Comment by kragen

Acid-free paper has a lot to recommend it: an archival lifespan of about 1000 years, low cost, and widespread equipment for printing it en masse very cheaply. Other media are superior in one or another way, but all are far more expensive.

It's probably worth mentioning https://github.com/za3k/qr-backup/ which is tested in practice rather than merely theoretical. It doesn't achieve very high density, though.

The theoretical information capacity of an uncoated 600dpi laser-printed page ought to be close to 600×600 bits per square inch, 23.6×23.6 bits per square millimeter in modern units. This is 33.7 megabits per US letter page or 34.8 megabits per A4 page. The bit error rate of a laser printer is quite low, under 1%, and the margins are maybe another 5% at most. So modest ECC ought to be able to deliver most of that channel capacity in practice. QR codes and OCR apparently don't come close.

As an exercise, 13 years ago, I designed a proportional 1-bit-deep pixel font for printable ASCII, based on Janne Kujala's work, that averages about 3½×6 pixels. This is about 20 bits per character, so a letter-sized page should hold almost a megabyte of human-readable ASCII text. I generated the King James Bible in it at 600dpi. It comes to about four pages. Printed out in a half-assed way at double size (300dpi) on a 600dpi printer, you can read it pretty easily with a good magnifying glass. I have not yet been able to get an even partly readable printout at full resolution. If someone else tries it, I'm interested in hearing your results.

http://canonical.org/~kragen/bible-columns.png (warning, 93+-megapixel image, 4866×19254)

http://canonical.org/~kragen/bible-columns-320x200.png (small excerpt from the above)

http://canonical.org/~kragen/sw/netbook-misc-devel/6-pixel-1... (the font as a 374×7 image)

http://canonical.org/~kragen/sw/netbook-misc-devel/propfontr... (the image generation program I regret having written in Python because it won't run in current Python)

http://canonical.org/~kragen/sw/netbook-misc-devel/bible-pg1... (test input text, public domain everywhere except the UK)