摘要
虚拟地理信息系统(VR-GIS)技术是地理信息系统(GIS)技术和虚拟现实(VR)技术相结合的新技术,是GIS技术从二维向三维甚至多维方向发展,从静态向动态发展的必然结果,是建立“数字地球”、“数字城市”关键技术之一。本论文以虚拟地理信息系统各种关键技术为研究对象,以构建出集大规模场景虚拟仿真和全面详实的三维地理信息说明于一体的软件平台为研究目标,以解决海量的和多源异构的三维空间信息数据组织、管理、实时绘制及VR、GIS无缝集成问题为主要研究内容,系统地进行了基于插件技术的软件集成方法、基于IOCP面向数字城市的3D模型数据引擎、基于GPU的虚拟城市加速渲染技术、VR与GIS无缝集成方法的研究。并根据研究成果,进行了一套面向城市/区域级大场景、多精度等级虚拟现实应用的软件开发平台——城域景观VRGIS一体化仿真平台(简称VRGIS平台)软件的研发。论文的主要工作和获得的成果如下：
(1)VRGIS平台的插件式软件架构设计
采用插件式软件架构对数据管理、三维渲染和3DGIS分析等数据处理方法进行集成,可有效提高这些功能的复用能力和复用程度。针对VRGIS平台应用领域宽泛的特点,充分考虑到插件技术的高封装、可复用等优势,建立了基于层次插件架构的软件开发模型,也为虚拟地理信息系统开发人员提供了即插即用的构件,以供二次开发使用。
(2)基于IOCP面向虚拟城市的3D模型数据引擎
城市三维模型数据是VRGIS系统中重要的一个地理对象,是虚拟环境的三维空间基础。为解决视点漫游过程中数据载入不及时、数据组织需同时满足VR高效渲染与GIS分析需求的难点,本文继承了GIS软件对空间数据管理的思想,对城市的三维模型进行分层组织,对其特点分别予以分析。充分考虑了模型数据的LOD、纹理映射、精度分布不均匀等特点,创新性的提出了面向建筑物层的Precision LOD R-tree及面向地面、道路、绿化混合层的Hilbert网格索引结构,建立了城市模型数据的高效索引。基于该数据索引,利用多线程及IOCP机制实时调度三维模型数据,实现基于外存的VRGIS数据引擎。利用LRU算法,建立模型缓冲区,实现三维模型数据的无延迟载入。
(3)基于GPU的虚拟城市加速渲染
数字城市应用对三维场景高效渲染提出了新的要求。为了满足这些要求,在采用LOD、VBO、显示列表、视锥裁剪等优化算法的基础上,本论文探讨了基于GPU的遮挡剔除算法,提出了基于GPU的billboard模型渲染算法及基于GPU的标志性建筑渲染算法,实现了基于GPU的粒子系统渲染。通过综合运用以上加速算法优化三维渲染引擎,提高虚拟地理环境系统的实时绘制效率。
(4)VR与GIS无缝集成方法的研究
VR与GIS的无缝集成主要表现在数据共享和功能融合。该平台将VR数据与GIS数据统一抽象化,且继承各自的特征,对外提供统一的访问接口,并保持独自的渲染与访问模式,从而实现了VR与GIS数据的统一与共享。本文深入研讨了VR与GIS的各种结合点,以数字高程模型为基础,实现了真正的三维量算及三维矢量路径分析等3DGIS功能,并以日照系列为例,引入日照物理模型,探索了阴影分析,日照分析及基于VR场景的日照数据生成方法,在应用上具有一定的创新意义。
本论文对虚拟城市关键技术进行探索和研究,创新点如下：
1)虚拟城市海量三维模型数据引擎的构建。分析城市三维模型的结构特点,创新性的提出了Precision LOD R-tree,并与Hilbert网格索引结合,利用IOCP多线程调度机制实现了一种面向虚拟城市三维模型的高效数据引擎,解决平台对虚拟城市海量模型数据的承载问题。
2)基于GPU的Billboard群渲染与建筑体图像融合算法。本文利用GPU编程技术,实现了Billboard群的高效渲染,属于技术创新。此外,建筑体图像融合算法将场景渲染优化、信息提取、导航定位等做了有机结合,是GPU图像融合算法在虚拟城市仿真应用中一次有效的尝试。
3)虚拟城市中3DGIS的无缝集成与实现。本文将VR场景作为3DGIS分析的基础数据源,实现三维地形的真实距离测量,表面积测量及体积测量,实现了基于三维矢量的路径分析功能。另外,在VR场景中引入物理模型进行3DGIS分析,进一步推动了VR与3DGIS的无缝融合。
VR-GIS is a new technique that integrates GIS with VR. It's a result of development of GIS technology from two-dimensional to three- and even multi-dimensional, and from static to dynamic. As one of key technologies that are used to establish "Digital Earth", "Digital City". This dissertation takes kinds of key technologies of VR-GIS as the main research object and the main aim is to construct an integrated platform of VR navigation and GIS analysis for city/region simulation. To solve the problems of massive data organization, management, real-time visualization and seamless integration of VR and GIS, this paper carries on the research about software integration methods based on plug-in technology, and spatial data engine of 3D city models based on IOCP technology, GPU-based hardware acceleration technologies, the method of integration of VR and GIS. Based on the research achievement, VRGIS platform has been researched, which can satisfy the high requirements of landscape simulation and management for large cities and regions. The main contents and innovations in this dissertation are summarized such as:
(1) Plug-in software architecture of VRGIS platform
Using plug-in technology to integrate the data processing method, such as data management, three-dimensional rendering and 3DGIS analysis, can improve the capacity and degree of methods reuse. Because of such domain characteristic that VRGIS platform has many application fields, fully considering technology advantages of the high component encapsulation and reuse, multi-layers plug-in architecture software development model has been established, which can provide VR-GIS developers a plug-and-play component library.
(2) Digital city 3D models data engine based on IOCP
