HUMAN-COMPUTER INTERACTION
SECOND EDITION
HUMAN-COMPUTER INTERACTION
SECOND EDITION
The cathode ray tube (CRT) is the predominant display device. A stream of electrons is emitted from an electron gun, which is then focused and directed by magnetic fields. As the beam hits the phosphor-coated screen, the phosphor is excited by the electrons and glows (see Figure 2.7).
This is the most common type, similar in operation to a standard television 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 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. 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. 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.
The resolution of screens using raster scanning is typically 640 ¥ 480 pixels, although higher resolutions are increasingly popular. 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.
Black and white screens are able to display greyscale by varying the intensity of the electron beam; colour is achieved using more complex means. Three electron guns are used, one each to hit red, green and blue phosphors. Combining these colours can produce many others, including white, when they are all fully on. 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.
An alternative approach to producing colour on the screen is to use beam penetration. A special phosphor glows a different colour depending on the intensity of the beam hitting it.
The colour or, for monochrome screens, the intensity at each pixel is held by the computer's video card. One bit per pixel can store on/off information, and hence only black and white. More bits per pixel give rise to more colour or intensity possibilities. For example, 8 bits/pixel give rise to 28 = 256 possible colours at any one time. 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. The system is therefore capable of actually displaying many more than the number of colours in the colourmap, but not simultaneously.
The CRT is a cheap display device, and has fast enough response times for rapid animation coupled with a high colour capability. Note that animation does not necessarily mean little creatures and figures running about on the screen, but refers in a more general sense to the use of motion in displays: moving the cursor, opening windows, indicating processor-intensive calculations, or whatever. As screen resolution increases, however, the price rises. Because of the electron gun and focusing components behind the screen, CRTs are fairly bulky, though recent innovations have led to less bulky displays in which the electron gun is not placed so that it fires directly at the screen, but fires parallel to the screen plane with the resulting beam bent through 90 degrees to hit the screen.
Although horizontal and vertical lines can be drawn perfectly on such a screen, also lines at 45 degrees reproduce reasonably well. However, lines at any other angle and curves have 'jaggies', rough edges caused by the attempt to approximate the line with pixels.
When using a single colour jaggies are inevitable. Similar effects are seen in bitmap fonts. The problem of jaggies can be reduced by using high-resolution screens, or by a technique known as anti-aliasing. Anti-aliasing softens the edges of line segments, blurring the discontinuity and making the jaggie less obvious.
Look at these two images with your eyes slightly screwed up. See how the second anti-aliased line looks better. Of course, screen resolution is much higher, but the same principle holds true. The reason this works is because our brains are constantly 'improving' what we see in the world: processing and manipulating the raw sensations of the rods and cones in our eyes and turning them into something meaningful. Often our vision is blurred owing to poor light, things being out of focus, or defects in our vision. Our brain compensates and tidies up blurred images. By deliberately blurring the image, anti-aliasing triggers this processing in our brain and we appear to see a smooth line at an angle.
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