The graphics screen

Abstract

The graphics screen is by far the most critical part of the whole CAD/CAM system. Computer hardware technology and the software techniques it supports are now well up to the task they have to perform on price and performance and both technologies continue to develop. Compared with the drawing board and the paper it replaces, however, the graphics screen still has deficiencies. To start with, it is so very much smaller than the drawing board. Figure 5.1 on page 44 shows a graphics screen in use. The size might be increased by some kind of projection system but if you did this you would not get a picture which was any easier to use, because it would be just a coarser picture. The problem is not in fact the size. The electronics which generates the picture on a display treats the screen like a rectangular array of squares to be filled in with black, white or a colour. Each square is called a pixel and is the smallest possible point which can be displayed. The number of pixels in the array is limited by fundamental characteristics of the circuitry. Current devices can achieve screens with about 2000 pixels from one side to the other and a corresponding number from top to bottom. Analysing an A0 sheet of paper in these terms, the smallest dot possible is about 0.2 mm across. The width of the paper is about 1180 mm so that there are about 5900 “pixels” from one side to the other, which means an A0 sheet of paper can hold three times the detail of a high resolution cathode ray tube (CRT) display.