Continuous Shape Transformation and Metrics on Regions

A natural approach to defining continuous change of shape is in terms of a metric that measures the difference between two regions. We consider four such metrics over regions: the Hausdorff distance, the dual-Hausdorff distance, the area of the symmetric difference, and the optimal-homeomorphism metric (a generalization of the Fréchet distance). Each of these gives a different criterion for continuous change. We establish qualitative properties of all of these; in particular, the continuity of basic functions such as union, intersection, set difference, area, distance, and smoothed circumference and the transition graph between RCC-8 relations. We also show that the history-based definition of continuity proposed by Muller is equivalent to continuity with respect to the Hausdorff distance. An examination of the difference between the transition rules that we have found for the Hausdorff distance and the transition theorems that Muller derives leads to the conclusion that Muller’s analysis of state transitions is not adequate. We propose an alternative characterization of transitions in Muller’s first-order language over histories.