blocking zones of a surveillance system are configured to prevent false alarms caused by motion within the zones , while allowing tracking of objects through the zones . an object that first appears within a blocking zone is not considered to be a reportable object until the object leaves the zone . all reportable objects are tracked , without regard to the blocking zones . objects that remain within their initial blocking zone are not deemed to be reportable objects , and therefore do not generate alarms . if an object initially appears within overlapping zones , the object is not deemed to be reportable until it leaves each of the zones at least once . the blocking zones do not mask the video images , and thus a complete record of activity is available for forensic purposes .