Canon copiers

[Robert J. Woodhead]

Paul writes:

>I'd like to know how they put the serial number into the copy.

Most likely by modulating the Least Significant Bit of the color
intensity in some (or all) color-filled areas of the image.  A
set of sync-bits, serial number, and some more sync bits.  Repeat
every line, shifted by 1 bit, and you'll get a pattern that is
hard to differentiate from the background unless you _know_
what to look for.

I wouldn't be surprised if there isn't a time and date stamp
as well, plus batch # info.