The system is therefore capable of actually displaying many more than the number of colours in the colourmap, but not simultaneously. The set of colours make up what is called the colourmap, and the colourmap can be altered at any time to produce a different set of colours. For example, 8 bits/pixel give rise to 28 = 256 possible colours at any one time. More bits per pixel give rise to more colour or intensity possibilities. One bit per pixel can store on/off information, and hence only black and white. The colour or, for monochrome screens, the intensity at each pixel is held by the computer's video card. A special phosphor glows a different colour depending on the intensity of the beam hitting it. These three phosphor dots are focused to make a single point using a shadow mask, which is imprecise and gives colour screens a lower resolution than equivalent monochrome screens.Īn alternative approach to producing colour on the screen is to use beam penetration. Combining these colours can produce many others, including white, when they are all fully on. Three electron guns are used, one each to hit red, green and blue phosphors. High-quality screens are available at up to approximately 1600 ¥ 1200 pixels, which offer both excellent resolution and large screen estate allowing many windows to be open at once.īlack and white screens are able to display greyscale by varying the intensity of the electron beam colour is achieved using more complex means. The resolution of screens using raster scanning is typically 640 ¥ 480 pixels, although higher resolutions are increasingly popular. Using a high-persistence phosphor, which glows for a longer time when excited, also reduces flicker, but causes image smearing especially if there is significant animation. Another way of reducing flicker is to use interlacing, in which the odd lines on the screen are all scanned first, followed by the even lines. This is repeated, at about 30 Hz (that is, 30 times a second), per frame, although higher scan rates are sometimes used to reduce the flicker on the screen. The electron beam is scanned from left to right, and then flicked back to rescan the next line, from top to bottom. This is the most common type, similar in operation to a standard television screen. As the beam hits the phosphor-coated screen, the phosphor is excited by the electrons and glows (see Figure 2.7).Ĭhapter 2 The computer Raster scan A stream of electrons is emitted from an electron gun, which is then focused and directed by magnetic fields. The cathode ray tube (CRT) is the predominant display device. In this section, we discuss the computer screen in detail, looking at the different types of cathode ray tube as well as the more recent screen technologies, and then move on to look at some less obvious output devices and the different nature of the interaction that they support.Ĭhapter 2 The computer 2.4.1 Cathode ray tube Showing 30 to 39 of 281 Ĭhapter 2 The computer 2.4 Output devices HUMAN-COMPUTER INTERACTION SECOND EDITION
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