16. A computer-readable storage medium capable of programming a processor to perform a method tracking body parts of a user to determine a state estimate of the user's position in a current frame of image data, comprising: (a) generating a plurality of skeletal hypotheses, the plurality of skeletal hypotheses defining a probability distribution; (b) selecting one or more skeletal hypotheses as being the most probable state estimates based on the probability distribution, where the probability distribution indicates one or more skeletal hypotheses as the probable state estimate; and (c) indicating that no state estimate is determined for the frame of image data where the probability distribution does not indicate one or more skeletal hypotheses as being probable state estimates.
19. A computer-readable storage medium as recited in claim 16 , said step (c) comprising the step of scoring the skeletal hypotheses and indicating that no state estimate is determined for the flame of image data where no skeletal hypothesis scores above a predetermined threshold value.