This invention provides self-clocking glyph shape codes for encoding digital
data in the shapes of glyphs that are suitable for printing on hardcopy recording
media. Advantageously, the glyphs are selected so that they tend not to degrade
into each other when they are degraded and/or distorted as a result, for example,
of being photocopied, transmitted via facsimile, and/or scanned-in to an electronic
document processing system. Moreover, for at least some applications, the glyphs
desirably are composed of printed pixel patterns containing nearly the same number
of ON pixels and nearly the same number of OFF pixels, such that the code that
is rendered by printing such glyphs on substantially uniformly spaced centers appears
to have a generally uniform texture. In the case of codes printed at higher spatial
densities, this texture is likely to be perceived as a generally uniform gray tone.
