Halftoning is the technique of converting continuous-tone images, such as those produced by silver halide film cameras, to strictly black and white images for reproduction in binary display devices, such as desktop inkjet printers. The earliest forms of halftoning were used in printers employing photo-lithography [1] and, as illustrated in Fig. 1.1, involved projecting light, from the negative of a continuous-tone photograph, through a mesh screen such as finely woven silk onto a photo-sensitive plate. Bright light, as it passed through a pin-hole opening in the silk screen, would form a large, round spot on the plate. Dim light would form a small spot. Light sensitive 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 finally have dots, where ink was to be printed, raised slightly above the rest of the plate producing an image such as that in Fig. 1.2.

Figure 1.2 Gray-scale image reproduced as an analog halftone.

In 1880 when the first analog halftoning process was perfected [1], the predominant form of printing was letter press or relief printing, which reproduced monochrome photographs as line drawings created by highly skilled craftsmen. With halftoning, publishers and printing houses had an easy and inexpensive way of reproducing continuous-tone photographs, making photo-lithographic presses the preferred printing technology. Halftoning also made photography a lucrative industry -- leading to a surge in technological innovation for photographic equipment [2].

As halftoning evolved, later versions of the process employed screens made of glass that were coated, on one side, by an opaque substance [2]. A mesh of parallel and equidistant lines were scratched in the opaque surface. A second mesh of parallel and equidistant lines were then scratched in the opaque surface running perpendicular to the original set. Screens would then differ in the number of lines per inch that had been scratched. While finer screens created better spatial resolutions (detail), the quality of the printing press would limit how fine of a mesh could be used. 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 [1]. This contact screen had direct control of the dot structure (Fig. 1.3) being able to control the screen frequency (the number of lines per inch), the dot shape (the shape of dots as they increase in size from light to dark), and the screen angle (the orientation of lines relative to the positive horizontal axis).

Figure 1.3 The screen frequency, dot shape, and screen angle for an analog halftone pattern.