algorithm performs equally well for meshes with extreme local refenement.     

Metamorphosis of arbitrary triangular meshes

Three-dimensional metamorphosis (or morphing) establishes a smooth transition from a source object to a target object. The primary issue in 3D metamorphosis is to establish surface correspondence between the source and target objects, by which each point on the surface of the source object maps to a point on the surface of the target object. Having established this correspondence, we can generate a smooth transition by interpolating corresponding points from the source to the target positions. We handle 3D geometric metamorphosis between two objects represented as triangular meshes. To improve the quality of 3D morphing between two triangular meshes, we particularly consider the following two issues: 1) metamorphosis of arbitrary meshes; 2) metamorphosis with user control. We can address the first issue using our recently proposed method based on harmonic mapping (T. Kanai et al., 1998). In that earlier work, we developed each of the two meshes (topologically equivalent to a disk and having geometrically complicated shapes), into a 2D unit circle by harmonic mapping. Combining those two embeddings produces surface correspondence between the two meshes. However, this method doesn't consider the second issue: how to let the user control surface correspondence. The article develops an effective method for 3D morphing between two arbitrary meshes of the same topology. We extend our previously proposed method to achieve user control of surface correspondence     

A fast registration algorithm of rock point cloud based on spherical projection and feature extraction

Point cloud registration is an essential step in the process of 3D reconstruction. In this paper, a fast registration algorithm of rock mass point cloud is proposed based on the improved iterative closest point (ICP) algorithm. In our proposed algorithm, the point cloud data of single station scanner is transformed into digital images by spherical polar coordinates, then image features are extracted and edge points are removed, the features used in this algorithm is scale-invariant feature transform (SIFT). By analyzing the corresponding relationship between digital images and 3D points, the 3D feature points are extracted, from which we can search for the two-way correspondence as candidates. After the false matches are eliminated by the exhaustive search method based on random sampling, the transformation is computed via the Levenberg-Marquardt-Iterative Closest Point (LM-ICP) algorithm. Experiments on real data of rock mass show that the proposed algorithm has the similar accuracy and better registration efficiency compared with the ICP algorithm and other algorithms.