Halftoning

Most printing processes consist of essentially binary image elements, i.e. the ink is either present or it is not. Exceptions are dye sublimation printers which can produce multiple ink densities on a give size dot, some ink jet and laser printers which can produce multiple size dots and gravure which produces multiple size dots of variable density.

• The problem of reproducing tonal values with a binary process has been around since the first printing presses.

• Similar issues occur with color displays when fewer colors can be displayed than are represented in the image. The method is to depict small areas of colors which cannot be individually be resolved by the eye. When mingled with one another they are perceived as the average of the colors.

• Such methods are referred to as dithering.

• For conventional printing processes, the most widely used method of dithering is halftoning.

• In the past, woodcuts and engraving on metal were employed to create the illusion of continuous tone.

• Traditionally, halftone screens were used since the associated physical screens were well suited for photographic methods using the process camera.

• Today, the dithered patterns generated by the screens can be reproduced digitally. In addition, other patterns such as stochastic or frequency modulated screening are possible with digital methods.

• Whether done digitally or mechanically, the tonal values are represented as a pattern of dots which average to the desired ink density. The larger the tonal value the larger the area covered by the dots.

• When digital halftone screens are to be output to a PostScript printer or imagesetter, it is necessary to specify screen frequency, or ruling, screen angle and dot shape.