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Term: Printed halftone

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Term: Printed halftone


Most photographs, paintings, or similar pictorial works reproduced in books, magazines and newspapers are printed as halftones. In a halftone, the continuous tones of the picture being reproduced are broken into a series of equally spaced dots of varying size, printed with only one color of ink. The outcome exploits an optical illusion: the tiny halftone dots are blended into smooth tones by the human eye. For color reproduction, multiple halftoned printing plates are created. In the four-color process, there are four plates: cyan, yellow, magenta, and black (CYMK). Inks in these colors are applied to the paper in separate passes in a printing press. Roughly speaking, the more dots or lines per inch (dpi, lpi) in the halftone, the greater the clarity on the printed page. The typical resolution for "newspaper quality" is 85 dpi/lpi; for higher quality printing 85-185 (or even more) dpi/lpi may be used.

From digital image to halftone. What level of resolution is needed in a digital image file to achieve good results when halftoning? To answer this question, most commentators speak in terms of output resolution. For example, the valuable UPDIG recommendations for photographers reports that "resolutions of 1.3–2 times the halftone screen for the project are considered safe. For example, if the images will be printed with 150-lpi halftones (often used for magazine printing), the appropriate image file resolution range would be 195 ppi to 300 ppi." This guideline assumes that the photographer knows in advance what the size of the printed image will be. If you know that from 195 to 300 ppi are required for this 6-inch magazine illustration, then you can say with confidence that your digital image ought to be 1,200 to 1,800 pixels on the long side.

From printed halftone to digital image The pattern of tiny halftone dots makes printed halftones challenging to scan. The spatial frequency of the halftone dot pattern and the spatial frequency applied by digital scanning and/or output devices interact, causing interference "waves" that manifest themselves as moiré patterns that degrade the image. There are a number of treatments that can mitigate or correct this degradation. A 1998 Library of Congress report identified four approaches to address this problem that were in use by various members of the digital library community at the time: Grayscale reproduction. For many illustrations, this approach offers a reasonable onscreen rendering for the user, although if the image is rescaled at display time (for example, reduced in size to fit the screen) some moiré patterns may result. Since printed output from a typical laser printer requires that grayscale images be halftoned by the computer (a similar but not identical process to the halftoning used in letterpress and offset printing), paper copies produced from these grayscale images may also suffer from moiré patterns. Descreening and rescreening. This approach removes the halftone dots and converts the image to grayscale, then rescreens it to produce a new halftone. Bitonal (one bit deep) capture at high enough resolution to reproduce the halftone dots. This approach requires capture resolutions at one or more multiples of the resolution of the original halftone screen. Randomization of the scanner "dot pattern." This process reproduces printed halftones as bitonal images to which a special diffuse dithering treatment is applied at scan time (or in post-processing a grayscale image). This reduces but does not eliminate moiré patterns.

The scanning of halftones is sufficiently complex to warrant book-length treatment, e.g., David Blatner, Steve Roth, Glenn Fleishman, Conrad Chavez, Real World Scanning and Halftones, now in its third edition (Pearson Education, 2004, ISBN: 0321241320).

Halftone (Wikipedia article)
LC report on reproduction issues (1998)
Scanning halftones, University of Missouri
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