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Abstract:

The term bio&gical motion has been coined by G. Johansson (1973) ' to refer to the ambulatory patterns of terrestrial bipeds and quadripeds. In this paper a computational theory of the visual perception of biological motion is proposed. The specific problem addressed is how the three dimensional structure and motions of animal limbs may be computed fro',n the two dimensional motions of their projected images. It is noted that the lhnbs of animals typically do not move arbitrarily during ambulation. Rather, for anatomical reasons, they typically move in single planes for extended periods of'time. This simple anatomical constraint is exploited as the basis for utilizing a "planarity assumption " in the interpretation of biological motion. The analysis proposed is: (1) divide the image into groups of two or three clements each; (2) test each group for pairwise-rigid planar motion; (3) combine the results fi'om (2). Fundamental to the analysis are two 'structure from planar motion ' propositions. Thc first states that the structure and motion of two points rigidly linked and rotating in a plane is recoverable fi'om three orthographic projections. The second states that the structure and motion of three points forming two hinged rods constrained to move in a plane is recoverable from two orthographic projections. The psychological relevance of the analysis and possible interactions with top down recognition processes are discussed.

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