Mesh smoothing method based on local wave analysis

As the mesh models usually contain noise data,it is necessary to eliminate the noises and smooth the mesh.But existed methods always lose geometric features during the smoothing process.Hence,the noise is considered as a kind of random signal with high frequency,and then the mesh model smoothing is operated with signal processing theory.Local wave analysis is used to deal with geometric signal,and then a novel mesh smoothing method based on the local wave is proposed.The proposed method includes following steps:Firstly,analyze the principle of local wave decomposition for 1D signal,and expand it to 2D signal and 3D spherical surface signal processing;Secondly,map the mesh to the spherical surface with parameterization,resample the spherical mesh and decompose the spherical signals by local wave analysis;Thirdly,propose the coordinate smoothing and radical radius smoothing methods,the former filters the mesh points’ coordinates by local wave,and the latter filters the radical radius from their geometric center to mesh points by local wave;Finally,remove the high-frequency component of spherical signal,and obtain the smooth mesh model with inversely mapping from the spherical signal.Several mesh models with Gaussian noise are processed by local wave based method and other compared methods.The results show that local wave based method can obtain better smoothing performance,and reserve more original geometric features at the same time.     

Shape distribution-based approach to comparing 3D CAD assembly models

As a key technology for three-dimensional (3D) model retrieval from model databases, the ability to perform comparisons of 3D models is important. In particular, during the product design phase, 3D CAD models should be robustly retrieved for design reuse or machining process planning. However, typical retrieval methods that are based on model names, codes, or annotations are limited in terms of their usability and robustness. Therefore, there are increasing requirements for shape-based 3D model retrieval techniques, as well as the ability to perform 3D model comparisons. In this paper, we propose a shape distribution-based 3D CAD assembly comparison method that has the ability to identify dissimilarities in 3D CAD assembly models by differentiating between assembly relationships and part-shape dissimilarities. This is different from existing methods, which are limited to only shape dissimilarities. We present experimental results for various test cases by comparing our proposed method and existing methods. Based on our experiment results, we found that our method can enable a comprehensive comparison of 3D CAD assembly models including assembly relationship dissimilarities, part-shape dissimilarities, and overall model dissimilarities.     

参数化模型到直接建模的模型转换方法

参数化建模和直接建模方法是当前主流的两种三维几何建模方法,两者在功能上有较强的互补性。为了实现两种模型之间的转换,更好地体现设计者的设计意图,提高设计效率,提出了基于曲面重构的转换方法。通过对参数化模型表面点集做Delaunay三角剖分,求解出模型表面几何方程,拟合重构出直接建模模型,并以包含特征信息的三角网格单元的跟踪标记和数据更新来实现直接建模操作。实验表明,该转换方法适用于大多数特征模型的转换,且具有较高的转换效率。     

Mathematical modeling for the design of a generic custom-engineered form mill

This paper presents a mathematical model for the geometric design of a custom-engineered form milling (CEFM) cutter and develops an accurate 3D surface-based generic definition of the form mill. The proposed geometric definition of the CEFM cutter is developed in terms of biparametric surface patches using newly defined 3D rotational angles rather than the conventional 2D angles. The non-uniform rational B-spline curve fairing and sweep surface approaches are used to design the cutting edge and flank surface, respectively, of the cutter. To validate the methodology, an interface is developed that converts the proposed 3D parametric definition of the cutter into an intermediate neutral CAD format and then renders the cutter model in any CAD modeling environment. The accuracy of the mathematical model is verified by evaluating the deviation plot between the surfaces of the proposed cutter model and the surfaces developed using digitized data of an existing actual CEFM cutter. The satisfactory comparison verifies the shape design methodology for custom cutters presented in this paper. The method described here offers a simple and intuitive way of generating shape design of custom cutters for possible use in machining process simulations, finite element analysis, and other applications.     

基于LCD和改进棋盘格模板的摄像机标定

为了克服传统棋盘格模板标定点检测算法对于遮挡、视角、照明和畸变的局限性,提出一种利用基准点标记代替传统棋盘格黑白方块的改进模板,同时提出模板标定点的相应全自动识别算法.为了提高标定模板的质量,标定模板通过LCD(liquid crystal display)显示.因为LCD具有极高的几何精度和纯平面性,从而提高标定精度.传统摄像机标定同时计算针孔模型和透镜畸变模型,从而2种模型耦合在一起,所得到的标定结果仅对训练数据是有效的,而算法对于新数据的标定误差增加.为了克服2种模型的耦合,提出一种分离地标定2种模型的方法.算法利用模板的射影不变量约束,即交比和直线的射影为直线,求解带有畸变的标定点的校正的坐标,然后线性地求解2种模型.实验结果表明算法稳定且精度高.     

