1. An object activity modeling method comprising the steps of: (a) obtaining an optical flow vector from a video sequence; (b) obtaining a probability distribution of a feature vector for a plurality of video frames, using the optical flow vector, wherein the feature vector is an d×L dimensional vector, d being a number of dimensions and L being a number of pixels in a video frame or in a region of interest; (c) modeling states, using the probability distribution of the feature vector; and (d) expressing the activity of the object in the video sequence based on state transition.