基于特征保持的三维模型四面体化及优化

三维模型四面体化是一种重要的有限元网格生成技术.介绍了一种特征保持的四面体网格生成及优化算法.首先使用三维模型主成分分析进行预处理,然后用体心立方构建初始四面体,接着通过拉普拉斯坐标改变模型边界切点的移动方式保持模型的局部特征,最后构造改进的密度能量误差函数优化四面体网格质量.实验结果表明,该方法可行、有效,且能很好地保持模型特征.     

语义特征模型网格重划的研究*

本文提出一个新的方法,基于原模型的网格来有效构造一个新网格。该技术利用语义特征模型具有操作局部化的特点,通过细胞元变化模型得出 (修改前/后)两个特征模型的相关部分,将原模型的复制网格节点直接关联到修正模型中,并结合对自由节点扩大自由集来进行优化过程。利用这种方法,可以在语义特征模型的几何属性中找到其固有的一致性,并且与修改前模型的网格相关联,最终在保持现有有限元的质量条件下快速产生完整质量网格。     

基于空间散列法的虚拟植物碰撞检测算法

根据虚拟植物模型复杂的特点,采用在精细的三角形网格模型上所创建的较粗糙的四面体网格模型,在虚拟植物模型上实现基于空间散列法的碰撞和自碰撞检测.该算法具有高效快速、无需复杂的数据结构等特点.实验结果表明,该算法能够有效地解决群体中植株的碰撞和自碰撞问题,并能够达到实时的处理要求.可同时适用于其它复杂的三维模型的碰撞检测问题.     

大规模网格模型间的快速视觉布尔运算

为了解决产品设计阶段中大规模网格模型间的布尔运算无法实现立等可得的速度瓶颈,提出了一种新算法。该算法利用离散化采样获得射线段点云模型,将三角面片间的3D布尔运算转换为射线段间的1D布尔运算,对相交处的交点进行高精度的求解和插值处理,使得布尔运算速度大为提高,从而大大提升复杂拓扑结构的产品设计效率。通过该算法所获得射线段点云模型可获得等同于基于三角网格的渲染效果,该方法可进行工程应用。     

大型三维几何模型在分布式协同设计中实时传输的关键技术

针对网络环境和分布式协同设计的实时性要求，在充分对比CAD模型和三角网格模型特性的基础上，提出了基于三角网格模型进行分布式协同设计，并讨论大型网格模型实时传输的关键技术，以有效的多分辨率模型为基础，在客户机／服务器环境下通过增量传输、选精化和特征映射，实现客户端驱动的产品3D模型的远程实时动态观察和特征参数的在线讨论与修改，系统可跨平台支持多个异地设计节点协同。     

四面体胞的尽可能刚性网格变形算法

提出一种基于四面体胞的尽可能刚性三角形网格变形算法。用户通过操作网格上的若干顶点以得到所需的模型变形结果。首先,算法对网格模型内部进行稀疏四面体化,以产生一个贴合模型表面的四面体胞集。在模型变形过程中,算法通过最小化相应的变形能量函数,以保持网格模型表面局部区域的刚性以及每个四面体胞的刚性,从而有效避免模型表面及其内部的扭曲。同时,针对大尺度编辑可能造成的模型局部塌陷,提出一种简单的四面体胞自适应剖分方法,根据模型局部体积的剧烈变化,自动剖分对应的四面体胞以增加模型内部的局部变形自由度,进而消除不正确的变形效果。此外,自适应的四面体胞剖分允许算法在初始时只需对网格模型进行稀疏的四面体化,而在变形过程中根据需要进一步提高四面体胞的局部稠密度,因而保证了算法的鲁棒性及其效率。实验结果表明,该变形算法可以有效保持模型的表面细节以及模型的内部体积,并能够有效避免模型形状在大尺度变形时的局部退化。     

基于虚拟原型的概念设计描述模型V-desModel

传统的概念设计方法由于缺乏真实感的交互手段,难以直观表达设计者的意图.基于虚拟原型的概念设计不仅能为设计者提供逼真的虚拟设计环境,而且充分体现了现代设计的成本低、周期短以及灵活性强等特点.针对概念设计的特点以及虚拟原型的特征分类,提出了基于虚拟原型的概念设计模型V-desModel,其核心是利用产品视图模型描述设计对象,将虚拟特征概念融入视图模型中,并采用可扩展"三维实体-约束图"来描述设计对象之间的约束关系.V-desModel模型能有效地支持基于虚拟原型的概念设计过程,较好地解决了概念设计中产品虚拟原     

