doug@xdos.UUCP (Doug Merritt) (01/03/90)
I just wrote a dithering program to display Usenix faceserver images
on a 4 bitplane device, referring to "Digital Halftoning" for expert
advice. Annoyingly, it only spends two pages on multibit displays, but
I eventually figured out that one could get 61 shades of grey from
a 2x2 ordered dither, assuming linearity.
In the process of fiddling with the grey scale to eliminate nonlinearity,
as suggested in the book (by eye, since I have no photometer), it occurred
to me that the nonlinearity of the output could be used to produce more
shades of grey.
After some tedious fiddling around to empirically create such a scale, I
ended up with a grey scale with around 180 distinct values, which produced
significantly better resolution of detail in the displayed faces, especially
in shadowed areas. Part of this was due to fixing the nonlinearity in
the dark end of the scale, but clearly having more distinct grey values
can be a Very Good Thing in general, even at the risk of nonuniformity
of step size. (A real risk in the absence of physical luminance
measurements.)
This seems like a very useful way to stretch low quality displays to
their limits, yet I haven't ever heard of this technique being used before.
Did I invent something new? Or is this very well known and I just missed
the references? Or...more likely, is it an old but obscure trick? Or
does no one care because linear response video output is common????
I don't think real highly of Ulichney's "leave it as an exercise to the
reader"; multibit halftoning is an important subject, and dealing with
all of the above issues is somewhat error prone. Several other programs
I've seen assumed 64 grey scales generated by the 2x2 dither on this 4
bitplane device. I had to correct several of my misunderstandings before
arriving at the accurate number.
(BTW my original goal had been simply to improve the speed of an existing
program by Thad Floryan; switching from "doing it by the book" to
using a highly optimized algorithm resulted in a 50-fold speedup...a
pleasant result. But then I discovered the seductiveness of improving
image quality as well.)
Doug
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Doug Merritt {pyramid,apple}!xdos!doug
Member, Crusaders for a Better Tomorrow Professional Wildeyed Visionary