This is a fantastic idea by Neue Digitale / Razorfish Berlin and it has been executed to perfection for Audi.
In 2010 Audi didn’t just create another beautiful calendar, but they created an unexpected experience! They printed a calender purely as landscape images, blank, no cars, nothing, and then prompted people to download and open the special Audi iPhone app while pointing the handset’s camera at the calendar, and voila, a car would fill up the empty space on the page!