RM

1. setup bk, this is an easy one coz the region is simple compared with others
2. Get edge inside monitored region. For each frame, white/black >a%? too many edges, need to change th. 3. Pixel based check all around one edge pixel, changed perc >%? Candidate target now, but removed or blocked?
4. check motion. if the corresponding part has motion there, it is a block.