Systems and methods for generating graphical bar codes (i.e., images that
contain inconspicuous graphical modulations that encode embedded information) by
halftoning with embedded graphical encoding are described. In one aspect, a graphical
bar code is generated by halftoning regions of an original image. The original
image regions incorporate errors diffused among regions of the original image and
computed based at least in part upon modulations in the graphical bar code corresponding
to a graphical encoding of a message. In another aspect, a graphical bar code is
decoded by generating a base image having halftone regions representative of an
original image. Regions of the base image are compared probabilistically to a set
of graphical code words to obtain a sequence of graphical code words corresponding
to a graphical encoding of a message. The sequence of graphical code words is decoded
to produce a decoded message.
