Virtual environment is a typical application of time-critical computing or time-critical rendering. This paper addresses the level of detail representing models in time-critical rendering. The authors present an automatic generation algorithm for models at multiple levels of detail based on triangle mesh simplification. The algorithm makes multiple passes over an existing triangle mesh, using local geometry and topology to remove venices that pass a distance or angle criterion. The holes left by the vertex removal are patched using a local triangulation process. Examples illustrate efficiency of the algorithm.