Nonlinear Image Processing

Digital Halftoning

DANIEL L. LAO

GONZALO R. ARCE

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

plate.

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3 76 DANIEL L. LAU AND GONZALO R. ARCE

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