How does a QR code work?
What look like small squares are a way of storing information. Let's take a QR code apart to understand how your phone reads it — and what actually changes when you update its destination.
From camera to information
You point your phone at a QR code on a table. The camera recognises the pattern and a link to the menu appears. Between those moments, the reader locates the code, identifies its grid and decodes the information represented by the light and dark modules.
Each small square is a module. Modules follow positioning rules: some help read the symbol, while the rest carry the content and error correction information. When the content is a web address, the phone can offer to open it.
A QR code stores the address, not the entire page. Reading an address and opening a website are separate operations: the first interprets the symbol; the second needs to reach the website.
What is inside a QR code?
In this illustration, the parts separate to show what each one does. In a printed code, they all sit on the same plane. You can take it apart, put it back together and select a function to highlight it.
- The three large markers help the reader find the code and determine its orientation.
- Alignment and timing patterns help locate the modules, even when the image is captured at an angle.
- This information identifies the error correction level and the mask applied to the data.
- The remaining area carries the encoded content and the redundancy used to recover data.
- A clear border four modules wide separates the symbol from its surroundings and helps readers recognise it.
The example's structure and matrix follow the open implementation by Project Nayuki. DENSO WAVE explains the quiet zone required around the symbol.
Why can it still work with some damage?
The code includes redundancy: extra information that helps recover data when part is lost or read incorrectly. There are four error correction levels — L, M, Q and H. Recovery depends on the level chosen and the damage encountered.
This does not mean you can cover any percentage of the pattern and guarantee a successful scan. The location of damage, affected patterns, size and contrast also matter. A central logo occupies space that is no longer available for reading modules.
Treat error correction as a tolerance margin and always test the final result. The technical explanation from DENSO WAVE on error correction describes recovery at the data level.
If the pattern is fixed, how can the destination change?
The content encoded in the symbol stays the same. A QR code containing the final address has that address fixed in it. If it is no longer useful, you cannot remotely rewrite the squares already on paper, a tile or a plaque.
A QR code with an editable destination contains a stable intermediate link. When someone opens it, a service looks up the associated destination and redirects them there. That association is what you can change in your account.
- Printed QR codeThe pattern stays the same
- Intermediate linkThe address stays the same
- Destination pageThe menu can change
Example: the QR code's link stays the same while its destination changes from the summer menu to the winter menu.
This is the principle behind AllMyQR: letting you update the destination when replacing the QR code on an object is difficult. A new address goes through the applicable checks. While it is pending, the previous approved destination remains in place, subject to the account's status and limits.
There is something to consider before making the object: redirection depends on the service, domain and plan terms. A code on a long-lasting object is not a guarantee of perpetual operation. Our website explains the work in progress on custom domains and portability.
Before making it physical
An editable destination cannot fix a poorly applied QR code. Before ordering a print run, engraving a plaque or finishing a piece, make a sample:
- Leave a clear margin. Keep clear space around the code, with no text or other elements intruding into that area.
- Check the contrast. The reader needs to distinguish light modules from dark ones, including in the lighting where the object will be used.
- Test the final size and material. Scanning on a screen is no substitute for testing print, glass, wood or your chosen surface.
- Try more than one phone. Check scanning at the expected distance, from different angles and with the object in place.
- Open the link. Confirm the address leads to the intended content and that you understand the terms for maintaining redirection.
The squares stay on the object. Choosing their link carefully is what lets you decide, later, where they will lead.