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The error correction is funny. If you don't use it, your message takes up less space, and the QR is smaller. If you have a fixed space to fit it in (e.g. printing to a sticker), that means your pixels can be physically bigger. For reliable reading, I found that physically bigger pixels helped more than the error correction itself.

Depending on the length of your message, sometimes you can turn up the error correction level without impacting the final QR dimensions, and in those cases it's a win/win.

This is actually "cameras can't focus close, and QR scanners aren't good at seeing a code which is tiny in the frame".

By making the pixels of the code bigger, the camera can be a bit blurry to get the code big enough, and then it'll read.

I mean, that's kinda the whole point? The precise details of the optic path don't really matter, bigger pixels will always be easier to resolve.

One might reasonably expect error correction to help with the decode of blurry inputs, but that wasn't really the case in my experiments.

The 'proper' fix is to either engineer cameras that can focus closer, or to engineer barcode scanning apps to be able to see a code which is small somewhere in the frame.

I think people are working on both as we speak.

It doesn't matter how objectively good your camera/decoder is, in absolute terms. There will always be some minimum size or maximum distance, where the detail can no longer be resolved (or errors no longer corrected)

Why? Is this something that's a side effect of some irrefutable physics law or a limitation of the current technology?

Both. You can't have a measurable feature smaller than the Planck length, but that's not relevant to QR decoding in any practical sense.

As for the practical limits, try using your phone to take a picture of something very far away, or very tiny, and see what happens.

If we're talking theory, the pixel size doesn't matter - it's the amount of information encoded in the QR code that matters (ie. excluding ECC bits!). In turn, that means it is theoretically possible to decode a QR code where each 'pixel' of the code is smaller than the pixel of the camera sensor observing the code.

While it is theoretically possible, nobody has yet done so...

Physically bigger pixels help a lot indeed. The encoding mode also helps make pixels bigger.

This is very convenient when you control the QR reader and need to represent long numeric identifiers like UUIDs.

For example:

  9728983f-7d7d-4189-b624-f92781e36650 (lowercase UUID):
    => length=36, 15 pixels between markers
  JWM9GFVXFN0RKDH4Z4KR3RV6A0 (base32 UUID):
    => length=26, 11 pixels between markers
  9728983F-7D7D-4189-B624-F92781E36650 (uppercase UUID):
    => length=36, 11 pixels between markers
  200924207194334734815443970355691218512 (decimal UUID):
    => length=39, 7 pixels between markers

The uppercase UUID has bigger pixels because it used a different encoding, and gets the same results as the shorter base32 uuid.

The decimal UUID is a longer string, but results in much bigger pixels because it can use numeric encoding.

I have a QR code base attendance tracker [1], where attendees show the code [2] on their phones (glares, etc.), in bad lighting conditions, etc. Bigger pixels means scanning stays quick. Same with parcel tracking [3] where field agents might need to scan QR codes in barely-lit hallways, etc.

[1] https://workspace.google.com/marketplace/app/qr_code_pass_fo...

[2] https://share.darkaa.com/!qntvtzNPWJ

[3] https://admin.trak.codes/

You can also use base45 [1]:

  -4J5BJ+%F$C881NIMV-IG2.C (base45, 132 bits in QR)
  JWM9GFVXFN0RKDH4Z4KR3RV6A0 (base32, 143 bits in QR)
  200924207194334734815443970355691218512 (decimal, 130 bits in QR)

It will make use of all allowed characters in the alphanumeric mode, while being significantly shorter than base32 and as dense as decimal. And decimal encoding in general needs bignum, because there is no suitable 10^k which is only slightly larger than powers of two so no convenient binary-to-decimal encoding exists (conversely, QR code itself does make use of the fact 2^10 is only slightly larger than 10^3 for this mode). Base45 always works on three-byte groups and maintains the similar efficiency in comparison.

[1] https://datatracker.ietf.org/doc/html/rfc9285

That's neat, especially for arbitrary binary data!

The error correction isn't meant for folks like us who think about things like "how can I make this message smaller to increase the readability of the QR code", it's for the people that take their entire tracking parameter-infected URL and dump it into a version 25 (117x117) code and plaster it onto a huge sign out front where a bird promptly shits on it and a teenager slaps a sticker on it and somehow the data remains.

I also have the impression that a common use of error correction is the "QR Code with a logo in the middle" style. With an appropriate level of error correction, you can carve an empty patch in the middle of the logo and slap a logo/icon there, and the QR code remains readable.

At least this is the way I did it years ago when I thought it would be fancy to have my contact data as a QR Code on my visit card.

Yes, though I doubt that’s what the creators had in mind.

Last year I did some experimentation along these lines and I also found that no error correction was best, at least when 3d printing QR codes. I was trying to find how small I could print it and still have it be detectable. Like the sibling comment, I suspect it is because of individually larger pixels.

The problem with that, as I understand it, is that your 3d print (or any print or screen) cannot get damaged at all. The pixels being bigger makes it easier to scan ...when it's fully in view and fully intact. Any pixel fails, and you can never recover the original data (besides some guesses, e.g. if the protocol identifier gets damaged a la htwps://)

Error correction doesn't seem to be for easier scanning, but for errors that might occur

(... and stupid logo overlays)

but this information is annoyingly hard to find. Hence my question about the mask xor overlay: does that actually help? Did anyone try it, or did it just sound like a good idea?

the idea that a single pixel of the design would get damaged absent the rest seems unlikely. What seems to be more common is that a direction of light/some flaw in the printing process causes many/all pixels to be flawed in the same way past some threshold.