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Because they cannot vary the thickness of
the film of ink in highlights and shadows, lithographic printing
presses are unable to reproduce the continuous tones of original
photographic images. However, if the original image is first converted
into a pattern of dots, the press can print a simulation that "tricks
the eye" into seeing what appear to be continuous areas of ink
coverage. An image converted in this way is called a halftone. Halftones are produced by screening the image mechanically or digitally
to create the matrix of dots. The image is screened once for each color
to be printed, so that a halftone for a four-color (CMYK) image will
consist of four screens as shown here. Place
the cursor over the continuous-tone image below to see the effect of
screening.
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