基于细分的网格模型骨架驱动变形技术

针对传统骨架驱动变形方法中模型细节特征不能得到有效保持的问题,提出一种基于细分的骨架驱动网格模型变形方法。首先,对网格模型待变形区域基于截交线进行局部骨架提取和控制网格构建,分别建立骨架与控制网格以及控制网格所对应细分曲面与待变形模型区域之间的关联关系;然后,将基本函数作用下的自由变形方法应用于骨架变形,通过骨架变形驱动控制网格变形,将变形前后控制网格所对应细分曲面的变化信息转为网格模型泊松梯度场的改变;最后,根据改变后梯度场重建网格模型。实例表明,该变形方法针对不同网格模型均可以得到较好的编辑效果,且细节信息在变形后都得到了有效保持。与传统骨架驱动变形方法相比,该方法除具备交互操作简单直观的优势外,同时能够更好保持变形模型几何细节特征,更为适合具有丰富几何细节的复杂模型的变形编辑。     

骨架驱动的作物叶片变形模拟

针对作物器官的变形问题,将骨架驱动物体变形的方法应用于作物器官的局部变形,提出了一种骨架驱动的叶片变形方法：生成叶片骨架模型,驱动骨架模型发生变形,并根据变形后的叶片骨架将变形操作应用到叶片曲面上,进而实现叶片曲面变形。应用该方法,分别对小麦叶片曲面进行弯曲和扭曲变形模拟。实验结果表明,基于骨架驱动的作物叶片形变方法能灵活控制叶片弯曲和扭曲程度,从而获得自然的叶片曲面变形效果。     

参数曲面的有限元网格化

基于推进波前法,本文提出了一种针对三维Trimmed参数曲面的有限元网格剖分方法,首先对曲面参数空间进行剖分,利用结点密度函数（与曲率有关）和生成内部结点的公式,使网格单元和结点能同时生成,然后把网格反映射到曲面上,从而实现参数曲面的三角形网格剖分。     

利用ABF++保角参数化的网格曲面刀轨规划

针对基于网格曲面参数化的刀轨规划中存在映射拉伸变形的问题,提出一种引入映射拉伸系数的网格曲面刀轨规划方法.首先采用ABF++保角参数化算法将网格曲面展平到参数域内;然后根据映射变形分析逐三角片地计算映射拉伸系数与梯度,并基于此将三维网格上的轨迹参数转换到参数网格上;最后根据参数域内的轨迹参数分别生成往复式刀轨与环形刀轨.以人脸模型为例进行了仿真与加工实验,验证了该方法的有效性.     

基于弹簧-质点模型的不规则曲面纹理映射

针对三角网格表示的不规则曲面的纹理映射问题,提出一种基于弹簧-质点模型的简单、高效、失真小的纹理映射算法。结合调和映射的参数化方法以及弹簧-质点模型的复杂平面展开方法,保持拓扑关系地将三维曲面投影于平面内;通过建立三角网格表示的投影面的弹簧-质点模型,将不规则曲面参数化于给定大小的矩形域;利用参数化的结果计算不规则曲面各顶点的纹理坐标,进行纹理贴图。实验结果表明,该算法能够实现纹理高效、均匀、变形小地映射于任意不规则曲面上。     

基于旋转模型的植物叶子卷曲变形模拟

为实现植物叶子卷曲变形实时仿真,提出了一种基于叶脉骨架的旋转模型。基于叶片图像构建叶子的三维模型,并利用轮廓中轴提取算法生成叶子主脉,由主脉节点自动分生二级叶脉。在此基础上,将叶脉骨架旋转模型拓展到整个叶膜,即叶片三角网格顶点绕叶脉骨架节点向量逐步进行空间旋转,从而实现整个叶片模型的动态变形。元宝枫仿真实验结果表明,该方法模拟的叶子卷曲变形效果具有较强的真实感,且算法的运行速度基本达到了实时性的要求。     

采用变形模型的图像序列可视外壳计算方法

针对基于图像序列变形模型三维重建中至关重要的初始模型计算问题,提出一种基于图像序列计算可视外壳的方法.该方法基于Snake活动轮廓变形模型,将真实物体可视外壳的计算问题转化为初始曲面通过内外力作用驱动收敛于目标物体的问题.以真实物体图像序列中提取的物体轮廓为输入,根据轮廓信息和光滑度信息计算内力及轮廓力大小,驱动球体初始曲面变形收敛于可视外壳;在变形过程中加入删除短边、分裂长边、对角线翻转等网格优化操作,以避免发生网格错乱.实验结果表明,文中方法有效地克服了传统的基于体元素细分产生网格算法网格质量不高的缺点,且参数可调,易于实现,占用内存少,生成的曲面更加光滑,细节恢复效果理想.     

