You are certainly to the first person to need this.  I would spend
some time looking for reasonable ways to implement this. Some ways may
involve very sophisticated, pinpointing key spots in the image and
seeing where they go. A simpler solution is just to team your camera
up with  a motion detector, that uses ultrasound, IR, sound detection
etc.

Pretty well anything you cook up on your own without mathematically
background is like to be both slow and inaccurate.  You want to be
able to calibrate it for sensitivity.  You don't want wind wiggling
leaves to trigger it.