A Method Of Rendering Complex Linear Map Symbols In Virtual Geographic Environments

The map symbol is the language of the map,and the linear symbol,as one of the three basic map symbols,plays an important role in the map and geographic information system.Traditional two-dimensional linear map symbol research has formed a mature system that can effectively enhance linear geographic elements and improve the efficiency of conveying cartographic information.Linear map symbols also have an irreplaceable role in the virtual geographical environment.For example,it emphasizes roads and rivers from a complex texture background and reveals the relationship between different line elements.Compared with 2D GIS,there are many difficulties in rendering 3D linear map symbols:the symbol fits perfectly on the 3D terrain surface,the rendering approach accurately draws linear map symbols with complex styles,and should guarantee real-time interactive rendering performance.Currently,the linear map symbols drawn by the existing rendering approaches including texture-based,geometry-based and shadow-volume-based approaches are too simple.Most of them treat vector lines as monochromatic symbols,ignoring the changes of geometry and color features,and these simple symbols fail to accurately express the specific meaning of different lines.In addition,these methods also have problems such as inaccurate fitting,significant saw-tooth phenomenon,insufficient rendering performance,and the like.To overcome the deficiencies of existing rendering algorithms,this study proposes a new screen-based scheme for rendering complex linear symbols in virtual geographic environment,supported by the National Natural Science Foundation of China—"Research on Overlaying Typical 2D Vector Features onto 3D Terrain Surface Algorithms"(41371365).Our method used the programmable rendering pipeline to calculate linear symbol styles during terrain rendering,can port a considerable part of 2D linear map symbols to 3D terrain surface.The main research contents and research conclusions of the paper are as follows:1)Designing methods for the encoding,transmission and real-time reconstruction of vector line data.In order to transfer vector data to the shader terrain shader,the vector data is encoded as a three-dimensional texture image.The uniform grid index is selected to establish the mapping between terrain cells and vector line segments,and the relationship is converted into an index texture image.Two texture images are passed into the fragment shader,and the corresponding texture coordinates are calculated according to the model coordinates of the fragments.The index values and node information are read using the sampling function to reconstruct vector line segments.2)Studying the calculation method of linear symbol style on the 3D terrain surface.The function method is used to draw linear symbols,and unique rendering algorithm is designed for each linear symbol.The position relationship between fragments and associated line segments is calculated in the 3D model space.The 3D cumulative distance is used to divide the period of linear symbols,and the vertical distance of the fragment to line segment is combined to determine which symbol region the fragment belongs to.Finally,the corresponding color is obtained from the symbol color table.3)Optimizing the rendering effect of linear map symbols in screen space.The local coordinate system of the fragment is established,and we can use the plane geometry to realize the circular transition of the symbol corner.Anti-aliasing on symbol edges and areas where color changes within the symbol is performed,and our method use the hermite function to calculate the color mixing ratio.Different symbol rendering functions are called in sequence according to the symbol priority,and the fragment colors are repeatedly calculated to form different overlap and connection relationships.A series of experiments show that the proposed rendering method can accurately draw complex linear map symbols on the 3D terrain surfaces.The symbol's period,corners,and intersections are all reasonably processed,and symbols fit precisely on the terrain surfaces.Furthermore,Anti-aliasing effect is satisfactory.Vector data is transmitted to the fragment shader,and the complex calculation process is transferred to the GPU,which ensures that the rendering performance of the proposed method meets the real-time interactive demand.