Step 4. Draw detection line (the yellow line as shown in the below picture) or detection region.
Detection Line: the yellow line should be within the blue detection region.
Detection Region: Draw A and B region. Make sure the two areas don’t overlap. If the target
enters from A region to B region, then it is counted as the entering number; If the target enters
from B region to A region, then it is counted as the exiting number.