Only applied to the scene where the targets are sparse and there is rarely any covering between targets, such as the perimeter without any guard. It will trigger alarm when the target passes through the warning line according to the set movement direction. It takes some time from the target appears to the target is confirmed, so it needs to leave some space when setting warning line, do not set it near the covering. Detect if there is any object crossing the warning line, it can support different direction selection, such as A->B, B->A and bidirectional, and it can activate record , snapshot and alarm according to the judgment result.