Demo video right here: https://dl.dropboxusercontent.com/u/2251898/SidLee_Knock.m4v

Only works on Chrome and Firefox (uses getUserMedia), should only be used on non touch devices.

It analyses the translation of the image of your webcam to detect if your screen has moved.

Best usage conditions:

- laptop with webcam on top

- laptop on a flat table (not on your laps)

- decent luminosity

- make sure you're not sitting before a monochrome background