My QR code generation library works well, but one of the features was generating a "colour" QR code. No, QR codes are not normally coloured, the idea is to just show the anatomy of a QR code, which parts are which. Wikipedia does a good job too.
As part of generating the code I have to create the data part, and padding, and then generate the error correction code (ECC) part, and apply various format control bits and fixed black/white units to make the image.
I have updated the library so it will make a QR code which shows what is what. It was a tad complicated by the fact that the error correction code is interleaved. This means that blocks of data and ECC are scrambled so that each block is actually spread out over the QR code. This means you can remove a chunk of the code - e.g. tear off a corner, and that is a small part of several separate blocks. You will notice the padding (green) below is spread out because of this interleaving.
Each block of data and ECC allows recreating of the data from a relatively small part of the overall block, so the distinction between data and ECC is not that relevant. But the colour coding shows how much is used for what quite nicely even so.
This has colour coding for :-
- Blue: the actual data for the content
- Green: padding bytes and bits
- Red: the ECC code
- Grey: formatting/control
- Black/White: the fixed pixels in this size QR code
So that is it, just in case you wondered...
(That one above is an NHS COVID-19 QR code).
P.S. I have been having more fun with custom padding bytes. This is not just changing units within the tolerance of the ECC (which can be done, if careful), it is changing padding so that the ECC is still valid.
Of course, if you want to get a bit meta, you can put one type of barcode in the padding of another type. This is a Datamatrix barcode in a QR barcode. A real Frankenstein barcode!
Technically the padding is meant to be a repeating pattern EC/11, and surprisingly this is not just a recommendation in the spec. But obviously nothing checks the padding on read, so allowing pretty padding and hidden data in the padding to work.