🚀 go-pugleaf

Some of the disagreement over whether this is possible may be due to
confusion over different dithering algorithms.

The Floyd-Steinberg algorithm, as someone mentioned, does error
diffusion and as such needs access to the entire image space.  The
other method suggested was ordered dither, and this indeed can be
implemented using the PostScript operators setscreen and settransfer.
(It is really setscreen that implements the dither; the transfer
function is used to compensate for ink spread etc., i.e. to even out
the results.)

What makes ordered dither suitable for a PostScript implementation
while error diffusion is not possible?  The distinction is that the
ordered algorithm overlays a threshold matrix over the image and uses
that matrix to determine how to color the cells.  This is analogous to
laying a "screen" over the image, which is what traditional halftoning
does.  In neither case does the algorithm call for adjusting the
values of neighboring pixels, so this technique will work despite the
fact that the spot function knows nothing about the image being
rendered.  (For more info, see Foley and Van Dam, 1st ed.  page
600-601.)

The materials handed out at the Adobe Developer's conference in June
of this year include a simple example of how to feed a threshold
matrix to setscreen.  (Unfortunately, there's a copyright notice
glaring up at me from the bottom of the page, so I don't feel free to
publish the example.  Perhaps someone at Adobe would be so kind?)

PostScript level II, according to the last specification I saw,
includes a simpler method of doing ordered dither.  My understanding
is that support for error diffusion is NOT part of that work, however.

-- Rich