> So is the rest all content? Not quite! There is also error correction, which is used to make sure that the QR code can still be read even if some parts are damaged, blurry, or missing. The error correction is generated by some fancy math, and we don't care about it here for the purpose of reading it by hand.
This should be explained better... I mean sure, the math can be hard.. but is the error correction appended at the end? After every byte? In the middle? Can we still read a qr code by hand if it has error correction (assuming it's not damaged, and we can skip the math part?)?
Barcodes use the last digit as a checksum. The checksum is some adding up the numbers and multiplying or something, then % 10 (only 0-9 possible values). As you can see with this math, the error checking has a 1/10 of a chance of coincidentally showing a correct checksum in cases where the data is incorrect. Either way, that last digit on barcodes is insignificant to the actual product's unique barcode ID.
Fun story: The barcode was ripped and the checksum # was frayed off of an item I was returning to Home Depot. I frantically tried to do the math quicker that the checker could manually look it up. But they won and I'll never forget it.
A few common barcodes including UPC work this way but mostly this is the exception. The 2D ones being discussed here like QR and Aztec use actual error correcting codes, so none of them. In the linear space Codabar (libraries) is a widely used example that doesn’t. GS-128 doesn’t, etc.
When I worked at a grocery store in the 2000s, you didn't need the checksum digit when keying in a barcode by hand. In fact, manually entering the checksum digit would result in a "product not found" error.
So I wonder if the checker had to manually look anything up, assuming it was only the checksum digit that was frayed off.
Perhaps some POS systems do require the checksum digit for manually-entered barcodes, though.
Barcodes are entered as text. Older barcode scanners used to plug into either the serial port or the PS2 keyboard port; often they had an inline adapter so you could plug it in between your computer and keyboard.
As a checksum, the last digit of the barcode is not part of the data (product number), it's metadata; when the barcode reader beeps, that means it has computed the check digit and it matches the digit on the barcode. The barcode data is sent to the computer, but the check digit is not. The check digit is not in the database. The computer using the UPC leaves it to the barcode reader or human to ensure the input is correct.
You'll might also notice that similar barcodes exist in multiple places as well--unopened cases of products might have an identifier that includes the UPC, and so does the price sticker on the shelf. Both of these include more data than just the UPC.
It is possible that a check digit could happen to be incorrect from the beginning. This (and more rarely, an accidental code reuse) does happen fairly commonly with ISBN, so I would expect the same for general UPC/EAN.
As a grocery checker, I also remember not typing the checksum. It isn't part of the product's ID, but manually typing in probably has a higher chance of error than scanning so what the heck??
Another fun fact: in the intro to The Simpsons, Maggie gets scanned by the clerk as a valid item with a price shown on the register.
the data in the code is encoded as a bitstream using possibly different alphabets, byte is just one of them.
Then, depending on the size and error correction level (not covered in the article where this is decoded from) the bitstream is split up into blocks, ecc data is calculated for both, and then the blocks are interleaved.
So this method will fail for any qr version above 5 for any level of ecc, and 3-5 with high levels of ecc.
An Artisanal QR Code [1] shows how to compute the checksum by hand using arithmetic similar to a long division. There are actually two slightly different codes (Reed-Solomon and BCH) for different pieces of information.
This should be explained better... I mean sure, the math can be hard.. but is the error correction appended at the end? After every byte? In the middle? Can we still read a qr code by hand if it has error correction (assuming it's not damaged, and we can skip the math part?)?