Digital Halftoning
Department of Electrical and Computer Engineering
University of Delaware
Newark, Delaware
13.1 Introduction
Since the introduction of photography in 1839, accurately reproducing an origi-
nal continuous-tone photograph without loss of tonal value or detail has been a
primary concern for the manufacturers of printing presses. At that time, the pre-
dominant form of printing was the letterpress, which was incapable of printing
intermediate tones and could only produce black and white images. Continuous-
tone, monochrome photographs could be reproduced only as line drawings created
by highly skilled craftsmen, usually on scratchboard. In 1880, the halftoning pro-
cess was invented, leading to a technical revolution in both photography and the
printing industry.
In terms of photolithography (a process introduced in 1855 [Des94]), halfton-
ing involved projecting light from the negative of a continuous-tone photograph,
through a mesh screen, such as finely woven silk, onto a photosensitive plate.
Bright light, as it passed through a pinhole opening in the silk screen, would form
a large, round spot on the plate. Dim light would form a small spot. Light sen-
sitive chemicals coating the plate would then form insoluble dots that varied in
size according to the tones of the original photograph. After processing, the plate
would have dots where ink was to be printed, raised slightly above the rest of the
Figure 13.1" The AM halftoning process.
Later versions of the halftoning process employed screens made of glass that
were coated on one side by an opaque substance. A mesh of parallel and equidis-
tant lines was scratched in the opaque surface and then a second mesh was
scratched perpendicular to them. Such screens would differ in the number of
lines per inch that had been scratched into the opaque surface; while finer screens
created better reproductions, the quality of the printing press would limit how
fine a mesh could be employed.
Later still, the glass plate mesh was replaced altogether with a flexible piece
of processed film, placed directly in contact with the unexposed lithographic film
[Des94]. This contact screen had direct control of the dot structure. The screen
controlled the screen frequency (the number of lines per inch), the dot shape (the
shape of the dots as the size increased from light to dark), and the screen angle
(the orientation of lines relative to the positive horizontal axis).
13.1.1 AM Halftoning
Today, printing is far more advanced with the introduction of nonimpact printing
technologies and the emergence of desktop publishing. Brought on by advance-
ments in the digital computer [Des94], the photomechanical screening process
introduced in 1880 has been replaced, in many instances, by digital imageset-
ters. In some instances, printing is no longer binary since continuous-tone dye-
sublimation printers are now readily available, but due to their speed and material
requirements (special papers and inks), they have not reached the widespread
acceptance that four color ink jet and electrophotographic (laser) printers have.
In these digital printers, the process of projecting a continuous-tone original
through a halftone screen has been replaced with a raster image processor (RIP)
that converts each pixel of the original image from an intermediate tone directly

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