| Virtual Reality Technology plays an important part in our daily life, especially the digital city technology, which applies in city planning, architecture designing, industrial simulating, geographic environment simulating, etc. Due to the rapid development and gradual improvement of the internet technology, online digital city system has become the main tendency of the virtual reality technology, to meet the practical need of the virtual reality industry. The standalone software of virtual reality has developed to a network system. Therefore, the users of this technology has extended from professional technical people to common people, which promotes the development of virtual reality technology greatly. However, the massive urban three dimensional(3D) model, as the foundation of the network system of virtual reality, has an extremely high demand to the web server, for its ability of saving, reading and dispatching massive spatial data.The main research objects of this dissertation are the spatial indexing of the global-scale nested quad tree, the rendering of the massive urban 3D model and the access efficiency model. The goal of it is to build up a network system of virtual reality to apply to the fast-querying of massive spatial data and the asynchronous network dispatching. The main research contents of this dissertation are to realize the asynchronous network dispatching of massive spatial data stably, accurately and quickly, and to build up the frame of the transmission of spatial data. At the same time, in the dissertation, it realizes the design of the asynchronous network dispatching and the rendering system of the model of 3D city, which is on the basis of the seamless integration of the research findings and VRGIS platform, and a server of distributed file management system applying the technology of Hadoop.The main results, the working machine in the dissertation achieved, are:(1) The design of the 3D city model’s rendering system and asynchronous network dispatching systemDue to the designed virtual reality system’s characteristics of network foundation and massive data, the server realizes the design by adopting a mature distributed file management system to store massive 3D spatial models, using AcitiveX and dynamic link library, and hooking the server with rendering engine.(2) The storage and query of the data, based on an nested grid index combining the quadtree and Hashing query technologyBecause of the global scale and scattering distribution of the 3D city model’s data, the nested grid index, combining the quadtree and Hashing Query technology, would be a suitable choice to solve the problem of the low efficiency in storing and querying massive spatial data. This nested grid index is to use the Quadtree to lock the querying scale of data quickly, and adopt the Hashing Query to locating the spatial data directly. As a result, the operating performance of the entire system would be improved significantly.(3) The asynchronous dispatching and rendering of the 3D city modelTo make a camera mode suitable for spherical browsing come true, the camera’s position is adopted to be the essential condition in this dissertation, and LOD is used to grade and optimize the texture of the city model. In the meantime, according to camera’s position, the grid for data exchanging can be conjectured, and the relation model between the downloading speed and downloads of the query data for one time. At last, the optimum data volume for one query can be calculated to keep a optimum downloading speed.(4) The realization and application of the 3D city model’s asynchronous dispatching and rendering systemIn this dissertation, the server’s plug-in is the seamless combination of the researching findings and the VRGIS platform. Thus, the server for web based on the HTTP protocol can be built up and linked to the distributed document manager. Finally, the asynchronous dispatching and rendering system of the 3D city model’s is realized. |
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