Twibright Optar

This is a program to store data on paper using a 600dpi b/w laser printer and a 600+ dpi scanner.

You need to install ImageMagick so that the resulting .pgm image can be converted into PostScript with the right dimensions (so each pixel is 3x3 600dpi pixels so that there is no unnecessary jitter).

Make sure you have libpng installed and if you type "libpng-config" on the commandline, there's a program which prints something.

Compile with make. Become root su - and type make install. Now you have optar, unoptar and pgm2ps installed on your system in /usr/local/bin. Later you can uninstall by typing make uninstall the same way as you typed make install.

Encoding (writing)

Run

./optar other_guys.ogg other_guys.ogg

which will produce files:

• other_guys.ogg_0001.pgm
• other_guys.ogg_0002.pgm
• other_guys.ogg_0003.pgm
• other_guys.ogg_0004.pgm
• other_guys.ogg_0005.pgm
• other_guys.ogg_0006.pgm

Now convert them into PostScript using the included pgm2ps tool: ./pgm2ps *.pgm

Print them using a 600dpi (or more) laser printer. Inkjet or dot matrix was never tested and will not probably work at the pre-defined data density. See "Changing the format" below.

Please note that the file will be padded by zeroes and the original length will be lost. Pack your data with tar if you store data that are sensitive to this.

Decoding (reading)

Clean and polish the scanner glass with rubbing alcohol and paper towel. Put yellow pages on the scanner lid to get sharper picture *). Insert the page so that the text on the bottom is upright. Scan the pages into PNG (not JPEG!) on 600dpi (or 1200dpi, slightly better):

• scan_0001.png
• scan_0002.png
• scan_0003.png
• scan_0004.png
• scan_0005.png
• scan_0006.png

Read the number sequence (format specification) from any of the papers and feed it as 1st argument to the optar, 2nd argument is the filename part before the underscore:

unoptar 0-65-93-24-3-1-2-24 scan > out.ogg

Then play out.ogg with mplayer. You should get first about 41 seconds from the Ogg Vorbis file.

*) In the scanner I tried (Canoscan), the lid didn't seem to be heavy enough to press the paper down completely - there were blurry spots in the picture. Without yellow pages I got 526 reparable bad bits bad from 3.2 million. With yellow pages the blurry spots were much sharper and I got only 261 reparably bad bits!

Please note the data are padded with zeroes so the original information about file length is lost. If your data format doesn't like this then first pack your data with tar.

A4 <-> US Letter

Change the convert parameters in pgm2ps (see comments). Change XCROSSES and YCROSSES in optar.h (see comments). Recompile. Then you can use US Letter instead of A4.

Changing the format

If your printer is low quality and you are getting irreparable bits, you can try to format the media to lower capacity. Unfortunately, setting by commandline is not implemented yet. Change XCROSSES and YCROSSES in optar.h to lower values which yields bigger pixels and lower capacity per page, but higher reliability. Make sure they are in roughly the same proportion as before, otherwise you get nonsquare pixels and unnecessary waste of channel capacity.

You can also change the decoding parameters in unoptar.c (look for MAGIC CONSTANTS) in attempt to read a difficult recording: unsharp_mask, unsharp_dist, sync_white_cut, white_cut, minmax_filter, pixel_blur, cross_trim.

Future improvement

• manpage could be written for optar and unoptar
• commandline help (-h) could be written for optar and unoptar
• the format could be made configurable. Now it's stored in the optar.h
• the magic constants could be changed by commandline options. Now they are stored in unoptar.c.
• Golay code decoding could be rewritten faster, using a sophisticated algorithm (Kasami algorithm?)
• Easy support for multiple pages per page, so it can be read by a digital camera. Currently it cannot since digital camera blurs at the sides of the picture.

Authorship and Licensing

© GPL 2007 Karel 'Clock' Kulhavy of Twibright Labs. See COPYING for the text of the GPL license.

e-mail: clock (at) twibright (dot) com

Twibright Optar homepage: http://ronja.twibright.com/optar/

Special note: The repository of optar at http://github.com/colindean/optar exists because I (@colindean) had difficulties finding the source and it seemed to be abandoned/dormant.