QR Code
Definition
A QR code is a two-dimensional barcode made of black and white modules in a square grid, with three large locator squares at the corners so a camera can find it and work out its orientation. It carries data rather than an image: a web address, a phone number, a wifi configuration or plain text. Capacity depends on the version and the character set, but a printed code comfortably holds an address of a hundred characters or so. Because a shorter address makes a sparser, easier-to-scan pattern, a code is often generated from a short link rather than a long one. Error correction is built in at four levels, and the highest lets roughly thirty per cent of the code be damaged or covered and still decode.
Why It Matters
A QR code closes the gap between the physical world and a web page in about two seconds, which a printed address never manages. When it fails, it fails completely and silently: a code printed at one centimetre on a poster read from three metres will not scan, and nobody will tell you. The destination is baked into the pattern, so a code on five thousand flyers cannot be corrected afterwards — unless it points at a stable address whose contents you can replace. That is the entire argument for aiming a code at something you control.
How It Works
A generator takes your text, encodes it into the module pattern with Reed-Solomon error correction, and outputs an image, usually PNG or SVG. The camera finds the three locator squares, corrects for angle and distortion, reads the modules and hands the decoded string to the operating system. If the string starts with https the phone offers to open it, and the browser then makes an ordinary request to the host. Nothing about the code is dynamic — it is a static picture of a string — so the only ways to change where it leads are to change what the address serves or to put a redirect in the middle. Print size matters too: a rough rule is that the code should be about one tenth of the distance it will be scanned from.
Real-World Example
A gallery publishes its exhibition catalogue as a PDF at nightfall-catalogue.99helpers.site and prints a QR code for that address on the wall label beside each work. Visitors scan, the catalogue opens on the phone, and the analytics show 340 opens in the first week. When a typo turns up in an artist biography, a corrected PDF replaces the old one at the same address and every printed code keeps working.
Common Mistakes
- ✕Printing the code too small for the distance it will be read from — a two-centimetre code in a shop window seen from two metres simply fails
- ✕Encoding a very long address with tracking parameters attached — the pattern becomes dense, and dense codes fail on cheap cameras in poor light
- ✕Inverting the colours or dropping the code onto a busy photograph — decoders need light modules lighter than dark ones and a quiet border of at least four modules
Related Terms
Short Link
A short link is a compact URL that redirects to a longer one. It exists to be typed, printed, read aloud or squeezed into a character limit.
Link Sharing
Link sharing means sending someone a web address that points at your file, rather than sending a copy of the file itself. One address, one source of truth, any number of recipients.
Permalink
A permalink is an address meant never to change, so that everything pointing at it keeps working. The permanence is a promise from whoever owns the address, not a property of the web.
UTM Parameter
A tag added to the end of a URL to record where a visit came from. Analytics tools read the tag and group the visit under the campaign you named.
PDF Hosting
Publishing a .pdf at a web address so it opens in the reader's browser rather than landing in their downloads folder. The header the host sends decides which of those happens.
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