面向广义优化的参数化设计方法研究

提出一种将广义优化模块与参数化设计集成的方法及其集成策略，详细探讨了产品广义优化模型和参数化模型的关系协调、变量映射构建、几何约束处理以及自动再生算法，借助于Pro／E参数化软件强大的干涉检验和三维可视化功能，实现了产品全过程和全性能的广义优化及优化结果的可视化。提出的方法在液压挖掘机设计中得到了验证。     

形貌视觉测量中立体拼接靶标的设计及应用

设计了一种正六棱柱形状的立体拼接靶标,以靶标侧面6个棋盘格的角点作为全局控制点。基于近景摄影测量技术,建立立体靶标的6个单元模型,通过计算模型内摄站间的相对位姿,推导出棋盘格角点在所属单元模型的局部坐标。以公共棋盘格为中介,确立相邻单元模型的坐标系转换关系。建立靶标的全局坐标系于1号棋盘格,推导该棋盘格平面与其像平面间的单应性矩阵,从而确立全局坐标系和1号棋盘格所处单元模型的坐标系的转换关系。依次递推实现全局坐标系和每个单元模型坐标系的转换,进而计算出全部靶标角点的全局坐标,再经光束平差算法获取精确值。以玻璃表面棋盘格的角点间距作为评价指标,拼接精度优于0.15 mm/m。基于立体拼接靶标的拼接试验表明,实体模型表面4个子区域的局部点云可被精确地拼接成整体点云。与基于全局控制点和平面靶标的拼接方法相比,本方法亦具有更高的拼接精度。     

EMD端点效应抑制的一种实用方法

根据工程上常见的非平稳随机信号可预测性差、波形不规则等特点,在研究和总结现有端点延拓方法的基础上,提出了一种实用的端点效应抑制方法——极值平移法。该方法以信号端点及其附近极值的特征作为延拓依据,分别在信号两端各添加2个极大值点和2个极小值点,以此达到抑制端点发散的目的。仿真信号和实测响应的分解结果表明,该方法计算效率高,可有效减轻端点效应对经验模态分解(EMD)结果的影响。     

网格曲面上测地B样条曲线交互操作与重用

针对现有曲面上自由曲线设计重用方法的不足,提出一种流形网格曲面上曲线几何变换方法,达到曲线重用与再设计的目的。网格曲面上的曲线用测地B样条表示,具有与欧氏空间中传统B样条相一致的明确数学模型;引入对数映射理论将给定的源曲线控制顶点映射到切空间,获得它们的法坐标,按照曲线迁移前后控制顶点法坐标保持不变的原则,建立曲线迁移前后控制顶点的对应关系,实现类似于欧氏空间中的平移、旋转和缩放等几何变换。以网格曲面上离散对数映射理论为基础,将欧氏空间中的对称定义拓展到曲面空间,提出曲面上曲线的广义镜像概念并给出具体的算法实现。法坐标很好地保持了控制顶点之间的测地距离和相对位置关系,因而也保证了曲线迁移重用过程中的形状保持性。试验结果表明,所介绍方法健壮、有效,能满足曲面上曲线的交互设计要求。     

Complete 3D surface reconstruction from unstructured point cloud

In this study, a complete 3D surface reconstruction method is proposed based on the concept that the vertices of surface model can be completely matched to the unstructured point cloud. In order to generate the initial mesh model from the point cloud, the mesh subdivision of bounding box and shrink-wrapping algorithm are introduced. The control mesh model for well representing the topology of point cloud is derived from the initial mesh model by using the mesh simplification technique based on the original QEM algorithm, and the parametric surface model for approximately representing the geometry of point cloud is derived by applying the local subdivision surface fitting scheme on the control mesh model. And, to reconstruct the complete matching surface model, the insertion of isolated points on the parametric surface model and the mesh optimization are carried out. Especially, the fast 3D surface reconstruction is realized by introducing the voxel-based nearest-point search algorithm, and the simulation results reveal the availability of the proposed surface reconstruction method.     

A new parametric control method for freeform mesh models

Parametric modeling technology is difficult to apply to freeform mesh models since there is no efficient way to impose geometric constraints. In this paper, we propose a novel method to control freeform mesh models parametrically. Our approach is to construct a control mesh that surrounds an object model and then impose constraints on it. The control mesh is parametrically controlled and the shape of the object model is modified by using an existing freeform deformation method. This paper is mainly concerned with automated construction of a control mesh and treatment of geometric constraints. Procedures for creating a control mesh are as follows: 1) determine the optimal orientation of the model, 2) project the model along three axes and extract contours, 3) create 2D control polygons for the contours, and 4) construct the 3D control mesh from the 2D control polygons. Geometric constraints are imposed on the edges and faces of a control mesh. Types of constraints are given by either a relative relationship between elements or an absolute displacement. A new control mesh is calculated by solving these constraints and the original model is modified accordingly. We tested our algorithms for two freeform models.     