3D models are important geographic objects in VRGIS system and are the three-dimensional spatial basis of virtual environment. To solve the difficulties of data loading delay when viewpoint updates, data management should satisfy the needs of VR navigation and GIS analysis, this dissertation uses the spatial data management idea of GIS software, several levels of city models are explored and their respective features are analyzed. Fully considering such characteristics that LOD, texture mapping, precision asymmetry, the data organization methods of Precision LOD R-tree and Hilbert grid respectively for building layer and hybrid layer are designed. Based on this spatial data index, an out-of-core VRGIS data engine is developed using the technologies of multi-thread and IOCP. As a result, there-dimensional models are loaded with low delay by building a memory buffer with LRU algorithm.
(3) GPU-based virtual city rendering acceleration technologies
Higher rendering performance requirements of virtual scene are proposed for kinds of digital city applications. In order to satisfy these requirements, on the basis of traditional optimize technologies such as LOD, VBO, display list, frustum culling, this dissertation discusses an optimal hierarchy occlusion culling algorithm based on GPU, designs a billboard rendering algorithm and an indicative building rendering algorithm based on GPU, implements particle system rendering based on GPU. It is useful for the optimization of three-dimensional display engine and the increase of visualization efficiency by using these algorithms.
(4) Seamless integration of VR and GIS
Data sharing and function unification are the main goals of the integration of VR and GIS. By abstracting VR data and GIS data uniformly and inheriting their traits, the platform provides uniform external access interface and keeps nodes have their own render and access mode. In this way, unification and sharing of VR and GIS data are achieved. This dissertation discusses in-depth the integration points of VR and GIS, realizes 3DGIS functions including three-dimensional measurement and path analysis based on the rendering of DEM terrain, takes sunlight as an example, imports the model of virtual sunlight, explores shadow generation method, sunlight analysis and sunlight data generation method based on VR scene, which has creative sense for VRGIS application.
This dissertation takes the key technologies of virtual city as research objects and the main innovations are summarized as follows.
1) Massive virtual city 3D models data engine has been constructed. By analyzing features of city models, the novel data management methods of Precision LOD R-tree and Hilbert grid respectively for building layer and hybrid layer are designed. With IOCP technology, the 3D models data engine has been developed, which enable VRGIS platform can process massive spatial data.
2) Billboard cluster rendering algorithm and building image fusion algorithm based on GPU. This dissertation realizes massive billboard objects rendering by making use of GPU programming technology. Furthermore, the algorithm of building image fusion combines rendering optimization and virtual navigation, which is an effective try of GPU image fusion used to virtual city simulation.
3) Realization and seamless integration of 3DGIS in virtual city. This dissertation takes the virtual scene as data source of 3DGIS analysis, realizes the functions of real distance measurement, surface area measurement, volume measurement and path analysis based on 3D vector data. Furthermore, the physical model is built in the virtual scene, which promotes the integration of VR and GIS.
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