B-QUADTREE. A DATA STRUCTURE FOR URBAN FLIGHTS AND WALKTHROUGHS J.L. Pina, F.J. Serón, E. Cerezo Grupo de Informática Gráfica Avanzada (GIGA) Departamento de Informática e Ingeniería de Sistemas Universidad de Zaragoza ABSTRACT A data structure based on urban blocks which vastly improves rendering speed in urban walkthroughs and flights is presented in this paper. The city is split by means of a quadtree partition and the block is adopted as the basic urban unit. One advantage of blocks is that they can be easily identified in any urban environment, regardless of the origins and structure of the entry data. Results are promising and lead to a factor three increase in rendering speed. KEYWORDS Visualization Urban Quadtree Block Walkthrough Fly 1. INTRODUCTION The aim of this paper is to find a data structure capable of interactively carrying out urban walkthroughs and flights without the use of predefined paths. In order to fulfill both possibilities, all the elements of the city are grouped into a basic unit: the block. Setting out from a typical Quadtree decomposition (Pajarola, R., 1998), a data structure that we named B-Quatree or Block-Quadtree was built. The use of this data structure leads to a great increase in FPS, both in flights and walkthroughs. One of the requirements imposed on the new data structure was that it was capable of being applied to urban data with low structure or no structure at all. Therefore, the proposed data structure can be applied to data acquired from several sources: 2D GIS, terrain measurements, etc. Another advantage of our method is that identification of buildings is not required. Our basic unit, the urban block, can be easily identified under any circumstance, which is not the case with individual buildings. Moreover, the use of blocks has an impact on the number of nodes in the structure: there are not as many there would be if we were using buildings, but there are enough for fast tree culling. 2. RELATED RESEARCH A great variety of solutions can be found in the bibliography related to urban walkthroughs and flights. The challenge still remains: to visualize a large amount of geometric elements all together and in real time. Methods can be classified into four main groups of techniques: LOD or level of detail, billboards, occlusion culling and preprocessing. LOD is one of the most frequently used techniques: continuous LODs have been tested (Dollner, J. and Buchholz, H., 2005), but these reduce rendering speed; so have hierarchical models (Funkhouser, A. and Séquin, C., 1993) that choose the proper resolution according to the node of the tree being displayed. However, it is very difficult to avoid bothersome drops between frames of different resolutions. Billboards (Sillion, F. G. and Drettakis, B. B, 1997) and impostors (Jeschke, S. et al., 2005), are based on the use of images of objects instead of the 3D objects themselves. They are very effective in reducing rendering time. The correct use of this technique allows for realistic visualizations with less graphics cost. In ISBN: 978-972-8924-39-3 © 2007 IADIS 138