Out-of-core isosurface extraction of time-varying fields over irregular grids

Abstract

In this paper, we propose a novel out-of-core isosurface extraction technique for large time-varying fields over irregular grids. We employ our meta-cell technique to explore the spatial coherence of the data, and our time tree algorithm to consider the temporal coherence as well. Our one-time preprocessing phase first partitions the dataset into meta-cells that cluster spatially neighboring cells together and are stored in disk. We then build a time tree to index the meta-cells for fast isosurface extraction. The time tree takes advantage of the temporal coherence among the scalar values at different time steps, and uses BBIO trees as secondary structures, which are stored in disk and support I/O-optimal interval searches. The time tree algorithm employs a novel meta-interval collapsing scheme and the buffer technique, to take care of the temporal coherence in an I/O-efficient way. We further make the time tree cache-oblivious, so that searching on it automatically performs optimal number of block transfers between any two consecutive levels of memory hierarchy (such as between cache and main memory and between main memory and disk) simultaneously. At run-time, we perform optimal cache-oblivious searches in the time tree, together with I/O-optimal searches in the BBIO trees, to read the active meta-cells from disk and generate the queried isosurface efficiently. The experiments demonstrate the effectiveness of our new technique. In particular, compared with the query-optimal main-memory algorithm by Cignoni et al. (1997) (extended for time-varying fields) when there is not enough main memory, our technique can speed up the isosurface queries from more than 18 hours to less than 4 minutes.