武汉大学虚拟现实实验室

>> 龚　俊，2006，虚拟建筑环境中的三维空间数据组织与管理关键技术研究

摘要

　　2006年2月16日的《自然杂志》刊登的“Virtual globes: The web-wide world”一文指出，既然真实世界是三维的，为什么科学探索不借助三维形式呢？由Google Earth虚拟地球引领的在线三维工具正使人们和空间信息的交互方式发生着深刻的变革。从World Wide Web到Web Wide World的变化反映了，人们同空间信息的交互方式正从没有反馈的单向信息服务朝在最接近现实的虚拟世界中进行互动式、协同式探索的方向发展。随着三维数据获取能力的日益增强以及三维GIS技术时代的到来，三维GIS技术在规划设计、建筑工程和城市管理等领域得到越来越多的应用，虚拟建筑环境(Virtual Building Environment, VBE)作为虚拟地理环境在建筑工程领域的特例也被广泛关注，将在楼房和基础设施等建筑物的整个生命周期包括规划、设计、施工、使用、维护到废弃等阶段中提供数据服务和分析应用，帮助人们在其中探索和发现新知识。值得特别提出的是，建筑文化遗产的三维数字化应用，已经成为虚拟建筑环境的研究热点和学术前沿。

　　相对于普通三维应用，虚拟建筑环境的数据规模大，数据类型丰富多样，场景的图形数据复杂度高，给数据管理和场景描绘带来了前所未有的困难，高复杂度的海量三维空间数据是阻碍应用的主要因素，三维空间数据组织与管理技术理所当然地成为虚拟建筑环境的核心技术。针对虚拟建筑环境的特殊性，本文从空间数据模型、空间数据组织、空间数据生产规划以及空间索引等关键问题入手，研究符合虚拟建筑环境应用的三维空间数据组织和管理方法。本文的研究内容和创新点包括：

针对虚拟建筑环境的核心问题――三维空间数据组织与管理，深入分析评价了三维空间数据管理模式、三维空间数据模型、三维空间数据组织方法以及三维空间索引等关键技术的研究进展、发展趋势和存在的关键问题。
针对虚拟建筑环境主要的数据源――多细节层次(LOD)真三维CAD模型，设计了一种实用的空间数据模型，在现有面向对象数据模型的基础上进一步淡化拓扑关系以简化模型，并扩展了CAD对象类型和LOD对象类型，CAD对象类型为集成CAD模型提供概念支持，弥补当前GIS三维建模能力的不足。将可见性裁减等实时渲染技术纳入空间数据模型框架中，确保数据模型能够高效地描述复杂建筑环境。
由于固有的CAD模型数据组织方法使得数据存储冗余，没有顾及高效可视化操作需要，因此成为虚拟建筑环境中集成应用CAD模型的瓶颈。本文提出了CAD模型数据法线向量信息的准确生成算法，保证CAD模型的高逼真度渲染质量，并且提出了CAD模型数据优化重组的方法。采用对几何坐标数据、纹理坐标数据和法线向量数据分别建立索引列表的方式重组点数据，通过集中存储相同材质面片和生成三角形条带来重组三角形面片数据，采用DDS图像压缩格式重组纹理数据。实验证明，重组方法实现了CAD模型数据的高度紧凑存储和可视化效率的显著提升。
针对高度复杂的虚拟建筑环境中三维模型细节层次的规划设计问题，本文以香港志莲净苑建筑群为例，提出了一种实用的LOD模型数据规划方法、每个细节层次模型精细程度的确定方法以及配套的数据生产工艺，应用严格的量化指标在限制模型数据量的同时保证了LOD模型数据成果质量，为建设具有几何上的高精度和视觉上的高逼真度的虚拟建筑环境提供了一条有效的途径。
本文针对基于R树的真三维空间索引普遍存在兄弟节点相互重叠和节点尺寸不均匀的问题，提出一种新的R树节点选择、节点分裂算法和插入算法。新的R树节点选择算法彻底地解决了传统算法无法正确处理对象落于两个兄弟节点重叠区域的情况，新的节点分裂算法采用了数据挖掘领域的k均值算法，以一分为多的分裂方式将溢出节点分裂为两个或两个以上的小节点，摆脱了一分为二的传统分裂方式的局限性，新的插入算法对插入操作造成范围变动的节点进行优化调整，保证树形始终合理。除了沿用传统的覆盖范围和重叠范围评定指标，本文还引入自定义的R树柯西值作为评定指标之一，保证节点形状的合理性。实验证明，采用新算法后，R树兄弟节点之间的重叠现象显著减少，节点尺寸更为均匀，薄饼状和长条状的节点明显减少，真三维空间查询效率提升至少50％以上，有利于海量三维空间数据的高效管理与实时可视化应用。

　　在上述研究成果的基础上，采用完全自主知识产权的VGEGIS软件技术，以香港志莲净苑仿唐建筑群数字化存档为案例，在微机平台上构建了一个虚拟建筑环境原型系统，验证了本文研究成果是可行性的。该原型系统不仅管理建筑群的几何模型和纹理数据，还汇集了大量的文档、图片、音频以及视频动画等建筑文物资料数据，以亲切直观的人机界面帮助人们全方位、多角度地了解志莲净苑建筑群，为建筑文化遗产的数字化存档工作提供了一种新工具。

