QR Code Guides

Seven guides sit in this category. Most circle the same gap: the code scanned fine, and then nothing happened.

A QR symbol carries text and nothing else

ISO/IEC 18004 specifies module layout, masking, and Reed-Solomon error correction. It says nothing about meaning. A decoder hands back a string, and every action after that belongs to the software holding it. A payload starting https:// is offered as a link, tel: goes to the dialler, BEGIN:VCARD goes to Contacts. With no rule for the prefix, the software shows the characters. That explains most reports of a code that reads but refuses to act.

The WiFi payload is a convention, not a standard

WiFi codes hold a string shaped like WIFI:T:WPA;S:MyNetwork;P:hunter2;; which came out of the ZXing project, not a standards body, and is not a registered URI scheme. Three things bite regularly. Semicolons, commas, colons, backslashes and double quotes inside the SSID or password each need a backslash escape, or the parser truncates the field at the wrong character. A password of only hex digits should be wrapped in double quotes, or some readers take it as a raw hex key. And the payload is usually byte mode encoded as ISO-8859-1, so an accented character survives the scan and still comes out wrong. Pasting the raw string into a text-based decoder shows which of the three happened.

What the Camera app may do that a web page may not

Android 10 and above, and iOS 11 and above, join a network straight from a WiFi code. That is not cleverness in the camera. It is an operating system component holding the network permission, acting on a parsed payload. A scanner in a browser tab holds no such permission, and no web platform API exists for writing a network profile to a device. A browser-based reader that prints the SSID and password as text is behaving correctly, and no setting changes that. Copy the password, open Settings, join manually.

Decoding without pointing a lens at anything

The decode step needs pixels, not a live camera. A screenshot works. So does a saved photo, a PDF page, or a code in an email on screen. The limits are resolution and quiet zone. A symbol needs a clear margin of four modules on every side, so crop tighter and a strict decoder rejects an image whose data is intact. Error correction recovers roughly 7, 15, 25 or 30 percent of codewords depending on the level chosen at encode time, enough for a coffee ring or a logo overlay. It cannot rescue a screenshot scaled down until neighbouring modules blur together.

Reading the destination before you open it

A printed square tells the eye nothing about where it leads, and a sticker placed over an existing code on a poster or parking meter is trivial to produce. So read the string first, which is the job of a scanner that shows the URL before following it. Find the registrable domain, the part immediately left of the first single slash after the scheme, and ignore the path, which is attacker-controlled decoration. Treat an xn-- prefix as a lookalike. Expand shorteners rather than trusting them.

Two guides run the other direction, into producing codes, including generating a batch from a spreadsheet column. Contrast is the usual failure. A transparent code dropped on a mid-tone photograph leaves the decoder nothing to threshold against, since it separates light from dark by brightness alone. As a rule of thumb, print the symbol a tenth as wide as the scanning distance: two metres out wants roughly twenty centimetres.