Study On The Methods Of Real-time Overlaying 2D Map Symbols On DEM

Map symbol is a sign or information carrier which is used to represent the spatial distribution,quantity,quality and other characteristics of geographic objects on the map.It is clear,intuitive,and highly abstract geographic objects.Now it is an important issue in geo-information sciences that how to mapping symbols effectively with high quality.By the rapid development of three-dimensional maps and three-dimensional map symbols,rendering three-dimensional scene has been the mainstream trend of cartography.The traditional two-dimensional map symbols and their associated data are still valuable resources for the geographic information industry.Two-dimensional map symbol,which is both quantitative and qualitative,is the primary means to abstract expressing geographic information.Due to the different dimensions,maps symbols may appear overhead or puncture phenomenon if the two-dimensional map symbols overlay the three-dimensional terrain directly.This paper try to study various rendering algorithm of overlaying the the two-dimensional symbols on DEM(Digital Elevation Model,DEM),including marker symbols,line symbols and area symbols.And then design simplification algorithms for every map symbol to reduce resources as much as possible and enhance rendering efficiency to meet the requirements of real-time roaming.The analysis of the final effect of the simplification algorithms will be performed through the experimental comparison.The research contents and achievements of this paper are following:(1)Overlaying two-dimensional marker symbols on DEMSince the marker symbols is difficult to distinguish after overlaying on maps,this paper proposed to present marker symbol by using billboard.This paper also use the piecewise function to adjust the size of billboard to reduce the screen area of close-marker symbols,which can improve the quality of rendering.When there is a larger number of marker symbols,this paper takes appropriate visibility culling policy,including the frustum culling,back-face culling,detail culling and occlusion culling,to reduce the number of marker symbols which will be drawn in current frame.In order to reduce the density of marker symbols and the complexity of visibility culling procession,combining spatial partitioning technique,GPU based hardware occlusion query and space-time continuum to further improve rendering efficiency.(2)Overlaying two-dimensional line symbols on DEMTo research on the methods of two-dimensional line symbols overlaying on DEM,this paper through the contrast analysis of the existing three overlaying methods including texture-based method,geometry-based method and stencil-shadow-volume-based method,and demonstrates advantages of stencil-shadow-volume-based method on overlaying the two-dimensional line symbol.According to the needs of stencil-shadow-volume-based method,this paper expands the algorithms which are used in two-dimensional line symbol tessellation,and based on the comparative analysis of building shadow volume structure by various algorithms.The previous work demonstrates the superiority of chain-based tessellation when building shadow volume structure.In order to solve the problem to the increasing amount of data which cause efficiency decreasing,this paper adopts some effectively simplified algorithm to reduce the number of inefficient line nodes and improve the efficiency of rendering line symbols.(3)Overlaying two-dimensional area symbols on DEMThis part uses the same method to overlay area symbols on DEM.When you construct a shadow volume,level the upper and lower surfaces first.And then apply the trapezoidal tessellation to split them for solving the problem of illegal polygon rendering.To present near fine and far coarse,drawing on the idea of progressive meshes,this paper proposes a method called view-depending progressive shadow volume meshes simplification algorithm to achieve a multi-resolution area symbol drawing.In the proposed algorithm,the new design of data structures can eliminate the ambiguity of vertex splitting.And splitting shadow volume to meshes resolve the issue of multiphase side.The algorithm also make fewer calculations edge weight calculation formula to improve the efficiency of rendering area symbols.The final result proves:drawing method used in this paper,is able to achieve high quality in overlaying two-dimensional map symbols on DEM,and simplification methods proposed in this paper can effectively reduce the cost of resources and enhance rendering efficiency.Through the study,the real-time overlaying two-dimensional map symbols on DEM has been solved.The result enriches the means of the symbolic expression two-dimensional vector data on three-dimensional scene.