We introduce a framework for streaming geometry in VRML that eliminates the need to perform complete downloads of geometric models before starting to display them. This framework for the progressive transmission of geometry has three main parts, as follows: 1) a process to generate multiple levels-of-detail (LODs); 2) a transmission process (preferably in compressed form); and 3) a data structure for receiving and exploiting the LODs generated in the first part and transmitted in the second. The processes in parts 1 and 2 have already received considerable attention. We concentrate on a solution for part 3. Our basic contribution is a flexible LOD storage scheme, which we refer to as a progressive multilevel mesh. This scheme, primarily intended as a data structure in memory, has a low memory footprint and provides easy access to the various LODs (thus suitable for efficient rendering). This representation is not tied to a particular automated polygon reduction tool. In fact, we can use the output of any polygon reduction algorithm based on vertex clustering (including the edge collapse operations used in several algorithms). The progressive multilevel mesh complements compression techniques such as those developed by M. Deering (1995), H. Hoppe (1996) or G. Taubin et al. (1998). We discuss the integration of some of these compression techniques. However, for the sake of simplicity, we use a simple file format to describe the algorithm.
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