基于映射法和Delaunay方法的曲面三角网格划分算法

提出一种曲面三角网格划分算法，该算法在曲面参数域中生成Delaunay类型的网格，然后将其映射到空间曲面，为了抵消映射过程中变形的影响，采用空椭圆准则代替传统的空间准则，并给出椭圆的构造算法以及椭圆圆心的定位方法，这些方法充分考虑到映射变形和求解速度，实验结果表明，该算法生成满意的曲面网格，具有一定的应用价值。     

基于细分曲面的泊松网格编辑

针对具有丰富几何细节的三维网格模型,基于直接坐标操纵的传统编辑算法在编辑过程中不可避免地存在细节特征无法得到有效保持的问题.综合基于细分曲面的空间变形方法以及微分域网格编辑二者优势,提出一种基于细分曲面的泊松网格编辑方法.首先建立待变形网格模型的包围网格,以包围网格所决定的细分曲面构造变形控制曲面;然后根据用户变形意图操纵包围网格,将对应细分曲面变化信息转化为对网格模型泊松梯度场的改变;最后根据变化后的梯度场重建网格模型.文中方法交互简单、直观,具有多分辨率编辑的优势,可以有效地保持网格模型的细节特征.丰富的变形实例证明了该方法的有效性和可行性.     

用卷积曲面生成脸部皱纹的方法

将卷积曲面应用于人脸建模,提出了在个性化人脸模型上生成皱纹的新方法。运用基于图像的建模方法将通用人脸网格变形并计算出纹理图像,变形后的网格再与一个核函数卷积生成卷积曲面,通过轮廓函数调整曲面的形状而产生皱纹。将纹理图像映射到卷积曲面上使皱纹具有真实感。实验结果表明本文的方法可以通过控制轮廓函数的参数生成不同外观的皱纹。     

Bidirectional Evolutionary Structural Optimization (BESO) based design method for lattice structure to be fabricated by additive manufacturing

Unlike traditional manufacturing methods, additive manufacturing can produce parts with complex geometric structures without significant increases in fabrication time and cost. One application of additive manufacturing technologies is the fabrication of customized lattice-skin structures which can enhance performance of products while minimizing material or weight. In this paper, a novel design method for the creation of periodic lattice structures is proposed. In this method, Functional Volumes (FVs) and Functional Surfaces (FSs) are first determined based on an analysis of the functional requirements. FVs can be further decomposed into several sub-FVs. These sub-FVs can be divided into two types: FV with solid and FV with lattice. The initial design parameters of the lattice are selected based on the proposed guidelines. Based on these parameters, a kernel based lattice frame generation algorithm is used to generate lattice wireframes within the given FVs. At last, traditional bidirectional evolutionary structural optimization is modified to optimize distribution of lattice struts’ thickness. The design method proposed in this paper is validated through a case study, and provides an important foundation for the wide adoption of additive manufacturing technologies in the industry.     

基于黎曼度量参数曲面四边形网格生成方法及UG 实现

论文给出了基于黎曼度量的参数曲面网格生成的改进铺砖算法。阐述了曲 面自身的黎曼度量,并且运用黎曼度量计算二维参数域上单元节点的位置,从而使映射到三 维物理空间的四边形网格形状良好。文中对原有铺砖法相交处理进行了改进,在运用铺砖法 的同时调用UG-NX 强大的二次开发库函数获取相应的信息,直接在UG-NX 模型的表面生 成四边形网格。算例表明,该法能在曲面上生成质量好的网格。     

一种参数多项式曲面片的逐点生成算法

在计算机绘图中，一般来说，曲线实际上是由折线代替，而曲面实为小平面拼接而成，在使计算量降到最低的情况下画出真正的曲线方面，已有许多文章研究了曲线的逐点生成方法，并取得了一定的进展，但是尚无有效的快速逐点生成曲面的方法，为了快速逐点生成曲面，在建立多项式函数递推计算公式和算法的基础上，给出了一种逐点生成参数多项式曲面片的算法，由于此算法中只用到整数加法运算，且点数的适当选取可使计算量达到极小，因此是一种很有效的算法，该方法还可以加以改进，而用于有理函数，这无疑对有理曲线曲面（如NURBS曲线曲面）的快速生成以及对计算机图形学的其他一些领域都是有意义的。     

一种基于逻辑栅格的网格自动生成算法

针对基于栅格法对表面变动较大的模型进行网格划分适应性差的问题,提出一种基于逻辑栅格的六面体网格自动生成算法。该算法对基于栅格法的思想进行改进,避开一次生成模型初始栅格,将模型离散为一系列截面,对相邻截面采用矢量求交方法形成一对截面的初始栅格,修正初始栅格形成两截面之间的一层网格,形成整体模型。采用改进的栅格法进行复杂地形拱坝坝肩块体网格自动生成,取得较好的效果,结果证明改进的栅格法对复杂地形适应性强,模型表面变动较大时能自动进行网格退化。     

Self-supporting rhombic infill structures for additive manufacturing

Recent work has demonstrated that the interior material layout of a 3D model can be designed to make a fabricated replica satisfy application-specific demands on its physical properties, such as resistance to external loads. A widely used practice to fabricate such models is by layer-based additive manufacturing (AM), which however suffers from the problem of adding and removing interior supporting structures. In this paper, we present a novel method for generating application-specific infill structures on rhombic cells so that the resultant structures can automatically satisfy manufacturing requirements on overhang-angle and wall-thickness. Additional supporting structures can be avoided entirely in our framework. To achieve this, we introduce the usage of an adaptive rhombic grid, which is built from an input surface model. Starting from the initial sparse set of rhombic cells, via numerical optimization techniques an objective function can be improved by adaptively subdividing the rhombic grid and thus adding more walls in cells. We demonstrate the effectiveness of our method for generating interior designs in the applications of improving mechanical stiffness and static stability.     

