>>2006,Gong Jun, Research on the Key Technologies of 3D Spatial Data Organization and Management for Virtual Building Environments

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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