A form of stochastic random distribution is used to print an invisible
watermark using a typical, e.g. 600 dpi, printer. The watermark contains
such information as "Do Not Copy," or other such information. While the
watermark is not visible on a first generation version of a document that
is printed, various techniques are used to print the watermark in such a
way that it becomes readily apparent when the first generation document
is photocopied. The preferred embodiment of the invention is concerned
with a technique for improving such techniques, for example so that gray
levels are improved to address variations from printer to printer.
Another preferred embodiment of the invention is also concerned with a
technique for performing error diffusion watermarking without the need
for performing a calibration step.