A computer system processes a video stream to detect a start of a first motion event candidate in the video stream, and in response to detecting the start of the first motion event candidate in the video stream, initiates event recognition processing on a first video segment associated with the start of the first motion event candidate. Initiating the event recognition processing further includes: determining a motion track of a first object identified in the first video segment; generating a representative motion vector for the first motion event candidate based on the motion track of the first object; and sending the representative motion vector for the first motion event candidate to an event categorizer, where the event categorizer assigns a respective motion event category to the first motion event candidate based on the representative motion vector of the first motion event candidate.