[nryknzlu] Communicating with blinking lights and a camera phone

Send data with a smartphone by blinking its camera flash and receive data by recording flashes with the camera.

Consider the difficult case where the camera sees many blinking flashes (many senders in its field of view) but is only interested in one of them with which it has pre shared a secret.  We need a robust protocol: both the sender and the receiver need not hold their device perfectly steady.

You could just call each other, but that would be too easy.