Motion primitives Motion primitives One natural primitive is qualitative properties of motion, which are quite highly stylised.
Periodicity is a particularly useful primitive.
As Niyogi and Adelson pointed out, the periodic motion of legs for walking or running people appears as a braid in a spatio-temporal cross section. Crude methods are sufficient to segment this braid well enough that its period and direction of motion can be estimated accurately. Similar braids must apply for walking, trotting and running animals, so that braids are a natural primitive. While the presence of a braid is a good cue that human or animal motion is occuring, it cannot be used directly to segment the motion, because occlusion prevents the motion of upper limbs forming a clear braid. However, a sequence of segmentation processes based around braids and straight traces appears to be quite successful.