Line crossing detection
Line crossing detection, also called a virtual tripwire, draws a line on the camera view and raises an alert when a tracked person or vehicle crosses it. The line can be directional, so entering triggers and leaving does not, which is what makes it useful on a fence, a gate or a threshold.
The line is drawn once per camera over the boundary it protects. On every frame the tracked subjects are checked against it, and a crossing in the configured direction is the event, with the frame and the time.
Its strength is precision: a road beside the fence is on the other side of the line and never triggers it. Its limit is that it protects one crossing point, so an area with several ways in needs a zone rule instead.
On Camzify it is line intrusion detection, licensed as an instance per camera with a notification window, and a failed check on a round when a line has been crossed since the last stop.