关键词: 地形渲染, 线状地图符号, 样式, 着色器语言, 虚拟地理环境

Abstract:

Vector data, as an important part of Geographic Information System (GIS), plays a significant role in three-dimensional(3D) GIS to indicate geographic objects, explain spatial relationships and enrich map information. However, in the study of 2D vector rendering onto 3D terrain surface, vector lines drawn by traditional algorithms are relatively simple and monotonous, mainly shown as monochromatic lines or multicolor parallel lines, which cannot meet the growing demands for visualization. With the rapid development of graphics hardware acceleration and the widespread application of Virtual Geographical Environment (VGE), in order to draw the complicated linear map symbol in 3D map effectively and efficiently, this paper proposes a screen-based method that renders the linear symbol onto the 3D terrain surface with the use of OpenGL Shader Language (GLSL). We transfer the vector data from the procedure to Graphics Processing Unit (GPU) by encoding the vector data into the texture data. The mapping relationships between terrain units and vector lines are stored in the index texture, and the mapping is used for looking up the related line segments in real-time. In addition, the node texture stores the attribute of vector lines. We calculate the position relationship between screen fragments and related line segments, and find out the area where the current fragment belongs to. If the current screen fragment has associated line segments, then we get the attributes of the related line segments from the node texture and set the fragment color. It is worth mentioning that we provide a variety of symbolic functions, which can render different types of vector line segments, including gradient lines, periodic lines, etc. In order to achieve better visual effects, we apply a series of detail optimization operations to related line segments, including anti-aliasing and corner optimization. Finally, we take the Purple Mountain as an example and render vector lines onto it using the proposed method. The experiments demonstrated that vector lines rendered by our method show great visual effect and our method can render some complicated and typical line symbols with high performance while keeping rendering efficiency at real time level.

Key words: terrain rendering, linear map symbol, style, shader language, virtual geographical environment