I also learnt the dubious skill of reading QR codes by hand. At work 1% of the QR codes that we received were not scannable. However when decoded by hand (thanks you this guide and others) they all contained the string we expected.
But there was an error in the error correction pattern. It would flip a bit on the message length. The QR data changed from a string of 4 characters to a string of 933+ characters, and either
- Be unreadable (xzing)
- Return 933+ characters of garbage (cognex)
- crash
- return the previous barcode scanned (iOS)
I really learnt something about Reed Solomon error correction that day. I have the QR code framed somewhere. Sometimes when bored I’ll scan it and confirm that it still brings up whatever I scanned last.
But there was an error in the error correction pattern. It would flip a bit on the message length. The QR data changed from a string of 4 characters to a string of 933+ characters, and either
- Be unreadable (xzing)
- Return 933+ characters of garbage (cognex)
- crash
- return the previous barcode scanned (iOS)
I really learnt something about Reed Solomon error correction that day. I have the QR code framed somewhere. Sometimes when bored I’ll scan it and confirm that it still brings up whatever I scanned last.