基于混合模型的虚拟装配关键技术研究

针对虚拟装配中采用多边形网格模型引起的组件定位困难和碰撞检测精度低等问题,提出一种采用精确几何模型及其对应的多边形网格模型的混合模型作为底层数据,支持精确装配的虚拟装配系统构造方法,并基于Parasolid和Vir-tools平台进行了实现。基于混合模型的碰撞检测能根据计算精度和时间要求自适应求解,在速度快的同时,满足高精度碰撞检测要求;基于混合模型的组件装配,采用离散采样点定义装配路径,通过装配几何约束交互式定义限制组件的运动范围,通过自由度规约、装配约束求解实现了零部件精确定位,通过四元数插值实现装配路径中零部件位姿的平滑过渡,能满足高精度的装配要求。该方法已经在开发的多个系统中得到验证和应用。     

基于运动对象局部场景截取的碰撞检测算法

为了解决虚拟心脏介入手术系统中的碰撞检测问题,提出了一种基于运动对象局部场景截取的碰撞检测算法.通过实时构建运动对象的局部场景截取体,可以获取该对象周围的局部场景数据,有效地降低了碰撞检测复杂性.并且它对虚拟场景中埘象的形变属性没有特殊要求,可适用于虚拟环境中包含可形变对象的情形.通过虚拟心脏介入手术中的碰撞检测实验表明,碰撞检测频率达到500Hz以上,并能精确给出碰撞点和发生碰撞的基本几何元素等碰撞信息,可以满足虚拟心脏介入手术对碰撞检测的实时性与精确性要求.     

结合轴对齐包围盒和空间划分的碰撞检测算法

目的 碰撞检测是虚拟现实,特别是虚拟装配中的关键技术。针对基于包围盒的碰撞检测算法的准确性和检测效率不足的问题,提出一种结合AABB轴对齐包围盒和空间划分的碰撞检测算法。方法 本文算法采用分步检测的方法,利用AABB算法来确定两包围盒的相交区域后,结合模型移动方向和运动趋势进行空间划分,利用碰撞检测的时空相关性,对时空相关的部分进行相交测试,通过将包围盒还原成三角面以及点的方式来保证检测的准确性。结果 本文算法与AABB层次包围盒二叉树算法、k-Dops包围盒算法以及BPS空间分割树算法进行对比实验分析。在碰撞的几何精度上,本文算法在大部分情况下与AABB算法和k-Dops算法的距离差超过阈值0.02,证明本文算法在碰撞几何精度上有明显的提高。在碰撞检测时耗上,随着碰撞检测难度的不断增加,本文算法在平移自由度下比AABB算法和BSP算法、在旋转自由度下比AABB算法和k-Dops算法的检测时间均降低了50%以上。在三角面数对算法碰撞检测时耗的影响上,当运动模型的三角面数较多时,本文算法表现出更高的稳定性。结论 结合AABB包围盒和空间划分方法的碰撞检测算法,在减少碰撞检测所需时间的同时提高了碰撞检测的准确性,可以满足虚拟装配技术中对碰撞检测算法准确性的要求,同时也能满足使用者实时性的交互习惯。     

面向虚拟装配的产品公差混合建模技术

为实现带公差的虚拟装配,应首先解决产品建模问题.通过分析虚拟装配对产品公差模型的要求,提出了虚拟环境下产品公差混合模型.该模型将产品基本公差信息与附加公差信息相结合,将普通几何拓扑信息与精确几何信息相关联,同时添加了装配特征等工程语义信息;详细阐述了产品公差混合模型的层次结构、模型信息来源、各个层次之间的映射方法及精度信息的表达方式;最后介绍了该模型的应用.     

