A QR Code is a matrix (two-dimensional) barcode. Arbitrary content may
 be encoded, with URLs being a popular choice :)
 .
 Each QR Code contains error recovery information to aid reading damaged
 or obscured codes. There are four levels of error recovery: Low, medium,
 high and highest. QR Codes with a higher recovery level are more robust
 to damage, at the cost of being physically larger.
 .
 The maximum capacity of a QR Code varies according to the content encoded
 and the error recovery level. The maximum capacity is 2,953 bytes, 4,296
 alphanumeric characters, 7,089 numeric digits, or a combination of these.