Facial hexahedral mesh transferring by volumetric mapping based on harmonic fields

Hexahedral mesh has obvious mechanical advantages over tetrahedral mesh, but it is no trivial task to generate hexahedral mesh for complex object shapes such as individual faces. This paper presents a novel method to generate patient-specific hexahedral meshes of facial soft tissue models, based on a volumetric cross-parameterization mapping from a standard hexahedral mesh to the individual model. The volumetric parameterization is constructed based on triple of the volumetric harmonic fields, which are adapted to be as close to mutually orthogonal as possible, to achieve some quasi-conformal effect. In addition, some piecewise constraints on the harmonic fields are added to ensure anatomical feature correspondence. Experimental results show that our approach works efficiently for facial soft tissue modeling, avoids element flipping and preserves mesh element angles to a significant extent.     

Feature-based modeling for automatic mesh generation

Automatic meshing algorithms for finite element analysis are based on a computer understanding of the geometry of the part to be discretized. Current mesh generators understand the part as either a boundary representation, an octree, or a point set. A higher-level understanding of the part can be achieved by associating engineering significance and engineering data, such as loading and boundary conditions, with generic shapes in the part. This technique, called feature-based modeling, is a popular approach to integrating computer-aided design (CAD) and computer-aided manufacturing through the use of machinable shapes in the CAD model. It would seem that feature-based design also could aid in the finite element mesh generation process by making engineering information explicit in the model.This paper describes an approach to feature-based mesh generation. The feature representation of a fully functioning feature-based system that does automatic process planning and inspection was extended to include finite element mesh generation. This approach is based on a single feature representation that can be used for design, finite element analysis, process planning, and inspection of prismatic parts. The paper describes several advantages that features provide to the meshing process, such as improved point sets and a convenient method of simplifying the geometry of the model. Also discussed are possible extensions to features to enhance the finite element meshing process.     

Assessing the maintainability of software product line feature models using structural metrics

A software product line is a unified representation of a set of conceptually similar software systems that share many common features and satisfy the requirements of a particular domain. Within the context of software product lines, feature models are tree-like structures that are widely used for modeling and representing the inherent commonality and variability of software product lines. Given the fact that many different software systems can be spawned from a single software product line, it can be anticipated that a low-quality design can ripple through to many spawned software systems. Therefore, the need for early indicators of external quality attributes is recognized in order to avoid the implications of defective and low-quality design during the late stages of production. In this paper, we propose a set of structural metrics for software product line feature models and theoretically validate them using valid measurement-theoretic principles. Further, we investigate through controlled experimentation whether these structural metrics can be good predictors (early indicators) of the three main subcharacteristics of maintainability: analyzability, changeability, and understandability. More specifically, a four-step analysis is conducted: (1) investigating whether feature model structural metrics are correlated with feature model maintainability through the employment of classical statistical correlation techniques; (2) understanding how well each of the structural metrics can serve as discriminatory references for maintainability; (3) identifying the sufficient set of structural metrics for evaluating each of the subcharacteristics of maintainability; and (4) evaluating how well different prediction models based on the proposed structural metrics can perform in indicating the maintainability of a feature model. Results obtained from the controlled experiment support the idea that useful prediction models can be built for the purpose of evaluating feature model maintainability using early structural metrics. Some of the structural metrics show significant correlation with the subjective perception of the subjects about the maintainability of the feature models.     

An automatic 3D mesh generation method for domains with multiple materials

This paper describes an automatic and efficient approach to construct unstructured tetrahedral and hexahedral meshes for a composite domain made up of heterogeneous materials. The boundaries of these material regions form non-manifold surfaces. In earlier papers, we developed an octree-based isocontouring method to construct unstructured 3D meshes for a single material (homogeneous) domain with manifold boundary. In this paper, we introduce the notion of a material change edge and use it to identify the interface between two or several different materials. A novel method to calculate the minimizer point for a cell shared by more than two materials is provided, which forms a non-manifold node on the boundary. We then mesh all the material regions simultaneously and automatically while conforming to their boundaries directly from volumetric data. Both material change edges and interior edges are analyzed to construct tetrahedral meshes, and interior grid points are analyzed for proper hexahedral mesh construction. Finally, edge-contraction and smoothing methods are used to improve the quality of tetrahedral meshes, and a combination of pillowing, geometric flow and optimization techniques is used for hexahedral mesh quality improvement. The shrink set of pillowing schemes is defined automatically as the boundary of each material region. Several application results of our multi-material mesh generation method are also provided.     

