Display of grey-scale data on a bilevel output device: addendum
, by Steve Cole and Joe Dellinger
In SEP-51, we discussed ways in which grey-scale seismic data could be
displayed on a bilevel output device such as a plotter. The data examples
included in the paper looked fine when produced by our laser printer, but
suffered noticeabley when reproduced in the report. Since our object was
to minimize the distracting texture created by the dithering process itself,
we created the images using the highest-frequency alternation of black and
white pixels the laser printer could produce. Unfortunately, reproduction
devices such as photocopiers are unable to "see" such high-frequency
variations. So just as we worked to correct for the characteristics of the
laser printer in the last report, we must now also take into account the
limitations of the reproduction process. This is done by reducing the
frequency of variation between black and white that is used to produce a
particular shade of grey, leaving clusters of black and clusters of white
pixels. This technique is called halftoning. There is a tradeoff: while
such halftoned pictures can be reproduced more faithfully, the originals
have a coarser, more distracting, texture. For images that are not going to
be reproduced by a low-resolution electrostatic printing process, the techi-
niques described in SEP-51 remain superior.
STANFORD