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Chapter 7:
Digital Halftoning for Printing and Display of Electronic Images
This chapter presents encoding methods, referred to as halftoning, that are used to reduce the number of quantization levels per pixel in a digital image, while maintaining the gray appearance of the image at normal viewing distance. Halftoning is widely employed in the printing and display of digital images. The need for halftoning encoding arises either because the physical processes involved are binary in nature or the processes have been restricted to binary operation for reasons of cost, speed, memory or stability in the presence of process fluctuations. Examples of such processes are most printing presses, ink jet printers, binary cathode ray tube (CRT) displays, and laser xerography. In most printing and display applications, the halftoned image is composed ideally of two gray levels, black and white. Spatial integration, plus higher level processing performed by the human visual system, and local area coverage of black and white pixels, provide the appearance of a gray level, or "continuous tone," image. Halftone techniques are readily extended to color and to quantization using more than two levels, but within this chapter there is space to cover these topics only briefly.
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