The colour QR

Three ordinary QR symbols, one per RGB channel, in one image. Here is how to generate it. Here is how to un-generate it. Both directions, written down, because a code nobody can read is the same as no code.

what it is

A QR symbol is monochrome: it uses one channel and wastes two. Put a different, complete, ordinary QR in the red plane, the green plane and the blue plane and one image carries three symbols at once. Every phone already contains a decoder that hardens each plane — finder patterns, alignment marks, format info, Reed–Solomon — and each plane fails or survives alone. Nothing new had to be invented for the hard part.

to generate

1. Take the payload P, N bytes.
2. Cut it into chunks of 2,100 bytes. The last chunk is the remainder.
3. For chunk i: prepend a 4-byte little-endian uint32 of i. That is the Fischer index.
   chunk 30 begins 1e 00 00 00 · chunk 31 begins 1f 00 00 00
4. Base64 the indexed chunk.
5. Render that base64 as an ordinary QR symbol. No custom encoder.
6. Compose three symbols into one image: chunk 3k to the red plane, 3k+1 to green, 3k+2 to blue. A plane with no chunk left to carry is filler and holds no symbol.

Order does not matter downstream. The index carries it.

to un-generate

1. Split the image into its R, G and B planes. Do not convert to grey — that destroys all three.
2. Run an ordinary QR decode on each plane separately. A plane with no symbol is filler; that is not an error.
3. Base64-decode each result.
4. First 4 bytes, little-endian, are the chunk index. The rest is the body.
5. Sort bodies by index, concatenate.
6. sha256 the result and compare. A chunk whose own Reed–Solomon failed never arrives, so a bad read is dropped, never merged.

measured, on the real stack

source            real JWST raw, CEERS F277W        65,536 B
chunking          31 x 2100 + 436 = 65,536          exact, no slack
codes             32 chunks / 3 per image           11 colour codes
code 11           R = chunk 30   G = chunk 31   B = filler
sha256 in         150760ca872afc9e41e433f9
sha256 out        150760ca872afc9e41e433f9          BYTE-EXACT

Read back independently from a 457 px crop of a screenshot — not the original file:

R plane   header 1e000000  ->  chunk 30   body 2,100 B
G plane   header 1f000000  ->  chunk 31   body   436 B
B plane   no symbol                                  <- filler, as labelled
                              2,100 + 436 = 2,536 B  <- matches "rows 246-255"

the reader

Below is the working decoder, not a description of one. Camera, or pick the image files directly — files decode at full resolution and are never downscaled.

chunks held