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.


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