“…Methods employing images [ 8 , 9 , 10 , 11 ] have likewise been proposed, as we can see from the fact that image processing is used in various fields [ 12 , 13 , 14 , 15 , 16 , 17 , 18 ]. These methods have gained superiority, as they do not affect the monitoring targets or the surrounding environment and allow for systems to be built at a comparatively lower cost.…”