QR error correction helps and hinders scanning (2021)

https://huonw.github.io/blog/2021/09/qr-error-correction/

by lucb1e • 3 years ago

78 45 3 years ago

huonw.github.io

lucb1e• 3 years ago

Combined one of the section headings in the text, which basically gives away the conclusion, with the caveat that it's about undamaged QR codes. As you can expect more damage (such as when tattooing one, or paper that goes around), higher error correction does make sense; same when overlaying graphics/logos.

I found this surprising. I'll be displaying QR codes on a screen, so for me L easily makes the most sense in order to be able to make it larger with fewer squares inside of it.

cratermoon• 3 years ago

The airline industry uses other kinds of 2D codes, like Aztec Codes[1] or Imagen PDF417 on your boarding passes, including the digital ones you have on your phone. For example: https://fwhibbit.es/wp-content/uploads/2017/07/3-1.png

1 https://en.wikipedia.org/wiki/Aztec_Code

sbierwagen• 3 years ago

The barcode on the back of North American driver's licenses is also a PDF417. Amusingly enough, it encodes all the text on the front of the license. Name, DOB, weight, height, etc. Retailers that scan your license for alcohol age limit checks get a lot of data they don't technically need.

labster• 3 years ago

Finally, a justification for lying about your weight on your license.

chocolatkey• 3 years ago

I remember that the WeWork location I went to used to scan (and likely siphon/store) all that data as part of the front desk check-in. And they really didn't need it either, but were very insistent on doing so before letting someone know I (a guest) was in the lobby

ProllyInfamous4• 3 years ago

ProTip™: US Passports are ACCEPTABLE FORMS OF ID (in all US Jurisdictions), and are not able to be scanned for these types of purchases.

MauranKilom• 3 years ago

The part that makes higher error correction even more pointless is that errors can only be tolerated well in certain parts of the QR code. If the position markers are damaged, a scanner likely won't even recognize that there is a QR code visible, which obviates any error correction. The article mentions this in the very last sentence/caveat.

It feels a bit like the error correction exists mainly so you can embed fancy logos in your QR code... Avoiding damage to some parts but not others seems unaligned with real world scenarios...

charcircuit2• 3 years ago

Since the position markers are constant they should be the easiest to recover if damaged. Considering how advanced computer vision is, a QR code can be identified without using them.

londons_explore• 3 years ago

Agreed - QR recognition/alignment algorithms can clearly be improved further, and then when they are, higher error correction codes ought to be more reliable as designed.

cbzoiav• 3 years ago

It's not a case of can they be. It's a case of can they cheaply be.

The camera app on your smartphone likely searches for them by default. That means feeding at least 1-2 frames a second through it.

Basic pattern matching for the position markers is a rounding error overhead vs processing a video feed. Fancy computer vision is not - on lower end devices you may not have enough resources to do both and on higher end devices it'll still impact power usage.

p3n1s2• 3 years ago

It feels a bit like the error correction exists mainly so you can embed fancy logos in your QR code

Absolutely not the case. QR codes, like most 2D matrix codes were designed for logistics and supply chain, their leap into the consumer mobile phone arena was an interesting one, but not the original design intent at all (QR was developed at Denso, the automotive parts and logisitics arm of Toyota). The purpose for the error correction is so that the symbol can still be read on a label even if part of it is misprinted or obliterated, it works very well in practice. Industrial scanning software is mature technology, and more sophisticated than you imply (besides, segmenting and aligning barcodes, any barcode, in greyscale images is pretty simple). Position markers are easy to reconstruct and damage can be tolerated.

barbegal• 3 years ago

It's the same for wireless protocols too. Transmitting with less bits per symbol makes the whole packet longer and potentially more prone to interference. Transmitting at a higher symbol rate can often reduce the amount of packets lost which is counterintuitive.

lettergram• 3 years ago

Why are we even using black and white tags?

https://austingwalters.com/chromatags/

charcircuit2• 3 years ago

Because printing binary black and white is cheaper to print and cheaper to read.

1letterunixname2• 3 years ago

That was the case in 2005, but now nearly every camera in public and industrial use supports color except save some in particular logistics uses that would need upgrading. The only reason it remains is legacy and convention. Yes, it requires different printers, but that's not an insurmountable supply chain change, especially if it's reduced to a few distinct colors that don't need to be mixed. This is a missed opportunity to encode more data into a given area and/or to make it more readable.

scrollaway• 3 years ago

Chromatags probably have their uses, but when put in practice like qr codes are, they don't have most of the "features". Monochrome means you can use cheap/shitty printers, you can theme them to go well with your brand, etc.

femto• 3 years ago

Same reason we use binary computers? The greater noise margin on each bit typically offsets the requirement to have more/smaller/shorter bits compared to a multilevel signal.

nwallin• 3 years ago

Printing. Printing is done in the CMYK color space. Cyan, Magenta, Yellow, blacK. When you print in color, it is basically four different printers printing onto the same piece of paper. Ideally everything is all lined up correctly, but if you want to do it on an industrial scale, good alignment costs time and money. Big printed color color posters tend to have shitty alignment. A color tag needs to have good alignment; if the cyan, magenta, yellow print runs are misaligned, chances are a color tag won't be able to read it. But the black run is always aligned to the black run. So it will always work.

Lighting. The Sun, incandescent, and halogen lights have pretty good color reproduction. But LED lights and fluorescent lights do not. LED lights and fluorescent lights are designed to look acceptable to human eyes. And they can be reasonably good at it. But cameras are not human eyes; color gets all fucky when you look at it through a camera.

Cameras. The tag you linked to uses the LAB color space; one channel is brightness, one channel is blue vs yellow, one channel is red vs green. Cameras almost always have BGGR arrays; on a 4 megapixel camera, you will have 1 million blue pixels, 2 million green pixels, and 1 million red pixels. So if you use a normal camera to read a tag in the LAB space, you have 4 million pixels reading the brightness channel, 3 million pixels reading the red-green channel, and 1 million pixels reading the blue-yellow channel.

kazinator• 3 years ago

The weasel word in the title is undamaged. But the story is actually about QR codes being damaged. The original image isn't damaged, but its impression is damaged in transit while being scanned. QR codes crammed with more information (such as more error correction) have smaller pixels for the same view angle, and those are more easily damaged during the scan. An undamaged QR code with large pixels (low information) might scan perfectly without any error correction at all, but as the information density increases, so does the potential for error (error not caused by damage in the QR code being scanned).

If we make a QR code so dense that the scanner cannot make out any of the pixels, it becomes completely unreliable, even if 80% of it is error correction bits.

Basically, error correction defends against processes that cause random erasure or flipping of bits. Error correction is not meant to defend against fundamental channel bandwidth problems.

vlovich1234• 3 years ago

Said another way if I understand correctly, erasure codes help as long as message + erasure fits into channel bandwidth. If it doesn’t, then erasure codes hurt. Is that right?

QR error correction helps and hinders scanning (2021)

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