| In Digital Valley,it is important to visually simulate the physiognomy of valley terrain. Visual simulation also provides a convenient and intuitive method for many other applications,e.g.,Valley Analysis,Flood Control and Disaster Prevention Analysis,Flood Routing and Civil Engineering Application. The visualization of complex terrain is a common key issue among these applications. And the key issue of terrain visualization is to solve the contradiction between complex terrain surface model formed by massive terrain data and the limited drawing ability of graphics hardware of the computer. At present,it is still very difficult to visualize the terrain with massive data characteristic,while keeping the visual effect at a satisfactory level and also sustaining high refresh rate,given today's computer. Utilizing existing hardware resource,working from the software perspective and constructing a suitable terrain model will be the way of solving this problem.The primary contradiction that the three-dimension visualization of the massive terrain should solve comes from two aspects: one is the contradiction between the limited computer processing capacity and complex geographical terrain data representation,the other is the contradiction between limited computer memory and massive terrain data. In view of these two principal contradictions,terrain visualization model,massive data management and real-time visualization interactive technology these three aspects are addressed emphatically in the dissertation.Summarily,the main studies and achievements of this dissertation are listed as following:(1) The terrain visualization development process and related basic knowledge are described in the dissertation,especially the related technologies and problems of every stage of terrain visualization,from data storage to date model construction,till terrain drawing.(2) For the massive terrain can not be loaded into visualization system memory at one time,the terrain must be divided into smaller tiles or it must be dealt with like Texture Clipmap. After discussing the four important terrain simplifying model at present,integrating the characteristic of massive terrain,the dissertation presents a new tessellation LOD simplifying model based on network (nLOD). (3) With the support of nLOD and integrating it with present mainstream technology network storage technology,the dissertation introduces a Massive Terrain Cluster Management System (TCMS),and discusses the organization and distributing algorithm of massive terrain according to the characteristic of terrain data and the need of terrain visualization.(4) Interactive technology of visualization system and terrain data management system is also a focus of the dissertation. According to the nLOD and TCMS mentioned above,the dissertation presents a set of strategies to optimize terrain visualization,including data compression,data transmission and data dispatching.(5) With the new features of computer graphics hardware,the computation of terrain visualization has been not only due to the CPU,but also due to the GPU. Therefore,on the reference of Texture Clipmap the dissertation studies and realizes the Geometry Clipmap algorithm proposed by Hoppe in order to deal with large scale terrain. The dissertation ameliorates the Geometry Clipmap algorithm according to its characteristic and proposes the Center-asymmetric Geometry Clipmap algorithm,which has better performance than the old one according to the comparison results.Above-mentioned research results have already been applied to some projects,partly or wholly,e.g."Digital Foshan","QingJiang Valley Three-dimension rendering System"and so on. The visualization effects of those applications were satisfactory. At last,a detailed conclusion on the whole work and the research results on this paper is summarized,and the improvable aspects and the proposals to the future like redundant data store,terrain shadow,terrain data compress and distributing application are discussed.
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