Research And Implement About Model Optimizing And Dynamic Download Technology Of Large Scale Virtual Environment Based On VRML

Virtual Reality was a sort of new interactive processing mode, it was integrated the cg technology, computer simulation technology, sensor technology, display technology and a lot of new productions, through the medium of vision, listen, touch sense organ act on user, in possession of interactivity, immersion and imagine characteristic, make user feel like being personally on the scene. For the moment, in the field of war, tour, product design and manufacture, teaching and training, art, recreation, medical treatment and so on, it have an extensive application and bring in a huge economic benefit . Today VR technology has been extended for several aspects , for example virtual cockpit simulation, virtual urbanism, virtual roam, virtual assembly, tridimensional visualization GIS, enhance reality technology .Virtual Reality Modeling Language went by the name of second generation Web language after HTML. It was used to description three-dimensional object and behavior. For the moment, it was a mainstream language for make tridimensional interactive net station base on WWW in the Internet, with platform independence. It could construct virtual world, could integrate text, image, sound, MPEG image media style and so on, could embed program code written by Java, JavaScript language and could through the medium of EAI extend it. VRML adopt object-oriented programming methods, actualize by object's hierarchical structure, package, reuse, message (event) transfer as well as hyperlink.In recent years, virtual scenes as digital city, virtual campus base on network more and more favor by people, for it could do without attend personally scene, with be able to learn local instance, as well as become a sort of in common use propagandistic instrument for recruit business and recruit students. VR roam system require strong reality sensation and immersion sense, so it is needs system be able to display wholeness object in real world,wherefore set-up three-dimensional model stand or fall direct influence virtual wandering effect, it is a foundation of established virtual reality system. Scene modeling is mostly included 3D vision modeling and tridimensional hearing modeling. Among others, vision modeling included geometry modeling, movement modeling, physics modeling, object behavior modeling. For favorable effect of the geometry modeling, we can utilize 3D max, MAYA tridimensional modeling software implementation and endue reality texture. For make the virtual scene actively, movement modeling is necessary too, it include object positional transfer, collision, scaling and distortion. At the same time, sound node support add sound file to VRML scene.But simple watch was insatiability for user's requirement yet. The most different between virtual reality roam and video play is user could participate in virtual scene and interaction control it, so it require event responsive real time very high. In spite of that today network and graphics hardware development faster, nevertheless, in company with virtual city scene scale increasingly more larger, scene model complexity increasingly more higher, ultimately result in browse and download speed sharp decline, had no means of actualize real time browse and interactive function, and user intolerability. Wherefore, it is important for optimize system model and files.For the moment, because of network bandwidth restrict, the"bottleneck"issue of cosmically scene's real time browse badly has been a hotspot for research of virtual reality application. The optimize policy be came up with many academic paper base on model and base on file execution efficiency. Judge from geometrical model, on the one hand cut geometry complexity,namely depress polygon patch number and the size of texture as best as one can; on the other hand, adopt LOD technology, namely on the basis of different stadium, foundation many different detail models, from the near to the distant take turns use from nicety to rough differ model description object detail, while the space ulterior, display coarseness model only, thereby achieve accelerating. Judge from file, adopt reuse(def,use)and Inline node improve code utilization ratio , and approve wrl file compress with gzip,in this way could reduce file volume,decrease versus network bandwidth require. This paper incorporation Jilin university virtual campus, expound the correlation technique and application actualize respectively. These approach although play the role of optimize action,but can not be able to solve this "bottleneck" issue at last. Wherefore, we need for from visibility point of view, use algorithm control the download of scene model, for improve the frame number per second of computer graphics.This paper brought a sort of dynamic whole number incremental algorithm base on AOI(area of interests)round field. Thereof basic idea is "if it is visible you need download, if it is invisible you need delete". First of all, administrate scene model with grid, split whole virtual scene with uniform grid and export a WRL file for each grid's scene. And then, control the visibility of scene with program.The function of algorithm is: while the view point is moving, the algorithm be able to determine which grids overlapped by AOI area. and only scan the bound of area, it be able to determine which grids are overlapped by current AOI area after the view point remove and download this part scene also be able to determine which grids are not overlapped by current AOI area after the view point remove and delete this part scene.Arithmetic download the grid's scene according as the relationship between grid's coordinate and currently view point and next view point's position, it is also fully in consideration of the time dependence and spatial correlation about the two neighbor AOI round fields, thereby we push out a discriminate that base on incremental computation use to grid select. For improve the speed of calculation, the circle radius"r", grid wide"d", and step"c"need select whole number witch equal to 2N. Because of"r"and"d"has been ascertained, the number of the grids which passing by circle are approach constant, so the algorithm complexity is approach constant too.Microsoft visual c++ 6.0 exploitation platform, utilize the EAI interface provide by BS contact actualized the algorithm which could control the display of VRML scene and improve the performance of browser.