| 3D urban model has began to apply in city programming, building designing and other domains as the most direct and actual form of city GIS. The work done in this paper derives from the need of 3D urban project of our laboratory. In this project, the terrain of urban area needs to be rendered in real-time in order to increase the reality of 3D urban model. While the total data of the terrain models has soared rapidly with the development of laser range technology. That brings great pressure to the rendering system of computers. One way to solve this problem is to perform level-of-detail (LOD) rendering for large-scale terrain models. Therefore this paper implements the real-time terrain rendering algorithm based on ROAM combining with the practice of our project and pay more attention to error metrics and maximal split depth.In the field of error metrics, lots of research are done in geometric screen distortion, this paper uses simplified error metrics to speed up rendering. The distance between viewpoint and mesh and the variance of terrain are used to get the needed resolution. According to the results of experiments the simplified error metrics can keep the terrain in a good view and reach the goal of our project.In the field of triangle bintree split, we add the control for it's maximal split depth. The recursion depth is coherent to the split depth of triangle bintree and at the same time it's maximal split depth determines the maximal resolution of terrain. Therefore we can control the recursion depth and the resolution of terrain based on the needs through controlling maximal split depth of triangle bintree. According to the results of experiments the control of maximal split depth of triangle bintree can improve rendering speed at the basic of keeping the terrain in a good view and reduce the burdens of computers.This paper introduces the concept of digital terrain model and the common terrain rendering technology and discusses the ROAM algorithm and it's implement, including terrain block, dual-queue optimization, error metrics, cracks and forced split. Subsequently, this paper introduces frustum cull and texture mapping technology and the final results of experiments and the discuss are given. |
