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The finder pattern and error correction bits mean you can deviate a lot from the standard squares. It works best if you don't do that but low density QR codes are so quick/easy/reliable to scan even with mediocre implementations that you can get away with a lot of evil.
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Replying to and
Most QR codes use the lowest level of error correction but if you turn it up to the highest level, it can cope with 30% of it being covered / cropped away. Can also get away with sticking fake data around the edges, especially if you don't do all of them. Can happily use circles.
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