[fartfeatures]

Point the camera at the door, use the camera to detect if the door is open.

[grahamj]

Mount the camera on the door facing down and watch for a moving floor

[fartfeatures]

That's a clever solution that would use less processing power. A camera pointed at the door provides the benefit of being useful for other purposes e.g. person detection / recognition and standard security camera purposes.

[lazide]

Until someone accuses you of looking down their shirt anyway.