1. A method of monitoring selected zones in a scene depicted in a video stream: receiving a definition of a zone of interest within the scene depicted in the video stream; and in response to receiving the definition of the zone of interest: determining, for each motion event detected in the video stream, whether a respective event mask of the motion event overlaps with the zone of interest by at least a predetermined overlap factor; identifying the motion event as an event of interest associated with the zone of interest in accordance with a determination that the respective event mask of the motion event overlaps with the zone of interest by at least the predetermined overlap factor; and generating the respective event mask for the motion event, wherein the generating includes: creating binary motion pixel maps for frames of a video segment associated with the motion event; and aggregating the binary motion pixel maps of frames of the video segment to generate the respective event mask for the motion event.