21cm | QR Puzzle Primer

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Quick primer

Straight to the clues. Minimal theory, puzzle-ready notes.

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Rabbit hole reference

Want the full nerdy route? Keep this article open beside these slides.

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QR primer deck

Section 1 / 8

What is a QR Code?

A QR Code is a structured image that machines can decode into data, often a URL. Building one by hand involves real math, but you do not need to do that for this puzzle.

Modules

QR Codes are made from a grid of squares called modules. Each module is either black or white, representing bits.

Finder Pattern

Finder Patterns help scanners lock onto code orientation. A white separator must surround each finder pattern.

Fixed Patterns

Each QR version has fixed patterns such as timing patterns, the dark module, and quiet zone. Versions 2+ also include alignment patterns.

Formatting Pattern

Formatting bits tell decoders how the payload was encoded and what mask was used. Version information is only needed for larger versions.

Formatting Pattern Encoding

The formatting pattern contains two redundant copies. It includes error-correction level bits, mask ID bits, and error correction bits for the format field itself.

Formatting pattern layout — Format bit placement

Formatting mask example — Masking example

Encoding Mode and Masks

Encoding mode is stored in the first data bits near the lower-right area. Data masking transforms bits for readability by scanners.

Version 1 mask patterns — Version 1 mask variants

More Info

There is much more to QR design than this summary, but this is all you need for this puzzle path. If this is your kind of challenge, check out 1420MHz.

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