1. A monitoring system for monitoring a person on a bed, the system comprising a camera for obtaining a set of images of a person on the bed; a motion vector estimator with an input coupled to the camera, configured to determine motion vectors as a function of location in each successive image of the set of images obtained during a time interval; a signal processing system with an input coupled to the motion vector estimator; wherein the signal processing system is configured to determine, within each successive image, (i) a map of image locations for blocks in each respective image associated with a body area of the person on the bed and (ii) a direction of a major axis of the body area on the bed from the determined map of image locations, wherein the direction is determined by a size of the body area on the bed along that direction of the major axis being larger than a size of the body area on the bed in all other directions, or at least not smaller, and wherein the direction of the major axis of the body area on the bed is adaptable in response to deviations of the major axis from a longitudinal direction of the bed; compute a measure of turning motion of the person on the bed, wherein the computation of the measure of turning motion of the person on the bed comprises (i) using (i)(a) the direction of the major axis of the body area on the bed and (i)(b) components of the determined motion vectors of each block in each respective image that are normal to the direction of the major axis of the body area on the bed and (ii) subsequently summing these components over the blocks from the respective map of image locations associated with the body area of the person on the bed within each successive image of the set of images; and detect pose state changes of the person on the bed based on the computed measure of turning motion of the person.