网格曲面近似“最直路径”的快速计算

为实现网格模型两点间最短路径的近似逼近,提出一种网格曲面上近似"最直路径"的计算方法.利用起点与终点邻域特征获得初始切割面,对网格曲面上的三角片逐步切割求交,扩展"最直路径";在求交过程中逐步调整切割平面的方向,直至终点.实验结果表明,文中方法快速、有效.     

基于特征可变性建模教学服务管理系统的研究与实现

可变性建模是软件产品线领域研究热点,现阶段的研究局限在需求阶段,缺乏完整的理论体系,从领域工程到应用工程缺乏详细的指导。针对这些问题,本文改进特征模型构建方法和流程,使其支持软件全生命周期,加强各模型的映射关系,增强流程可操作性,保障模型间的一致性。最后,得到需求阶段和设计阶段的可变性模型,并成功应用到教学服务管理系统二次开发中。通过构件开发和效率比较,验证本文改进方法作为软件复用分支理论是实际可行的,并且能够提高开发效率。     

从表面重构的体数据实现三维网格剖分

针对有限元分析中网格最优化问题,本文提出一种改进的生成四面体网格的自组织算法。该算法首先应用几何方法将三角形表面模型重新构造成规定大小的分类体数据,同时由该表面模型建立平衡八叉树,计算用以控制网格尺寸的三维数组;然后将体数据转换成邻域内不同等值面的形态一致的边界指示数组;结合改进的自组织算法和相关三维数据的插值函数,达到生成四面体网格的目的。实验对比表明,该方法能够生成更高比例的优质四面体,同时很好地保证了边界的一致。在对封闭的三维表面网格进行有限元建模时,本文算法为其提供了一种有效、可靠的途径。     

基于二维轮廓序列的膝关节三维重建

提出了一个基于二维轮廓序列的四面体网格生成方法,用于医学图像三维几何模型重构.该方法首先对各选定的断层图像提取目标轮廓并做分支匹配等处理,然后生成各轮廓内部平面域的三角网格,最后在相邻断层之间根据三角网格连接四面体单元.该方法被应用于人体膝关节虚拟手术系统的三维几何建模,得到的膝部股骨模型包含494个节点和2 046个四面体单元,膝部脂肪模型包含2 854个节点和14011个四面体单元,这些模型被成功地应用于膝关节手术仿真,从而证明了该三维模型重建方法的可行性和有效性.     

CAD mesh model segmentation by clustering

CAD mesh models have been widely employed in current CAD/CAM systems, where it is quite useful to recognize the features of the CAD mesh models. The first step of feature recognition is to segment the CAD mesh model into meaningful parts. Although there are lots of mesh segmentation methods in literature, the majority of them are not suitable to CAD mesh models. In this paper, we design a mesh segmentation method based on clustering, dedicated to the CAD mesh model. Specifically, by the agglomerative clustering method, the given CAD mesh model is first clustered into the sparse and dense triangle regions. Furthermore, the sparse triangle region is separated into planar regions, cylindrical regions, and conical regions by the Gauss map of the triangular faces and Hough transformation; the dense triangle region is also segmented by the mean shift operation performed on the mean curvature field defined on the mesh faces. Lots of empirical results demonstrate the effectiveness and efficiency of the CAD mesh segmentation method in this paper.     

保特征的联合滤波网格去噪算法

目的 在去噪的过程中保持网格模型的特征结构是网格去噪领域研究的热点问题。为了能够在去噪中保持模型特征,本文提出一种基于变分形状近似（VSA）分割算法的保特征网格去噪算法。方法 引入变分形状近似分割算法分析并提取噪声网格模型的几何特征,分3步进行去噪。第1步使用变分形状近似算法对网格进行分割,对模型进行分块降噪预处理。第2步通过分析变分形状近似算法提取分割边界中的特征信息,将网格划分为特征区域与非特征区域。对两个区域用不同的滤波器联合滤波面法向量。第3步根据滤波后的面法向量,使用非迭代的网格顶点更新方法更新顶点位置。结果 相较于现有全局去噪方法,本文方法可以很好地保持网格模型的特征,引入的降噪预处理对于非均匀网格的拓扑结构保持有着很好的效果。通过对含有不同程度高斯噪声的网格模型进行实验表明,本文算法无论在直观上还是定量分析的结果都相较于对比的方法有着更好的去噪效果,实验中与对比算法相比去噪效果提升15%。结论 与现有的网格去噪算法对比,实验结果表明本文算法在中等高斯噪声下更加鲁棒,对常见模型有着比较好的去噪效果,能更好地处理不均匀采样的网格模型,恢复模型原有的特征信息和拓扑结构。     

