I tried reading them manually about ten years ago, but gave up because of the mask. The mask pattern needs to be applied (xor) before you can even tell as much as the data type...
Does anyone know how much it actually helps QR code readers to have this mask pattern applied?
Last month, I tried to find out what the optimal error correction setting is (answer: none, if the matrix isn't damaged, like in my use-case where it's shown on a screen), which wasn't easy to find. I didn't find anyone who actually went out into the real world and tried out different settings and different scanning implementations to see what the effect is of choosing a good mask or choosing a good error correction value. In the end, a theoretical answer was enough for my application because I can assume the QRs to be pristine, but in a perfect world we also wouldn't need a mask setting. I then tried to do some research of my own by reading a small QR off my screen with different error correction levels and an intentionally shaking camera to simulate read issues when someone points a smartphone, but found no significant difference between any of the settings. With there being at least four alignment markers, a huge quiet area, and a "timing pattern", does it really matter if there are white blobs due to not having a mask?
(The secondary reason why I gave up trying to read QRs visually is because I noticed they always have the URL written below as fallback anyway. Since then, I did see a few times where there was no fallback, but it's very rare)
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...
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.
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.