[Printers] Dithering

[Seth David Schoen]

The Purdue research reminds me that dithering algorithms definitely do
provide a possible channel for watermarking, because there may be
ambiguities or choices about where to put a particular colored dot, and
the choices made may be used to encode information.

This is particularly interesting with regard to some printers like the
Xerox Phaser 8400, which does not appear to print yellow tracking dots
of the DocuColor type, but does appear to produce cyan regions by
dithering with yellow -- and the yellow dots form repeating patterns
which could easily be compared with tracking dots.

A good way to rule out the possibility that dithering algorithms are
being used for tracking would be to compare identical pages printed on
two printers of the same model, and see whether they make the same
dithering choices.  However, does anyone know a way to get a printer
like the Phaser 8400 to print (what is from its perspective) true, pure
cyan?  I want to know whether the printer adds yellow on its own
initiative, or if, perhaps, the PDF interpreter on the computer is
sending it some color (perhaps from an RGB instead of CMYK color space)
that is not actually 100% pure cyan.

-- 
Seth Schoen
Staff Technologist                                schoen at eff.org
Electronic Frontier Foundation                    http://www.eff.org/
454 Shotwell Street, San Francisco, CA  94110     1 415 436 9333 x107