Digital Prints

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Mastering Digital Printing
This is where dithering comes in. In the
dictionary, dithering means “nervously
excited or confused.” Dithering is simply
an alternative to halftoning and is the
process of breaking down a continuoustone
image into a bunch of tiny, confused,
excited little spots in a “stochastic”
or random arrangement. Dithering,
sometimes in combination with halftoning,
has been successfully implemented
by inkjet and color laser printers to output
a full range of tones and image detail.
HP, for example, combines halftoning
with what it calls PhotoREt Color
Layering Technology on many of its desktop
inkjets. PhotoREt layers the color dots
on top of each other and dithers them with error diffusion, which is a common dithering
method (others include ordered-matrix dithering and threshold dithering). Error diffusion
means that the error in creating a specific color—say green, which has to be made up of the
only colors the printer has available, primarily for green: yellow and cyan—is spread to the
adjacent dots. If one is too green, the next one over is made to be less green. And so on. If you
stand back and look at the print, it all balances out, and what you see is “green.” (Note that
there is no green ink in 99.9 percent of all inkjet printers; Canon’s i9900 is the lone desktop
exception at the time of this writing. All the green—or any of the other colors of the rainbow—must
come from a visual blending of primary colors that the printers do have.)
Epson employs its own proprietary algorithms (an algorithm is the mathematical set of
instructions the printer software uses to control and precisely place the ink droplets) for
what it calls AcuPhoto Halftoning, actually a type of error-diffusion-type dithering.
Canon uses what it calls Precision Color Distribution Technology for its dot layering technique
to ensure uniform color.
Moving away from inkjets, the Xerox Phaser 7750 color laser printer uses a combination
of digital halftoning and a special dithering pattern to render the image (see Figure 2.16).
Why is all this talk about dithers and halftones important? Because the type of screen rendering
will partially determine the “look” of an image when printed using that particular screening
or halftoning technology. This is a big part of what makes up a print’s “digital signature.”
When you get experienced enough, you will be able to spot the differences between the specific
types of digital output. And you can make your purchase or service choices accordingly.
The bottom line is that when you’re at the upper end of digital printing quality, including
inkjet, you’ve pretty much entered the world of continuous-tone imaging. The dots
touch with no space between them, and the four or six (or more) colors are layered next
to or on top of each other to blend together and form a smooth image. The dividing line
between continuous-tone and screened images, at least with high-quality, 8-bit digital
printing, is disappearing.
Figure 2.15 A simulation comparing
halftone screening (top) with frequency
modulated screening (bottom), 30x
magnification.
Eyes courtesy of Martin Juergens; panels
courtesy of Wasatch Computer Technology