I have tried all the events detection mechanisms that the firmware 5.1.6 has, and all of them miss a lot of events. They cannot keep sending emails even on a continuous motion present. Therefore I as DelBoy (and a lot of other users) do not trust the reliability of the 2CD2332 firmware 5.1.6 motion detection and I have reverted to continuous recording and motion detection for the email alerts. The line detection could be very usefull if it was reliable. Instead when I draw the area in the motion detection panel, I draw the perimeter of the area I want detected. In this way I imitate the line detection but with a thicker line instead of the thin line in the line detection.
Good to know this tip with the motion detection. I just have the whole thing enabled like it came from the factory. My cameras are using firmware 5.2.0 if that makes any difference.