“…Given an isovalue, one can then descend the hierarchy, pruning empty subtrees based on this minmax scheme. In recent years, many other methods for accelerating the search phase were proposed [3,4,6,7,15,20,19] all of which have a complexity of On, where n is the size of the dataset. In 1996, we introduced the span space [10] as a mean for mapping the search onto a two-dimension space.…”