| Virtual Reality(VR)generally refers to wearable virtual reality.It simulates a variety of human perception functions including visual,auditory and tactile by interactive devices such as helmet-type displays and hand controllers,which can make users fully immerse themselves in the virtual world,and interact with objects in the virtual world with natural behavior.In the field of geographic information,the application of VR effectively mitigates the deficiency of traditional geographic information—it is passively perceived—and enables people to actively analyze the deep meaning contained in geoscience data.Thus,VR has become an important direction for future development of three-dimensional(3D)geographic information systems(GISs).As a new generation of 3D-GIS platform and an important carrier of “Digital Earth”,virtual globes can realize unified management and visualization of multi-source and multi-scale spatial data,including image,topographic,atmospheric,oceanic and urban data.The combination of VR and virtual globes—VR globes—can not only represent global to local geographic environments and outdoor to indoor geographic environments and enable people to view virtual scenes in an immersive manner at any location on Earth,but also expand the traditional human–computer interactivity from a two-dimensional(2D)screen space to a 3D real space.Thus,complex interactive strategies can be designed with VR globes to satisfy the requirements of the application of different spatial data.VR globes can realize interactive visualization of multi-source and multi-scale spatial data.From the current research,to achieve the combination of VR technology and virtual globes technology,there are at least three aspects need to be addressed in visualization.Firstly,in terms of the VR interaction,the existing VR system mainly uses the location tracking technology to realize the mapping of user behavior from real space to virtual space.Due to the area within which the user can move in the real space cannot be infinitely extended,the realization of VR navigation in the entire virtual globe space within a limited real space is the first problem to be addressed.Secondly,in terms of the VR rendering,the special binocular rendering mechanism of VR requires the 3D scene be continuously drawn twice within one frame,while the complexity of earth scenes,especially city scenes that contain large numbers of fine building models,is much higher than that in normal visualization systems.Therefore,how to ensure the rendering efficiency of VR globes under the binocular rendering mechanism is the second problem to be solved.Finally,in terms of the VR sickness,the most effective way to reduce it is to stabilize the frame rate,so that the frame rate will not lower than the screen refresh rate.However,the complexity of earth scenes and the randomness of the user browsing make it impossible to stabilize frame rate by pre-optimizing the scene.Therefore,how to stabilize the frame rate in real time during the VR globes rendering process is the third problem to be settled.In order to solve the above problems,the main research contents of this dissertation are as follows:The first chapter is the introduction,focusing on the development of 3D-GIS to VR,the development of virtual globes to VR,and the background and significance of VR globes.It shows that VR-GIS is an important direction for the future development of 3D-GIS.As a new generation of 3D-GIS platform,virtual globes develops to VR,which can not only further exert the advantages of virtual globes in spatial data expression,but also provide strong support for virtual globes convert to more professional 3D-GIS platform.The second chapter introduces the existing theories and methods in virtual globes.It mainly includes a global discrete grid model for realizing the unified organization and management of multi-source heterogeneous spatial data,a global multi-scale spatial data model for realizing 1:1 modeling of real earth in computers,and a global 3D scene rendering method for realizing efficient rendering for global 3D scenes.The third chapter proposes a scene interaction method for VR globes,which is used to solve the problem of roaming interaction between VR users and global multi-scale 3D scenes,including two parts.Firstly,a real space to virtual globe space mapping model is designed.The model establishes the positional mapping relationship between real space and virtual globe space,so that users can interact with three-dimensional scenes of any scale in the world.Secondly,a VR viewpoint correction algorithm is proposed.This algorithm has introduced a technology of collision detection and response,which can make users roam and interact in complex scenes with a large number of irregular obstacles,such as near-surface and indoors.The fourth chapter proposes a binocular parallel rendering method for VR globes,which is used to solve the problem of low efficiency rendering of VR globes under binocular rendering mechanism,including two parts.Firstly,a binocular parallel rendering model is designed.This model realizes the parallel rendering of the binocular scene by assigning the rendering tasks of the VR left and right cameras to different CPU cores.Secondly,a resolution synchronization algorithm is proposed to conduct realtime synchronization on the resolution of scene in the rendering process and thus avoids the problem of erroneous binocular stereo matching.The fifth chapter proposes a time-critical visualization method for VR globes,which is used to solve the problem of VR sickness caused by frame rate instability,including two parts.Firstly,a rendering time estimation model is designed,which realizes accurate estimation of the rendering time of VR globes.Secondly,a scene dynamic optimization algorithm is proposed,which dynamically adjusts the LOD structure of scene based on the estimation of rendering time to ensure the final scene can be completed within a limited time,thus achieving the purpose of stability of frame rate and reduction of VR sickness.The sixth chapter discusses the design and implementation of the VR globe prototype system.The system is mainly based on several algorithms and methods proposed in the dissertation,and combined with osg Earth,Open VR and other techniques.It mainly includes three modules: data loading and scene modeling,binocular rendering and stereo displaying,scene roaming and human-computer interaction.The seventh chapter presents the conclusions and prospects for future work. |
PDF Full Text Request