基于分层块状全局搜索的三维点云自动配准

提出了一种分层块状全局搜索到临近点局部搜索的改进迭代最近点(ICP)算法,用于进一步提高ICP算法的配准速度并消除点云缺失对点云配准的影响。该配准方法在粗略配准之后,以点云块为分层单元对模型点集进行选取,并对选取的少量模型点进行全局搜索获取其对应最近点;然后,以这些模型点对应的最近点作为搜索中心,在场景点集中进行局部搜索,获取这些模型点的大量临近点的对应最近点;最后,剔除错误对应最近点对,并求取坐标变换。与基于KD-Tree的ICP算法和基于LS+HS(Logarithmic Search Combined with Hierarchical Model Point Selection )的ICP算法相比,该配准算法对Happy bunny扫描数据的配准速度分别提高了78%和24%;对Dragon扫描数据的配准速度分别提高了73%和30%。这些结果表明该算法可以快速、精确地实现三维点云间的配准。     

基于形状特征的自由曲面建模技术

针对自由曲面重构建模中存在的问题,对曲面数字化及离散点参数优化技术进行了研究。基于自由曲面的形状特征,提出了一种曲面自适应数字化技术。建立了基于显式的最小二乘重构误差的参数优化模型,运用子空间共轭梯度内部反射牛顿优化技术实现了大规模数据点的参数优化。研究结果表明,自适应数字化技术大大提高了曲面的采样效率;参数优化模型合理且优化算法收敛迅速。     

基于CenSurE star特征的无人机景象匹配算法

针对传统局部不变特征的景象匹配算法冗余点多、实时性差、抗几何变换不突出的情况,提出基于CenSurE-star的无人机(UAV)景象匹配算法。首先采用Cen Sur E特征星型滤波器(CenSurE-star)提取基准图和实时图中的特征点,并生成FREAK二进制描述符;然后将汉明距离作为特征点的相似性判定度量,采用K近邻距离比值的方法提取匹配点对;最后利用基于RANSAC的定位模型得到空间几何变换关系,实现图像匹配并获取定位点经纬坐标。算法性能评价实验表明,本文算法不仅相对于SIFT、SURF、ORB算法,对各种变换具有更好的鲁棒性,而且相对于改进的SIFT、SURF算法处理时间有更大程度的缩短,算法定位误差在0.8个像素内,尺度误差在0.02倍内,旋转角度误差在0.04°内。基于算法进行外场飞行实验,实验证明算法定位精度较高,可以适应地貌信息较少的环境,并能满足无人机视觉辅助导航的需求。     

四轮独立驱动四轮转向电动试验样车的研制

研制出一辆四轮独立驱动四轮转向(4WD-4WS)电动试验样车,4个车轮分别由彼此独立的直流电机驱动,后轮转向机构由步进电机驱动跟随前轮转向。阐述了底盘总成、电气总成的结构及特点;采用等转矩、等功率两种模式分配四轮驱动力,进行了直线行驶与转弯行驶的基本试验。结果表明,研制的4WD-4WS电动样车设计结构可靠、性能良好,为系统地进行动力学性能试验,以及寻找最佳的驱动力分配模式和最佳的四轮转向模式提供依据。     

特征保持点云数据精简

由于三维扫描设备采集的点云数据庞大,本文提出了一种特征保持的点云精简方法以在减少冗余数据的同时更好地保持原始曲面的几何特征。首先,利用K均值聚类法在空间域对点云全局聚类,对点云构建K-d树并以K-d树的部分节点作为初始化聚类中心。然后,用主成分分析法估计点云法矢和候选特征点,遍历每个聚类,若类中包含特征点则将该类细分为多个子类,细分时将聚类映射到高斯球。最后,基于自适应均值漂移法对高斯球上的数据进行分类,高斯球上的聚类结果对应为空间聚类细分结果,各聚类中心的集合为精简结果。以多个实物模型为例验证了算法的有效性。结果表明,本文方法精简的点云在平坦区域保留少数点,在高曲率区域保留更多的点。相比于非均匀网格、层次聚类、K均值点云精简法,该方法对包含尖锐特征的曲面精简误差最小,更好地保留了原始曲面的几何特征。     

估计多视点摄像机姿态的两步法

本文提出一种估计多个视点位置上的摄像机姿态的新算法,即两步估计法。该算法首先采用第一、二两幅图像上的特征匹配点重建出物体的局部模型,然后以局部模型作为世界坐标系的中心,利用物体模型上的空间点与图像特征点的对应关系,求解每个视点位置上的摄像机姿态。真实图像的实验结果表明,本文所提算法是有效的,重建模型上特征点的重投影误差小于1个象素。     

基于点搜索的多曲面体清根轨迹生成方法

针对多曲面体清根加工轨迹的生成问题，建立了基于搜索初始点在参数域和物理域内搜索刀心点的点搜索算法。在搜索过程中，以多曲面体为研究对象，根据其结构特点确定搜索初始点和搜索中心，将参数域内搜索初始点与搜索中心的连线作为搜索方向在参数域内进行搜索。在物理域内生成参数域内对应搜索点的偏置点，判断偏置点到达多曲面体的距离是否满足距离条件，若满足距离条件则为刀心轨迹上的点。算例与实际加工结果表明，所提出的搜索算法搜索过程稳定，可以快速生成高质量的清根加工轨迹。