Most non-rigid motion is much simpler than is generally thought, having many fewer degrees of freedom than, for instance, 3-D shape. The relatively circumscribed nature of nonrigid motion can be seen by examining it from the perspective of vibration modes. The fact that a small number of parameters are sufficient to describe non-rigid motion mean that it is plausible to use sensory data to recover a nearly complete physical model of an object, so that we can, for instance, predict its response to impinging forces. One perceptual task of considerable interest is estimating material properties such as stiffness, strength, and mass by watching, touching or listening to collisions. We derive equations which allow estimates of these material properties to be extracted from visual and tactile data, and speculate on the use of aural data.
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