虚拟维修中表面间碰撞检测问题研究

基于几何约束的虚拟样机中,表面间的碰撞检测是实现维修操作的必要条件.目前有一些公用的碰撞检测工具包,但这些碰撞检测工具包一般支持多面片模型,并没有利用CAD模型中的表面信息,无法确定发生碰撞的表面.该文针对虚拟维修操作的特点,提出了一种新的场景图数据结构.在该场景图下采用表面-三角面片映射和表面-对象映射,在通用的碰撞检测工具包的基础上实现表面间的碰撞检测.最后,描述了约束管理系统的结构,利用该系统可对碰撞检测系统中发生碰撞的表面进行约束识别和求解,从而有效地支持虚拟维修中的装配和拆卸操作.     

虚拟装配环境中碰撞检测算法的研究综述与展望

虚拟装配系统可对机电产品进行装配仿真,生成装配顺序与装配轨迹,而碰撞检测技术正是对装配顺序与装配轨迹的正确性进行验证。把虚拟装配环境的碰撞检测算法归类为:基于时间域的碰撞检测算法、基于几何空间的碰撞检测算法、基于图像空间的碰撞检测算法。对这几类算法的研究现状进行了综述,根据研究现状分析了碰撞检测算法中存在的问题及研究难点,并对碰撞检测算法的研究趋势进行了展望。     

基于CAD网络分布式虚拟装配环境实现

复杂装配体中零部件碰撞检测实时响应是虚拟装配中的瓶颈,针对以上不足,提出基于CAD网络分布式的虚拟装配环境。,在CAD服务器端的装配体中,构建包括自身的零件两两精确碰撞检测所需时间表,包容盒快速碰撞检测得到所需进行精确碰撞检测的零件对及其总时间,贪婪法快速分配零件对任务给各客户机;利用面向连接的TCP/IP套接字(Socket)协议进行数据传送,在CAD客户机端进行零部件碰撞检测计算,碰撞结果通过网络返回CAD主机,解决了碰撞检测的实时性响应问题。并在散货装船机装配过程仿真中得到成功应用。     

虚拟手术中基于可碰撞集的软组织自碰撞检测算法

快速精确的碰撞检测是保证手术仿真真实性的重要条件.由于虚拟手术仿真过程中的高复杂性和高计算性,使得现有的碰撞检测已经不能满足实时性与复杂性的要求,为此提出一种新的软组织自碰撞检测方法.该方法根据软组织的曲率提取可碰撞集,基本几何元素间的自碰撞仅在该可碰撞集中发生,基本几何元素相交检测采用基于特征检测的方法.通过试验表明,该方法不仅能有效地提高碰撞检测的速度,而且能对人体软组织进行相对精确的检测.     

一种基于半透明颜色叠加与深度值的碰撞检测算法

为了快速验证虚拟装配仿真过程中零部件的可装配性,改进了一种基于图像空间的快速碰撞检测算法。该算法首先通过半透明颜色叠加实现对无碰撞装配体的快速滤除,并确定出潜在的碰撞区域;然后利用像素深度值求出遮挡对象与待装配物体在其运动方向的最小分离距离,弥补了基于图像空间的碰撞检测算法只能判断物体间是否发生碰撞而不能求距的缺点;最后针对求距环节提出了一种像素区域划分策略,以提高算法的检测精度。测试结果表明,所提算法在整体上能够满足虚拟装配系统实时性和精确性的要求。     

基于OSP场景图结构的虚拟装配碰撞检测算法的研究与实现

快速、精确的碰撞检测对提高虚拟环境交互的真实性和增强虚拟环境的沉浸感有至关重要的作用,而虚拟装配环境的复杂性、实时性和精确性又对碰撞检测提出了更高的要求;为实现虚拟装配系统中交叉表面间的精确碰撞检测,提出一种基于OSP结构的虚拟现实碰撞检测方法;该结构第一层为对象节点,主要用于粗略的碰撞检测,第二层为表面节点,第三层为多面片节点,第二层和第三层主要用于精确的碰撞检测,采用层次的碰撞检测算法,以提高碰撞检测的实时性和精度;应用结果表明该方法能有效解决虚拟装配系统表面间的精确碰撞检测问题,满足系统实时交互的应用要求.     

