Information embedding in document copies

摘要

A photocopier (10) scans a document and so modifies the resultant image as to embed in a resultant copy information that is visually imperceptible. To do so, it identifies regions (32, 33) that consist primarily of text-character parts. It embeds a message bitstream by selectively darkening the text pixels in such regions in accordance with the values of successive bits in the bitstream.