青铜器纹饰特征增强及自动提取

目的 青铜器是我国的文化瑰宝,然而出土青铜器大多破损、变形,需要修复以进行保护。随着3维激光扫描技术及数字几何处理研究的发展,文物数字化修复技术得到了广泛的重视。在青铜器修复过程中需要将相邻碎片的纹饰对准,以保证纹饰的连续性,从而保证修复质量。因此,青铜器纹饰特征的有效提取是青铜器修复过程中的一项重要工作,鉴于青铜器纹饰特征一般具有比较明显的尖锐边,本文提出并实现了一种青铜器尖锐特征增强及自动提取算法。方法 首先,为了减少网格均匀度对特征提取的不利影响,提出一种加权法向距离;其次,为了增强尖锐特征提取效果,提出一种逆双边滤波算法,并利用该算法获得反锐化掩膜,增强法向距离间的差异性,使得大的更大,小的更小;最后,采用Otsu算法自动确定分割阈值,依据该阈值把网格顶点分为特征点集和非特征点集,实现青铜器纹饰特征的提取。结果 对实际3维激光扫描获得的青铜器模型,分别采用本文算法和Tran等人提出的尖锐特征自动提取算法进行了纹饰特征提取,包括采用两种算法进行了纹饰特征增强前后纹饰特征提取实验,本文使用的3个模型点数在6 000至80万之间,这些模型都可以在1 s到10 s之间得到最终的提取结果,具有较高的效率。同时,本文算法可以更为准确地提取尖锐特征点,且得到的特征点更为连续,有利于进一步的处理。结论 采用本文提出的青铜器纹饰提取算法,能够自动、高效地提取青铜器纹饰特征。     

三维实体仿真建模的网格自动生成方法

有限元网格模型的生成与几何拓扑特征和力学特性有直接关系。建立网格模型时，为了更真实地反映原几何形体的特征，在小特征尺寸或曲率较大等局部区域网格应加密剖分；为提高有限元分析精度和效率，在待分析的开口、裂纹、几何突变、外载、约束等具有应力集中力学特性的局部区域，网格应加密剖分。为此，该文提出了基于几何特征和物理特性相结合的网格自动生成方法。该方法既能有效地描述几何形体，又能实现应力集中区域的网格局部加密及粗细网格的均匀过渡。实例表明本方法实用性强、效果良好。     

同态滤波下RGB-D点云配准方法

目的 为了提高彩色物体配准的精度,针对3维点云颜色信息易受光照条件影响的问题,提出一种基于光照补偿的RGB-D（RGB Depth）点云配准方法。方法 引入同态滤波算法,并将模型对象的3维点云转化成线性点序列,从而对颜色信息进行光照补偿,以提高颜色信息的一致性;获取模型的颜色和几何特征并加权组合成混合特征,以此定义源点云的特征点,并运用K近邻算法搜索其对应点;用奇异值分解（SVD）得到配准的刚性变换矩阵。结果 进行传统的迭代最近点法（ICP）算法、深度信息与色调相结合的算法以及本文算法在不同的光照强度组合的模型配准对比实验,结果显示,在网面凹凸均匀的大卫模型上,配准时间及特征点匹配平均误差方面均约减少到对比方法的1/2;在网面光滑的barrel模型和网面凹凸不一致的阿基米德模型上,特征点匹配平均误差约分别减少到对比方法的1/6和1/8。此外,与Super 4-Points Congruent Set（Super 4PCS）、彩色点云配准算法在不同组合光照强度下进行对比实验,针对4种不同的网面结构模型,本文算法的SIFT特征点距离平均误差全距约减少到对比方法的1/5。结论 利用同态滤波算法抑制光照影响,提高了颜色信息的一致性,在一定效果上消除了光照强度不均匀对3维点云配准精度的干扰。