Out-of-core real-time haptic interaction on very large models

In this paper we address the problem of fast inclusion tests and distance calculation in very large models, an important issue in the context of environments involving haptic interaction or collision detection. Unfortunately, existing haptic rendering or collision detection toolkits cannot handle polygonal models obtained from 3D digitized point clouds unless the models are simplified up to a few thousand polygons, which leads to an important lack of detail for the scanned pieces. We propose a data structure that is able to manage very large polygonal models (over 25M polygons), and we explain how this can be used in order to compute the inclusion of a point into the solid surface very efficiently, performing several thousand point-in-solid tests per second. Our method uses a data structure called EBP-Octree (Extended Bounding-Planes Octree), which is a very tight hierarchy of convex bounding volumes. Based on a spatial decomposition of the model using an octree, at each node it defines a bounding volume using a subset of the planes of the portion of the polygonal model contained at that node. We use the EBP-Octree in a haptic interaction environment, where distance tests and the orientation of collided triangles must be accurate and fast. We also demonstrate that the proposed algorithm largely meets the interactive query rate demanded by a haptic interaction (1 kHz), despite being executed in a single CPU thread on a commonly available computer.     

A collisions evaluation method in virtual environment for collaborativeassembly

Collision detection is critical for collaborative assembly simulation to assist design processing. However, collisions between polygonal models may not reflect collisions between real objects in reality because of polygonal approximation and designed tolerance. This problem reduces the reliability of simulation in collaborative assembly and sometimes even results in wrong conclusions. To solve the problem, we propose a novel collision evaluation algorithm based on generalized penetration depth, approximation information, and tolerance information. Given two interfered polygonal models, generalized penetration depth is calculated using relative motion information. Then two thresholds, which are based on approximation information and tolerance information, are integrated to evaluate collisions between polygonal models. In order to distinguish the status of collisions for further analysis, the collisions between polygonal models are categorized into three types by evaluation algorithm, namely real collision, potential collision, and fake collision. Computational efficiency and accuracy of the evaluation algorithm are verified in a virtual collaborative assembly environment.     

虚拟手术实时物体碰撞检测和软组织变形研究

针对虚拟手术系统中实时碰撞检测和软组织变形的问题进行了研究。对于实时碰撞检测,使用了包围盒检测结合三角面片检测的方法来检测物体之间的碰撞,并且找到相互碰撞的两个三角面片。对于软组织变形,使用了一种基于胡克定律的弹簧质点模型来虚拟仿真变形,并提供了控制点移动的方法。采用优化虚拟手术中的物体碰撞检测方法,减少了不必要的计算和增强了系统的精确度;简化了软组织变形的算法,相对加快了有限元模型运算速度。对推动虚拟手术的发展具有积极的意义和作用。     

Incremental algorithms for collision detection between polygonalmodels

Fast and accurate collision detection between general polygonal models is a fundamental problem in physically based and geometric modeling, robotics, animation, and computer-simulated environments. Most earlier collision detection algorithms are either restricted to a class of models (such as convex polytopes) or are not fast enough for practical applications. The authors present an incremental algorithm for collision detection between general polygonal models in dynamic environments. The algorithm combines a hierarchical representation with incremental computation to rapidly detect collisions. It makes use of coherence between successive instances to efficiently determine the number of object features interacting. For each pair of objects, it tracks the closest features between them on their respective convex hulls. It detects the objects' penetration using pseudo internal Voronoi cells and constructs the penetration region, thus identifying the regions of contact on the convex hulls. The features associated with these regions are represented in a precomputed hierarchy. The algorithm uses a coherence based approach to quickly traverse the precomputed hierarchy and check for possible collisions between the features. They highlight its performance on different applications     

