I just came across an interesting nugget in a Slashdot article today, discussing why Photoshop refuses to handle images of US currency. Apparently, many high-end photocopiers also refuse to copy banknotes, but how do they detect when you’re attempting to do so? Image identification is a tricky process at the best of times.
As it turns out, there’s quite an elegant explanation. Modern banknotes include a little “constellation” of five circles in a unique pattern. It seems photocopiers (and now Photoshop) just have to look for this signature pattern to know that it’s a banknote they’re looking at. What I find fascinating about this is the way that the banknote designers have incorporated the constellation into their designs. You can see some examples in European notes here, and read Markus Kuhn’s more complete description in the extended entry. Apparently this technique is also used on the new US$20 note. (Can anyone spot it?)
The battle between currency producers and counterfeiters is an ongoing one, and becoming increasingly high-tech. My favourite anti-counterfeiting solution still remains the plastic notes from Australia, though.
Markus Kuhn writes:
For those of you curious about how this algorithm detects a banknote, here is a slide of a short talk that I gave to our local research group soon after I discovered the “EURion Constellation” two years ago while experimenting with a new Xerox color photocopier and a 10 euro note.
The algorithm looks in the blue channel of a color image for little circles and most likely examines the distance distribution encountered. I have discovered a small constellation of just five circles (a bit like Orion with the belt starts merged) that will be rejected by a Xerox color photocopier installed next door from here as a banknote. Black on white circles do not work.
These little yellow, green or orange 1 mm large circles have been on European banknotes for many years. I found them on German marks, British pounds and the euro notes. In the US, they showed up only very recently on the new 20$ bill. On some notes like the euro, the circles are blatantly obvious, whereas on others the artists carefully integrated them into their design. On the 20 pound note, they appear as “notes” in an unlikely short music score, in the old German 50 mark note, they are neatly embedded into the background pattern, and in the new 20 dollar bill, they are used as the 0 of all the yellow 20 number printed across the note. The constellation are probably detected by the fact that the squares of the distances of the circles are integer multiples of the smallest one.
I have later been told that this scheme was invented by Omron and that the circle pattern also encodes the issuing bank.