Thanks, that's it! Actually it is not hard to unlock the phone by double tapping it (albeit you also have to swipe afiter that), and I wouldn't use Qt Facelock myself, but the code for face recognition could be (or not, I'm not a dev at all) useful to make the front camera prevent lock screen when you are staring at the screen of the N9.

The whole point lies in the difference between unlocking and preventing lock under certain conditions (automatically). Unlocking the device is just neat as it is, but preventing the phone to lock when you watching it, without requiring a tap every 20 sec, would be useful.