Lattice structure lightweight triangulation for additive manufacturing

Additive manufacturing offers new available categories of geometries to be built. Among those categories, one can find the well developing field of lattice structures. Attention has been paid on lattice structures for their lightweight and mechanical efficiency ratio, thus leading to more optimized mechanical parts for systems. However this lightness only holds true from a mass related point of view. The files sent to additive manufacturing machines are quite large and can go up to such sizes that machines can freeze and get into malfunction. This is directly related to the lattice structures tendency to be of a high geometric complexity. A large number of vertices and triangles are necessary to describe them geometrically, thus leading to larger file sizes. With the increasing use of lattice structures, the need for their files to be lighter is also rising. This paper aims at proposing a method for tessellating a certain category of such structures, using topologic and geometric criteria to generate as few as possible triangles, thus leading to lightweight files. The triangulation technique is driven by a chordal error that controls the deviation between the exact and tessellated structures. It uses interpolation, boolean as well as triangulation operators. The method is illustrated and discussed through examples from our prototype software.     

Osteochondral Integrated Scaffolds with Gradient Structure by 3D Printing Forming

Recently in the area of biological manufacturing and rapid prototyping manufacturing, the bone scaffolds based on the additive manufacturing in repairing bone defects have been paid more and more attention. In the process of preparation, path planning directly affects the structure, performance as well as the final bone cell culture conditions. Due to the special natural bone scaffold structural characteristic, the traditional rapid prototyping(RP) path planning is not fully suitable for the preparation of bone scaffolds. In this paper, based on the 3D printing extrusion forming technology, a method of path planning for osteochondral integrated scaffolds with gradient structure is put forward, which provides a theoretical basis for bone-scaffold modeling and practical preparation. The implementation of the path planning processing system makes it possible to process data automatically from the initial stereo lithography(STL) model of the actual bone defect part by computer X-ray tomography technique(CT) scan or modeling,to generate the path code and to generate the final machining information after post-processing. This work provides some guidelines for independent research and development of automation equipment for biological manufacturing preparation and software technology.The experiment and test results have verified the validity of the path planning method and the good properties of the bone scaffolds with gradient structures.     

基于耦合锯齿混沌映射的虚拟光学加密系统

文中结合时空混沌系统具有初值和控制参数的高度敏感性,并能产生大量非相关的伪随机序列的特性,提出了一种基于耦合锯齿混沌映射生成虚拟光学加密系统的随机模板的方法。该方法基于交叉耦合映像格子模型,局部状态演化方程采用锯齿混沌映射,构成时空混沌系统。实验结果表明,当耦合系数ε的取值小于0．01时,基于交叉耦合映像格子模型的锯齿混沌系统可以生成具有均匀分配的混沌序列。经Matlab仿真实验证明,该加密系统密钥的灵敏度高,在一定的密钥长度下可以抵抗暴力破解。     

Robust uniform triangulation algorithm for computer aided design

This paper presents a new robust uniform triangulation algorithm that can be used in CAD/CAM systems to generate and visualize geometry of 3D models. Typically, in CAD/CAM systems 3D geometry consists of 3D surfaces presented by the parametric equations (e.g. surface of revolution, NURBS surfaces) which are defined on a two dimensional domain. Conventional triangulation algorithms (e.g. ear clipping, Voronoi-Delaunay triangulation) do not provide desired quality and high level of accuracy (challenging tasks) for 3D geometry. The approach developed in this paper combines lattice tessellation and conventional triangulation techniques and allows CAD/CAM systems to obtain the required surface quality and accuracy. The algorithm uses a Cartesian lattice to divide the parametric domain into adjacent rectangular cells. These cells are used to generate polygons that are further triangulated to obtain accurate surface representation. The algorithm allows users to control the triangle distribution intensity by adjusting the lattice density. Once triangulated, the 3D model can be used not only for rendering but also in various manufacturing and design applications. The approach presented in this paper can be used to triangulate any parametric surface given in S(u,v) form, e.g. NURBS surfaces, surfaces of revolution, and produces good quality triangulation which can be used in CAD/CAM and computer graphics applications.     

A robust efficient tracing scheme for triangulating trimmed parametric surfaces

An efficient, robust parametric trimmed surface triangulation method is presented. Efficiency is gained during trimmed curve tracing by minimising the number of cells processed. Key feature is the efficient tracing algorithm, and knowledge of orientation of the trimming curves is not required. The method is applicable to NURBS surfaces and operates on the untrimmed surface, constructing a rectangular parametric grid onto which the trimming curves are traced. This approach also minimises the occurrence of degenerate triangles and copes with holes independently of the grid size.