Visual perception of moving parts

There are countless three-dimensional interpretations of a set of points moving in a two-dimensional image. A unique visual interpretation of motion thus requires assumptions about the types of structure likely to be found in the three-dimensional world. We propose that the human visual system favors articulated structures in its interpretations. An articulated structure is a rigid body with moving parts that themselves are rigid and rotate in fixed planes with respect to the body. (A bicycle is an example.) We claim that an image consisting of just two moving points is seen as an articulated structure (when their motion is consistent with one), even though countless other interpretations are possible, including a rigid one of a rod moving in space. An experiment is presented in support of our claim, and a well-known display from an old experiment is reinterpreted as a special case of an articulated structure.