“…For instance, some methods are based on motion analysis: in [3] a technique based on the observation of the motion of particles is described; such particles are initially evenly placed over the image domain, and then moved according to optical flow. In [4] a technique based on motion heat maps is presented, together with a set of indicators for measuring motion entropy and, finally, classifying motion as normal or abnormal. Finally, in [5] a method is presented, that is capable of detecting the precise contour of a crowd.…”