Collision detection of a moving polygon in the presence ofpolygonal obstacles in the plane

This paper presents a new approach for the following collision detection problem in the plane: Let a simple polygon P rotate at a center &ogr; with constant angular velocity ω and translate towards a set of polygonal obstacles S with constant velocity ν. Given P and S as well as their initial positions, and given also the velocities of P, determine whether or not P will collide with any element of S and report the collided elements of S if collisions occurred. An O(mn) worst-case optimal algorithm is proposed to solve this problem, where n is the number of vertices of P and m is the number of vertices of the obstacles in S     

融合神经网络的布料碰撞检测算法

目的 针对当前在虚拟环境中布料柔体碰撞检测效率慢和准确性低的问题,提出一种根节点双层包围盒树结构和融合OpenNN （open neural networks library）神经网络加速预测碰撞检测的算法。方法 首先改进了碰撞检测常用的包围盒技术,提出根节点双层包围盒算法,减少包围盒的构造时间。其次使用神经网络优化碰撞检测技术,利用神经网络可以处理大量数据的优势,每次可以检测大量基本图元是否发生碰撞,解决了碰撞检测计算复杂性高的问题。最后准确地找到碰撞粒子并做出碰撞响应。结果 在相同的复杂布料模型情况下,根节点双层包围盒算法在运行速度上比传统混合包围盒算法快,耗时缩减了5.51%~11.32%。基于OpenNN算法的总耗时比根节点双层包围盒缩减了11.70%,比融合DNN （deep neural network）的自碰撞检测算法减少了6.62%。随着碰撞检测难度的增大,当布料模型的精度增加84%时,传统物理碰撞检测方法用时增加96%,融合DNN的自碰撞检测算法用时增加90.11%,而本文基于神经网络的算法用时仅增加了68.37%,同时表现出更高的稳定性,满足使用者对实时性的要求。结论 对于模拟场景中简单模型的碰撞,本文提出的根节点双层包围盒算法比传统的包围盒方法耗时短。对于复杂模型,基于OpenNN神经网络的碰撞检测算法在效率上优于传统的包围盒算法和融合DNN的自碰撞检查算法,而且模拟效果的准确性也得以保证,是一种高效的碰撞检测方法。     

基于点云模型的虚拟手术系统建模及碰撞检测

建模与碰撞检测算法是虚拟手术具有良好实时性的前提。通过CT得到病人身体患部的点云数据,再基于八叉树剖分算法,采用层次包围球结构,对手术部位的软组织及器官进行建模。为了提高碰撞检测的实时性,将手术器械物理模型简化为一个小球或一条直线与软组织交互,而几何模型保持不变,这样在不影响虚拟手术的视觉效果同时又提高了碰撞检测的速度。实验结果表明,算法能准确检测出虚拟手术器械与虚拟模型接触的点,而且碰撞检测实时性显著提高,简化后的平均碰撞检测时间仅为虚拟手术的快速碰撞检测算法中未简化的方法平均时间的10%。     

Collision Detection for Animation using Sphere-Trees

The detection of collisions between moving polyhedral objects is one of the most computationally intensive tasks in the computer animation process. The use of object-oriented techniques to encapsulate data within the objects' structures compounds this problem through the requirement for inter-object message passing in order to obtain geometric information for collision detection. The REALISM system decreases the time for collision detection by using a three stage process. The first stage identifies objects in the same locality using a global bounding volume table. The second stage locates regions of possible collision using a sphere-tree data structure (a hierarchical tree of spheres based on octree-type spatial subdivision). The final stage finds intersections between polygonal faces of the objects that are contained within the intersecting pairs of leaf nodes. Hence the algorithm uses a spherical geometry approximation rapidly to locate regions of potential collisions and then uses a local intersection test with actual object geometry information. The system is therefore fast and accurate. Tests for various geometric objects support this and show performance improvements of jive times over traditional polyhedral intersection tests.