6 . 2 Dots and Pictures
A dot or mark is some visual blip, smudge, or collection of pixels that appears on the printed page or display. Certain kinds of images and image manipulation techniques depend on the manipulation of tiny dots, especially the images reproduced with the technique known as halftoning.
Photographs are images with continuous shades of black, white, and gray. These shades are tones. Halftoning is a technique, originally developed for the printing industry, to reproduce continuoustone images using printing techniques capable of black and white only, not shades of gray.
In halftoning, a dot pattern is created by rephotographing the original photograph through a screen. The dots blend together, both on the paper and in our eye, to give a convincing illusion of continuous tones. In the magnified section of the following figure, you'll notice that the brighter an intensity desired, the smaller the size of the dots. In addition, the darker the tone, the larger the dots. Your eye spatially fuses the small dots so that they appear as continuous tones.
The size, shape, and orientation of the holes in the screen used in creating a halftone can dramatically affect the resulting image. Digital halftoning is a computer graphic simulation of the rephotographing process. PostScript, a page description language, uses the same terminology (that is, with the setscreen command) to allow the precise manipulation of halftone screens.(3) The resolution, orientation, and shape of the dot may all be manipulated. (See Section 5 . 1 . 2 PostScript in Chapter 5 Document Standards.)
A new form of halftoning, called stochastic halftoning, uses a semi-random arrangement of dots to create gray levels and color. The results offer smoother gradations of color.
Another technique used to create the simulation of continuoustone images is dithering. Dithering is sometimes considered a type of halftone. Unlike halftones, which vary the size of the dot, dithering varies the number or density of the dots within a fixed array. For example, a dither pattern that uses a threebythree array of dots can represent 10 different intensity levels by displaying more, less, or no dots in the grid area. The trade-off is resolution: the more gray levels you wish to display, the larger the array necessary, and, therefore, the less resolution available. Each grid area represents a single pixel. Dithering results in much poorer quality images than halftoning. But for devices such as screen displays, which cannot vary the dot size with sufficient resolution, it is very useful.(4) The way in which the dots within the dither grid are turned on and off can greatly affect the overall image, as illustrated below.
Another interesting consequence of simple mark manipulations is the advent of inexpensive resolutionenhancement devices for laser printers. The typical laser printer has a resolution of 300 dots per inch (dpi). Resolutionenhancing computer boards that plug into the controller section of a laser printer are now available. They can enhance the printing resolution from 300 to 600 or 1000 dpi by manipulations of the dot shape and some other tricks with the laser.
Resolution enhancement has spawned a small cadre of new products. They fall into two general categories: edge smoothing and gray-scale enhancement. Edge smoothing is used by Apple's FinePrint, DP-Tek's Super Smoothing Technology (SST), HP's Resolution Enhancement Technology (RET), and LaserMaster's TurboRes. Gray-scale enhancement products use various forms of halftoning to improve photographic imagery. Some of the products using this approach are Apple's PhotoGrade, DP-Tek's LaserPort Grayscale controller, and XLI's Super LGA. Simply put, both categories use very fine control of the laser's energy level (for vertical resolution) combined with a true resolution increase (horizontal) to improve the print quality.(5)