关键词：拟建筑环境，虚拟地理环境，三维可视化，空间数据组织，LOD模型，CAD模型，三维R树，三维城市模型

Abstract

On February 16 this year, Nature magazine published a paper titled “Virtual globes: The web-wide world”, which points out that Life happens in three dimensions, so why doesn’t science? Online 3D tools led by the Google Earth “virtual globe”, are profoundly changing the way we interact with spatial data. The change from Word Wide Web to Web Wide World, shows that unilateral information services without feedback is developed into interactive and collaborative exploration in virtual world most close to reality. Because 3d data acquisition technique is increasingly boosted and the time of 3D GIS has come, the technology of 3D GIS is more and more applied to the fields of plan, design, construction and civil management et al. As the special case of VGE in the fields of AEC (architecture, engineering and construction), Virtual Building Environment (VBE) has been broadly concerned, and will provide data services and support decision-making across the lifecycle of building and infrastructure investment: planning, design, construction, use, maintenance and decommissioning. Especially, 3d digital application of architectural heritage, has become research focus and academic front in VBE.

Compared with ordinary 3d applications, the amounts of data are large-scale, data types are rich and graphical complexity of scene is greatly high in VBE, which bring great difficulty to data management and interactive visualization. Highly complex, large-scale 3d spatial data is main bottleneck of VBE applications, so 3d spatial data organization and management certainly becomes the core technology for VBE. Aiming at the characteristics of VBE, this dissertation researches on the key ones of the core technology, such as spatial data model, spatial data organization, data production planning and spatial indexing et al. The main research work of this dissertation is as follows:

Aiming at 3d spatial data organization and management that is the core problem of VBE, this dissertation thoroughly analyzes and evaluates research progresses, development direction and existing problems of its key technologies, such as 3d spatial data management mode, 3d spatial data model, 3d spatial data organization method and 3d spatial index et al.
Aiming at LOD models of full 3d CAD model that is one of main data sources for VBE, a kind of practical spatial data model is designed, which simplifies spatial topological relations and extends CAD class and LOD class. CAD class is used to integrate CAD model, and LOD class for multi-scale phenomena in VBE. Spatial data model also absorbs some real-time rendering techniques, for example visibility culling, which can efficiently describe complex building environment.
Inherent storage method of CAD data makes data redundant, and doesn’t concern efficient visualization, which is a practical bottleneck to integrate CAD model. This dissertation puts forward the algorithm of accurately generating normal vector in CAD model and promises high-fidelity rendering quality, then presents optimal method of reorganizing CAD model data. Point data is reorganized by generating index list respectively for geometric coordinates, texture coordinates and normal vectors. Triangular facet data is reorganized by collecting the faces with the same material attribute and generating triangle strip. Texture image data is reorganized in the format of DDS. Experiments prove that data storage can be tight highly and rendering efficiency can be boosted markedly after adopting reorganization method.
Aiming at planning and designing LOD of 3d model in highly complex VBE, based on building complex of Chi Lin Nunnery in Hong Kong, this dissertation introduces a useful method to plan the level of details for 3D city models and to determine the fidelity of each level of details. Moreover, the corresponding techniques for LOD modeling and full quality control is presented. Quantitative criteria are applied in order to limit the amounts of data and promise the quality of data production, which provides an effective approach to produce 3D city models with high-precision and high-fidelity.
Since the 3d spatial indices based on R-tree are usually limited by their weakness of low efficiency due to the severe overlap of sibling nodes and the uneven size of nodes, new R-tree algorithms are proposed, including node-choosing, node-split and insertion algorithms. The new node-choosing algorithm completely solves the problem that traditional ones cannot properly handle the condition when the object is in the overlap area of two sibling nodes. The new node-split algorithm introduces the k-mean algorithm and split the overflowed node into two or more small nodes, not limited to traditional one-into-two mode. In the new insertion algorithm, nodes are optimally adjusted whose overlay area have changed because of insertion so that the tree shape remain rational all the time. Besides traditional criteria of overlay and overlap, this dissertation also introduces self-defined R-tree Cauchy value as one of criteria, in order to remain rationality of node shape. Experiments prove that the overlap of R-tree sibling nodes is minimized drastically, the sizes of nodes maintain even, strip-shape and cake-shape nodes decrease greatly and the efficiency of full 3d spatial query is boosted at least 50%, which is beneficial to efficient management of 3d spatial data and real-time visualization.

Based on the above research work and VGEGIS software with independent intellectual property, in order to realize digital documentation of Tang-style building complex of Chi Lin Nunnery in Hong Kong, a VBE prototype is built on personal computer, and the building model is high-fidelity and highly accurate. In the process of interactive roaming in VBE, the fresh rate remain more than 15 frame per second, which can prove that these research work is feasible. This prototype manages not only geometric model and texture image, but also a large amount of attribute information such as text, graphics, audio and video, and provides friendly human-machine interface to help users learn building complex of Chi Lin Nunnery comprehensively. It provides a new tool for digital documentation of architectural heritage.

Key words: Virtual Building Environment, Virtual Geographic Environment, Geovisualization, Spatial Data Organization, LOD Model, CAD Model, 3D R-Tree, 3D City Model