COMPUTER GRAPHICS

the use of computers to produce pictorial images. The images produced can be printed documents or animated motion pictures, but the term computer graphics refers particularly to images displayed on a video display screen, or display monitor. These screens can display graphic as well as alphanumeric data. A computer-graphics system basically consists of a computer to store and manipulate images, a display screen, various input and output devices, and a graphics software packagei.e., a program that enables a computer to process graphic images by means of mathematical language. These programs enable the computer to draw, colour, shade, and manipulate the images held in its memory. A computer displays images on the phosphor-coated surface of a graphics display screen by means of an electron beam that sweeps the screen many times each second. Those portions of the screen energized by the beam emit light, and changes in the intensity of the beam determine their brightness and hue. The brightness of the resulting image fades quickly, however, and must be continuously refreshed by the beam, typically 30 times per second. Graphics software programs enable a user to draw, colour, shade, and manipulate an image on a display screen with commands input by a keyboard. A picture can be drawn or redrawn onto the screen with the use of a mouse, a pressure-sensitive tablet, or a light pen. Preexisting images on paper can be scanned into the computer through the use of scanners, digitizers, pattern-recognition devices, or digital cameras. Frames of images on videotape also can be entered into a computer. Various output devices have been developed as well; special programs send digital data from the computer's memory to an imagesetter or film recorder, which prints the image on paper or on photographic film. The computer can also generate hard copy by means of plotters and laser or dot-matrix printers. Pictures are stored and processed in a computer's memory by either of two methods: raster graphics and vector graphics. Raster-type graphics maintain an image as a matrix of independently controlled dots, while vector graphics maintain it as a collection of points, lines, and arcs. Raster graphics are now the dominant computer graphics technology. In raster graphics, the computer's memory stores an image as a matrix, or grid, of individual dots, or pixels (picture elements). Each pixel is encoded in the computer's memory as one or several bitsi.e., binary digits represented by 0 or 1. A 2-bit pixel can represent either black or white, while a 4-bit pixel can represent any of 16 different colours or shades of gray. The constituent bits that encode a picture in the computer's memory are called a bit map. Computers need large processing and memory capacities to translate the enormous amounts of information contained in a picture into the digital code of a bit map, and graphics software programs use special algorithms (computional processes) to perform these procedures. In raster graphics, the thousands of tiny pixels that make up an individual image are projected onto a display screen as illuminated dots that from a distance appear as a contiguous image. The picture frame consists of hundreds of tiny horizontal rows, each of which contains hundreds of pixels. An electron beam creates the grid of pixels by tracing each horizontal line from left to right, one pixel at a time, from the top line to the bottom line. Raster graphics create uniform coloured areas and distinct patterns and allow precise manipulation because their constituent images can be altered one dot at a time. Their main disadvantage is that the images are subtly staircasedi.e., diagonal lines and edges appear jagged and less distinct when viewed from a very short distance. A corollary of television technology, raster graphics emerged in the early 1970s and had largely displaced vector systems by the '90s. In vector graphics, images are made up of a series of lines, each of which is stored in the computer's memory as a vectori.e., as two points on an x-y matrix. On a vector-type display screen, an electron beam sweeps back and forth between the points designated by the computer and the paths so energized emit light, thereby creating lines; solid shapes are created by grouping lines closely enough to form a contiguous image. Vector-graphics technology was developed in the mid-1960s and widely used until it was supplanted by raster graphics. Its application is now largely restricted to highly linear work in computer-aided design and architectural drafting, and even this is performed on raster-type screens with the vectors converted into dots. Computer graphics have found widespread use in printing, product design and manufacturing, scientific research, and entertainment since the 1960s. In the business office, computers routinely create graphs and tables to illustrate text information. Computer-aided design systems have replaced drafting boards in the design of a vast array of products ranging from buildings to automotive bodies and aircraft hulls to electrical and electronic devices. Computers are also often used to test various mechanical, electrical, or thermal properties of the component under design. Scientists use computers to simulate the behaviour of complicated natural systems in animated motion-picture sequences. These pictorial visualizations can afford a clearer understanding of the multiple forces or variables at work in such phenomena as nuclear and chemical reactions, large-scale gravitational interactions, hydraulic flow, load deformation, and physiological systems. Computer graphics are nowhere so visible as in the entertainment industry, which uses them to create the interactive animations of video games and the special effects in motion pictures. Computers have also come into increasing use in commercial illustration and in the digitalization of images for use in CD-ROM products, online services, and other electronic media.