Halftoning Introduction

Image halftoning is the process of converting a high-resolution image to a low-resolution image, e.g. a 24-bit color image to a three-bit color image or an 8-bit grayscale image to a binary image, for printing and display. Until the late 1990s, printing presses, ink jet printers, and laser printers were only able to apply or not apply ink to paper at a given spatial location. For grayscale printing, the ink dots were black. For color printing, a cyan, magenta, and yellow ink dot is possible at each spatial location. Many color printing devices can also produce a black ink dot. Low-cost liquid crystal displays (LCDs) have the same limitation in that they can only turn a pixel on or off.

Halftoning is more complicated than simply truncating each multi-bit intensity to the lower resolution. Simple truncation would give poor image quality because the quantization error would be spread equally over all spatial frequencies. Instead, binary halftoning would try to compute a pattern of binary dots to achieve the illusion of a multi-bit image. Halftoning may produce a low-resolution multi-bit image, as occurs in display devices, halftone image codecs, photocopiers, and laser printers.

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