关键词: 背景差分法, 感兴趣点, 光流法, 熵, 聚众事件

Abstract: With the need of security surveillance，the detection of abnormal events in video surveillance has been paid more and more attention.This paper proposes an approach of detecting gathering abnormal events.Its focus is optical flow changes of the points of interest.When foreground obtained by background subtracted is more than the setted threshold and the distance among objects in a certain range，the points of interest are extracted，using the optical flow method tracks the points of interest then caluates entropy，once the entropy is greater than the setting threshold，it is judged to report a gathering event and to give an alarm；otherwise，this frame’entropy is set a fixed value，continuing processing next frame of the background subtracted.Its effectiveness has been verified through the real shot of the video sequence.

Key words: background subtracted, points of interest, optical flow, entropy, gathering events