QR code guide
What I learned while building this tool, written down. Enough to work with QR codes without reading the spec yourself.
How it works
How the blank space works
Most of a QR code is meaningless padding. Cancelling the mask over it turns that region white. The method and the measurements.
Anatomy of a QR code
Corner squares, timing line, alignment patterns and format strip, each pointed out on a real code.
Choosing an error level
What L, M, Q and H actually change, with numbers, and which to pick for screens, print and outdoor use.
What a QR code can hold, and how much
URLs, Wi-Fi logins, contacts, messages. Why the same size code holds several times more or less depending on what you put in it.
In practice
Making it scan reliably
Knowing the order a scanner works in tells you what is safe to restyle. Concrete limits for colour, size and print dimensions.
Making a QR code in colour
Scanners ignore hue and only see brightness. Which colour pairs work, which fail, and what happens when you invert the code.
Putting a logo in a QR code
Why a code still scans with a logo on top, and where that stops working. How large it can be, and what it must never cover.
Before you send a QR code to print
Why a code that works on screen fails on paper. File format, black settings, minimum size and test prints, as a checklist.
Static versus dynamic QR codes
How codes whose destination you can change later actually work, what that convenience costs, and why printed codes sometimes die.
Avoiding QR code scams
Fake codes on parking meters, restaurant tables and letters. What to check before you scan, and what never to do after.