The QR Code (or Quick-Response Code) is spread widely in Japan and is the most popular open 2D-Code for Mobile Tagging.
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Example
Generator
Twitter about QR Codes
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Scanning
Scanning a QR Code is easy.
Capacity
In theory a QR Code can store up to 4296 characters. For Mobile Tagging 40-60 characters are feasible (depends on the physical size of the QR Code). Otherwise average phones will not be able to detect them anymore.
QR Code with 20 characters
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QR Code with 80 characters
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QR Code with 200 characters
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QR Code with 400 characters
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Pitfalls
Using QR Codes for Mobile Tagging has some pitfalls, that you should definitely avoid.
Most problems arise because the tester has a good phone (like Nokia's N95) with a high quality camera, but the actual user has an average phone with an average camera.
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Whitespace / Quitezone
Make sure you leave an empty space (marked in red) of around 4 modules on each side.
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Amount of data
You should not encode more than 60 characters in a QR Code. Otherwise detection will be too bad with average phones (see "Capacity").
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Color
You should always use black and white. If contrast is too low code detection will be impossible.
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Design
Some people add logos in the middle or change colors. You should not do that (although it looks very cool) as detectability goes down and people will have difficulties to recognize it as a QR Code.
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Size
The code should never be smaller than 2cm (with ~40 characters encoded). The more characters you encode the larger the code has to be. But you should never encode more than 80 characters.
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Blogs about QR Code
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