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题名:
分布式虚拟环境中实时可见性问题的研究及其应用
作者: 刘列明
答辩日期: 1999
专业: 计算机应用技术
授予单位: 中国科学院软件研究所
授予地点: 中国科学院软件研究所
学位: 博士
关键词: 虚拟现实 ; 分布式虚拟现实 ; 可见性问题 ; 可见性预处理 ; 分布式交互仿真 ; 阴影生成
摘要: 信息技术是现代文明的技术基础,是开展科学研究和技术开发的重要支撑手段,是高技术中的关键技术。信息技术的发展,直接影响着社会生产力和综合国力的变化。近50年来,由于半导体、计算机和通信技术的飞速发展,数字化的信息已经渗透到了与人们生活密切相关的各个领域。然而,以纯数字化形式存在的信息并不符合人们习惯的可用感官接受的表示方式。长期以来,人们一直在致力于突破这种纯数字化的交互方式,以获得一种图文并茂、生动形象,使人有身临其境之感的和谐的人机交互环境。正是在这一需求的推动下,近二三十年来,虚拟现实技术迅速兴起,并取得了令人瞩目的成就,被认为是21世纪可能使社会发生巨大变化的几大技术之一。虚拟现实技术是一系列高新技术的汇集,涉及人工智能、计算机图形学、人机接口技术、多媒体技术、传感技术以及高度并行的实时计算技术等领域。分布式虚拟现实(Distributed Virtual Reality,DVR)技术将虚拟现实技术与网络技术相结合,构造出基于网络的虚拟环境。在这个环境中,位于不同物理位置的多个用户或多个虚拟环境通过网络相联结,并共享信息。虚拟现实系统运行在分布式环境下有两方面的原因,一方面是充分利用分布式计算机系统提供的强大计算能力,另一方面是有些应用本身具有分布特性,如多人通过网络进行游戏和虚拟战争模拟等。实时性与真实感是提高人在虚拟环境中“身临其境”的沉浸感以及实现人机和谐的关键。本文讨论的实时可见性算法(包括相对于视点可见与相对于光源可见算法)正是针对这两个方面而提出。具体而言,可见性预处理算法是面向实时的要求;实时阴影算法在面向实时的同时,又面向真实感要求。因此,本文在内容上可分为两大部分:第一部分是基于建筑结构的可见性判断;第二部分是基于点光源的大规模动态场景中阴影的实时动态生成。归纳起来,本文的主要工作包括以下几个方面:1)在以房间等建筑结构为基础的虚拟环境中,景物空间被自然划分为许多个独立的区域,区域之间通过portal(墙上的门或窗)相连。在这样的虚拟环境中进行漫游时。位于某一区域中的观察者其视线必然被局限在这一区域中,而其它区域中的景物只有通过相互之间的portal才具有可见性。因此,在所有的景物多边形中,相对于观察者而言实际可见的只是一部分多边形。本文提出一种改进的基于房间结构的可见性预处理算法。其目的就是通过确定每一房间区域中的可见面超集并将其以portal结构为基础组织成链表结构,漫游的过程可被简化为在超集范围内精确选择可见面进行绘制,实现实时的要求,同时能方便地对portal对象进行实时的交互与管理。2)阴影的生成对增进虚拟场景的真实感以及提高观察者对虚拟场景的理解力与可交互性无疑都有着重要的意义。然而,实时阴影的生成一直是一个难于解决的问题,目前现有的算法普遍表现为生成速度较慢,尤其对于具有高度复 杂度的大规模分布式虚拟场景而言更是如此。速度问题成为阻碍阴影算法投入实时应用的首要瓶颈。本文提出了一种基于点光源的实时阴影生成算法。该算法可适用于较高复杂度的分布式虚拟场景。实验结果表明,该算法有较高的应用价值。3)本文的阴影生成算法以BSP树为基本数据结构。因此,BSP树本身的优化问题直接关系到阴影生成算法的效率。当选取的BSP树结构很差时。在其上运行的阴影算法的效率会大打折扣,耗时较多,从而影响到实时性。BSP树的优化问题是一个NP完备问题,故很难求出最优解。同时,最平衡与最少节点这两个条件也不能同时满足。本文在深入分析了阴影生成算法的效率的基础上,提出了一个基于贪婪算法的BSP树优化算法,该算法能在较少的时间内取得接近最优的BSP树解。该算法为实时阴影计算的高效运行奠定了坚实的基础。 4)BSP树结构在计算机图形学中的应用越来越广泛。但目前对于BSP树的分析更多地偏向于应用,缺少形式化的表述,以至于当需要对BSP树进行更精确的分析(如时间与空间效率等)时,缺少合适的理论基础。为了更好地对BSP树进行理论上的分析,本文对BSP树结构进行了形式化的定义,并以此为基础揭示了BSP树的许多内在性质,完成了对BSP树同构性的严格证明。该证明确保了本文提出的BSP树优化算法的正确性。这些定义与证明对深入了解BSP树的特性有着重要的意义,同时也为BSP树在其它领域中的应用提供了理论参考。5)在这些算法的基础上,作者设计并实现了几个分布式虚拟现实的原型系统。这几个系统涉及到虚拟教育、虚拟军事演练以及虚拟文物修复等领域,在整个虚拟现实的应用领域中具有充分的代表性,同时也能充分表现出分布式虚拟现实技术的应用与发展的方向。随着计算机硬件水平的不断提高,虚拟现实与分布式虚拟现实技术必将得到越来越广泛的应用,从而给人机交互乃至整个信息技术领域带来一场巨大的变革。本文希望能在这一方面有所推动。
英文摘要: Virtual Reality(VR) is a kind of simulation where a virtual world is created to simulate the real world we live in and human interaction with the objects in the virtual world could be conducted through multiple sensorial channels. It has been developed very quickly from its naissance and will be one of the primary technologies that can lead the society by great changes in the 21 st century. Distributed Virtual Reality(DVR) is an improvement of VR which combines Virtual Reality with Network. A virtual environment is then constructed based on the network, in which several users in different places can communicate and share the information by the virtual network-based environment. DVR is based on the virtual environment, so the great computing power of distributed computer system can be obtained, and the distributed applications can be realized such as the multi-user network game and virtual war simulation. The key of "immersion" in Virtual Reality is Real-Time and Realism. A good real-time VR system can react to people's behavior as soon as possible and realism can make produced image more lifelike and vivid. Both of them can make people feel in the real world while in the virtual environment. Realism is reflected by the complexity of the virtual environment and the natural relations between the virtual objects such as the shadows that show the occluding relations between objects. Visibility problems discussed in the paper is a key problem directed to the realization of both real-time and realism of a virtual environment. These two categories of visibility problems discussed in this paper are actually identical in essential. They are both the visibility problems among objects relative to some reference point(or line, polygon). So, the solutions for the two problems can be used as references of each other. In the thesis we give an in-depth study on the two categories of algorithms, and then proposed feasible real-time algorithms for both visibility preprocessing and shadow generation. The real-time visibility preprocessing algorithm presented in the thesis is based on the structure of