QR codes on beer glasses

The theme is half friday and half geeky. So, the American advertising agency BBDO New York has placed a white QR-code on branded glasses of Guiness beer. A link from the code leads to the corporate page, from where you can tweet about another drunk pint or update your status on Facebook (it also mentions downloading some exclusive beer content, but this is not so much fun)



The most important feature of the idea is that the code is visible only when the glass is filled with black Guiness (we assume that there is no other dark one). In the case of the usual light beer, the QR code is simply unreadable.
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It is clear that all this is being started for the sake of increasing Guiness's citation in the social. networks, but this idea never ceases to be interesting.