(increases complexity, but more robust to damage or occlusion).
Text
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Image <img>
Image <svg>
Simpler codes are easier to scan:
Keep the input as short as possible.
If using a URL with a common top-level-domain (such as .com), some readers work without an initial http:// or https://, but probably best not to rely on this.
All-numeric text is the most compact, but all upper-case alphanumeric (with some symbols allowed) is more compat than mixed-case.
Do not use upper case for the URL past the domain name without first checking the server responds correctly.
Include a written alternative for the URL.
For WiFi QR codes, use the format: WIFI:S:my_ssid;T:WPA;P:my_passphrase;;