`STOCHASTIC IMAGERY`

This is a discussion about reproducing continuous-tone imagery with constant-tone media, such as ink and toner.

Newspapers and magazines use a rigid pattern of dots in varying sizes to convey the gray or color tones of photographs and drawings. These standard halftones represent the conversion of a continuous tone image such as a photograph into a printable version made up of larger and smaller solid dots in straight and regular rows. The process is ingenious but rather crude, the best anyone was able come up with in the 19th and for even most of the 20th century. A whole industry has been built around it. It produces the artifact of the dotted halftone grid overlaying all imagery that most of us have grown accustomed to.

But there is another way of simulating continuous tone, long known to artists. Stippled drawings are made up of tiny dots and are labor intensive. Unlike standard halftones, the dots are the same size; their disposition is intended to reflect the relative values of the various parts of the image.

Stochastic (stow-KASS-tick) images look much like fine stippling, but they are produced entirely and only by computer. As with stippling, the dots are all the same size, and their closeness conveys the appropriate degree of darkness. ("Stochastic" refers to the randomness in the dots' positions.) The dots in standard halftones are not random at all but oriented on a grid.

Stochastic images take better advantage of any printing press's capacities and so produce finer, more artful results than standard halftones ever can. The dots can be as small and as close as a particular printing device can print, rather than a multiple of that smallest dot as with standard halftones. Resolution is higher with stochastic halftones. They can be black-and-white or color. With color printing there's never a concern for moiré artifacts that plague standard halftones. Some color laser printers print stochastic halftones.

I am particularly interested in lower resolutions for standard publication. I create my own stochastic images which printers treat as line art.

Of course, the computer screen doesn't suffer from the same constraints as the printing press. On a printed sheet all the red ink must be the same red ink, not a lighter shade here and a darker one there, and the same for all the blue ink, and the yellow, and the black. But on screen, every pixel can display a different shade of red or other color or combination of colors. Yet even in the 72 dpi environment stochastic images can have a place. They are much smaller files than their standard counterparts so they load much quicker and take up less server space. They can be very dramatic and artful. And what looks good at 72 dots per inch, the screen resolution, looks even more impressive at 300 or even 600 if the press can handle it. When monitor resolution increases to rival that of printing, then onscreen stochastic imagery will come to even more prominance. The higher the resolution, the better stochastic images look. But even at lower resolution they surpass ordinary halftones and could greatly enhance the photos in daily newspapers.

For this image, an actual black-eyed susan was scanned.
The stochastic halftone of the greyscale scan was made in Digital Darkroom.

Digital Darkroom still works under Mac OS 8.6.

compare: standard vs stochastic halftones

color stochastic images

johnkyrk@johnkyrk.com

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