Out-of-core Multi-resolution Terrain Modeling

In this chapter we discuss issues about Level Of Detail (LOD) representations for digital terrain models and, especially, we describe how to deal with very large terrain datasets through out-of-core techniques that explicitly manage I/O operations between levels of memory. LOD modeling in the related context of geographical maps is discussed in Chapters 3 and 4. A dataset describing a terrain consists of a set of elevation measurements taken at a finite number of locations over a planar or a spherical domain. In a digital terrain model, elevation is extended to the whole domain of interest by averaging or interpolating the available measurements. Of course, the resulting model is affected by some approximation error and, in general, the higher the density of the samples, the smaller the error. The same arguments can be used for more general two-dimensional scalar, or vector fields (e.g., generated by simulation), defined over a manifold domain, and measured through some sampling process. Available terrain datasets are becoming larger and larger, and processing them at their full resolution often exhibits prohibitive computational costs, even for high-end workstations. Simplification algorithms and multi-resolution models proposed in the literature may improve efficiency, by adapting resolution on-the-fly, according to the needs of a specific application [32]. Data at high resolution are preprocessed once in order to build a multi-resolution model, which can be queried on-line by the application. The multi-resolution model acts as a black box that provides simplified representations, where resolution is focused on the region of interest, and at the level of detail required by the application. A simplified representation is generally affected by some approximation error, which is usually associated with either the vertices, or the cells of the simplified mesh. Since current datasets often exceed the size of main memory, I/O operations between levels of memory are often the bottleneck in computation. A disk access is