典型二维矢量地图要素在三维地形表面的贴合渲染算法研究

3D GIS, in its earlier development, focuses on making the virtual scene looks like a real natural world. With the expansion of application scenarios, it becomes more and more important to introduce the abstract ability of 2D map to 3D GIS, by overlaying 2D vector map elements on the surface of 3D terrain model to outline spatial phenomenon or pattern of earth surface. The primary elements on 2D topographical map include simple solid polyline, symbolized polyline, simple polygon (surrounded by simple solid polyline and filled by only one single color), and complex polygon (surrounded by symbolized polyline or filled by vector or raster texture pattern), and more. Specific mathematical models are researched respectively for continuously spreading typical types of 2D vector map elements over the 3D surface, which ensure 2D graphics seamlessly fit to 3D irregular surface. That means 2D graphics wouldn't suspend above, puncture into, or submerge beneath the 3D surface, and wouldn't lose shape or be distorted. For simple solid polyline and simple polygon, the parallel algorithm for rendering them on 3D surface is proposed to cover the shortage of existing methods. For symbolized polyline, the spreading distance from the line's origin and its symbol pattern are both used to construct the symbolization rule. For complex polygon, the rule, and then the mathematical model, to spread its interior texture over 3D surface are explored. For all the four types of 2D map elements, the methods to synthesize or to simplify rendered pixels according to sighting distance are researched. Based on achieved mathematical methods, by constructing appropriate spatial data model and data scheduling mechanism, in terms of core thought of data parallel and pipeline parallel algorithms, new parallel algorithms are proposed to improve rendering performance of 2D map elements on 3D surface. The research work enriches focuses in the field of 2D vector graphics and its application in 3D GIS.

为使三维GIS在具备真实感场景表达能力的同时，对空间现象或规律，兼具二维矢量地图的抽象与概括表达能力，拟把典型二维矢量地图要素的投影目标从平面拓展到三维几何表面。针对常用的二维矢量地图要素：无符号折线、简单多边形、符号化折线和复杂多边形，分别研究提出其在三维不规则地形几何表面的连续铺展数学模型和方法，消除二维矢量可能会出现的"悬浮"空中、"穿刺"地表、"淹没"地下及变形走样等现象；进一步提出二维矢量在三维表面贴合渲染效果根据视距远近而综合调整的动态模型，以使得渲染效果具有一定的空间距离感；通过建立合适的空间数据模型以及数据调度机制，并基于数据分块以及流水线处理的有关计算方法，建立三维地表及其表面二维矢量地图要素无走样紧密融合的高性能并行渲染算法。在理论方面丰富和发展三维GIS中二维矢量在三维表面投影的有关数学模型和方法，在应用方面丰富和发展三维GIS中有关高性能并行地理计算的实现技术。

在三维 GIS 中，虚拟地理场景通常由数字正射影像和数字高程模型构成。由于影像提供的信息纷繁复杂，容易造成视觉错感而使观察者在场景中迷失。作为地理信息系统的重要组成部分，矢量数据在虚拟地理环境中对空间现象或规律可以实现很好的抽象和概括表达。传统的矢量线渲染算法（几何方法、纹理方法、模板阴影体）难以满足绘制高质量矢量要素的需求。项目针对典型二维矢量要素在三维地形表面贴合渲染的问题，主要开展如下几个方面的研究：1、研究二维矢量要素在三维地表的贴合渲染的数学模型；2、研究基于屏幕空间的矢量线渲染算法的视觉误差处理；3、研究复杂二维线状地图符号在三维地表的高效率渲染；4、研究虚拟地理环境中线要素注记文字自动化放置算法。获得的主要研究成果如下：提出了一种基于屏幕像素投影反算的矢量渲染算法，渲染精度达到1个像素，且矢量线渲染不受地形模型影响；建立了矢量线边缘处理规则，解决了基于屏幕算法在矢量边缘处的锯齿及单像素宽度要素的断续现象；在可编程渲染管线基础上，实现了在三维地形表面高效率、高质量绘制具有复杂样式的线状地图符号,符号的绘制考虑了多种视觉变量，如形状、颜色、透明度等；考虑三维地图的可视化特征，同时兼顾传统的地图制图偏好，探索并设计了一套三维地图线要素注记放置的规则，设计并实现了一个高效的注记放置算法，降低了注记参考线的实时计算量，避免了注记被遮挡，提高了沿线分布的文字注记可读性。将相关算法模型和应用到虚拟地理场景中，在整体效果和渲染效率上都取得令人满意的效果。