1. A monitoring system for monitoring a person on a bed, the system comprising a camera for obtaining 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 the image; a signal processing system with an input coupled to the motion vector estimator; wherein the signal processing system is configured to determine image locations associated with a body area and determine a direction of a major axis of the body area from the determined image locations; compute a measure of turning motion, the computation comprising summing components of the motion vectors that are normal to the major axis from respective locations within an image; detect pose changes based on the measure of turning motion.