Increasing Update Rates in the Building Walkthrough System with Automatic Model-Space Subdivision and Potentially Visible Set Calculations

摘要

Pre-processing some building models can radically reduce the number of polygonsprocessed during interactive building walkthroughs. New model-space subdivision and potentially visible set (PVS) calculation techniques, used in combination, reduce the number of polygons processed in a real building model by an average factor of 30, and a worst case factor of at least 3.25. A method of recursive model-space subdivision using binary space partitioning is presented. Heuristics are developed to guide the choice of splitting planes. The spatial subdivisions resulting from binary space partitioning are called cells. Cells correspond roughly to rooms. An observer placed in a cell may see features exterior to the cell through transparent portions of the cell boundary called portals. Computing the polygonal definitions of the portals is cast as a problem of computing a set difference operation on co-planar polygons. A plane-sweep algorithm to compute the set operations, union, intersection and difference, on co-planar sets of polygons is presented with an emphasis on handling real-world data. Two different approaches to computing the PVS for a cell are explored. The first uses point sampling and has the advantage that it is easy to trade time for results, but has the disadvantage of under-estimating the PVS. The second approach is to analytically compute a conservative over-estimation of the PVS using techniques similar to analytical shadow computation. An implementation of the Radiosity lighting model is described along with the issues involved in combining it with the algorithms described in this dissertation. (kr)