In printing, the simulation of a continuous-tone image (shaded drawing, photograph) with dots. All printing processes, except for Cycolor, print dots. In photographically generated halftones, a camera shoots the image through a halftone screen, creating smaller dots for lighter areas and larger dots for darker areas. Digitally composed printing prints only one size of dot.
In order to simulate variable-sized halftone dots in computer printers, dithering is used, which creates clusters of dots in a "halftone cell." The more dots printed in the cell, the darker the gray. As the screen frequency gets higher (more cells per inch), there is less room for dots in the cell, reducing the number of shades of gray or color that can be generated.
In low-resolution printers, there is always a compromise between printer resolution (dpi) and screen frequency (lpi), which is the number of rows of halftone cells per inch. For example, in a 300 dpi printer, the 8x8 halftone cell required to create 64 shades of grays results in a very coarse 38 lines per inch of screen frequency (300 dpi divided by 8). However, a high-resolution, 2400 dpi imagesetter can easily handle 256 shades of gray at 150 lpi (2,400 / 16).
PRINTER RESOLUTION &
MAXIMUM SCREEN FREQUENCY
Gray or --At printer resolutions--
Cell Colors 300 dpi 1200 dpi 2400 dpi
4x4 16 150 lpi 300 lpi 600 lpi
8x8 64 38 lpi 130 lpi 300 lpi
16x16 256 19 lpi 75 lpi 150 lpi
Analog Vs. Digital
The analog world of commercial printing prints dots in varying sizes. The digital world prints in grids of dots. Increasingly, digital printers use techniques that overlap dots to achieve greater variability in dot sizes.