building. When a building set with many rooms is built, all the models are loaded and rendered simultaneously in the environment. While a user is walking through a room, he/she can see all the models in the room, but those in other rooms may be invisible. Therefore, it is unnecessary to render the invisible objects in other rooms. To improve the rendering speed, a method of selective rendering is employed. Since the "room" is the main structure in our system, it is appropriate to adopt the visibility preprocessing based on the "room". Only the rooms visible from the viewpoint are rendered. The data structure is a link of portals organized with rooms, and with this structure people can walk through in the virtual environment and interact with the portals conveniently. Shadow generation plays an important role in realistic image synthesis but generation of shadows is quite time-consuming. In this regard, shadow generation for a dynamic environment in real-time rendering such as that for a virtual environment in VR applications turns to be a quite difficult task. In this paper, we present a new shadow generation algorithm directed to real-time rendering for a complex dynamic scene. The algorithm takes advantage of the fact that in many cases, only a small number of polygons change their shape relative to the lights and result in a corresponding change to the shadow calculation while most of the polygons remain static relative to the lights. So we are able to process the static and dynamic objects separately and thus allow the number of polygons to be calculated as less as possible. In order to improve the performance of shadow generation in our approach, a preprocessing is carried out for the static scene in the environment in terms of a Tiling method to determine precisely the shadow-relation sets. And when the real-time calculation is conducted, the relative static polygons that have occluding relations with the dynamic objects are merged with the dynamic ones. Then the merged BSP tree can be used to produce dynamic shadows by applying Shadow Volume BSP tree (SVBSP) method. The experimental results show that our approach is efficient. A method for the optimization of BSP trees based on the algorithm is also presented. In different applications, we have two choices to optimize the BSP trees. One is to get the minimum size, and the other is to get the maximum balance. Here in our algorithm, we will have to make the previous choice because although the balance is important for set operations and merging, in our application, the procedure of shadow producing takes most of the computation. And to produce the shadows, each node in the BSP tree should be processed once to construct shadow volumes. So, the number of nodes is crucial for the efficiency of the algorithm and the optimization in the paper dictates the minimum size of a BSP tree. An improved greedy algorithm is utilized in the algorithm to get an optimal BSP tree. The main idea is to use the merge of BSP tree to simplify the production of a tree. The experimental results show that we can get near optimal BSP tree(<2n), and the time complexity is significantly reduced. As an application of the study on the visibility problems, three prototypes of DVR system are introduced in the thesis. They are the distributed VR teaching environment, battlefield simulation based on DiS(distributed interactive simulation) and restoration of church St. Paulo. These three systems are representative for the applications of DVR. DVR proved useful in all these fields. A virtual teaching environment is presented in the thesis based on the distributed simulation with a goal of research on the implementation of VR in education. Implementation of VR in battle simulation can save a lot of expenses of ammunition, and the restoration of the famous ruins provide an attractive way for people to visit the past. The main technologies we utilized are introduced in the thesis. Being developed on the PC platform, these systems could be employed in many application fields.
语种: 中文
内容类型: 学位论文
URI标识: http://ir.iscas.ac.cn/handle/311060/7464
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Recommended Citation:
刘列明. 分布式虚拟环境中实时可见性问题的研究及其应用[D]. 中国科学院软件研究所. 中国科学院软件研究所. 1999-01-01.
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