HUMAN-COMPUTER INTERACTION SECOND EDITION
Dix, Finlay, Abowd and Beale

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Chapter 2 The computer Light pen Page 66

The light pen is similar in principle to the touchscreen, in that the user points directly to items on the screen rather than manipulating a device to move the screen cursor; again it is a direct input device. It differs from the touchscreen in that a particular pointing device is used -- the light pen. The pen is connected to the screen by a cable and, in operation, is held to the screen and detects a burst of light from the screen phosphor during the display scan. The light pen can therefore address individual pixels and so is much more accurate than the touchscreen. It can be used for fine selection and drawing in a way that the touchscreen cannot. Problems with the light pen are that it too is tiring on the arm, and is a fragile device, easily broken, or lost on a busy desk. Both the light pen and touchscreen also suffer from the problem that, in use, the act of pointing actually obscures the display, making it harder to use, especially if complex detailed selections or movements are required in rapid succession. For this reason, and one of increased cost, neither device is as popular as any of the ball or joystick input devices.

Chapter 2 The computer Digitizing tablet Page 67

All digitizing tablets are capable of high resolution, and are available in a range of sizes from A5 to 60 ¥ 60 in (1.52 ¥ 1.52 m). Their sampling rate can vary between 50 and 200 Hz, affecting the resolution of cursor movement, which gets progressively finer as the sampling rate increases. The digitizing tablet can be used to detect relative motion or absolute motion, but is an indirect device since there is a mapping from the plane of operation of the tablet to the screen. It can also be used for text input; if supported by character recognition software, handwriting can be interpreted. Problems with digitizing tablets are that they require a large amount of desk space, and may be awkward to use if displaced to one side by the keyboard.

Chapter 2 The computer Touch pad Page 67

Touch pads are very small compared with tablets, usually around 2--3 inches (50--75 mm) square. They were first used extensively in Apple Powerbook portable computers but are now used in several other notebook computers and can be obtained separately to replace the mouse on the desktop. They are operated by stroking a finger over their surface, rather like using a simulated trackball. The feel is very different from other input devices, but as with all devices users quickly get used to the action and become proficient. However, even experienced users continue to report problems if another finger accidentally touches the pad causing the motion detection to become confused and the cursor to jump across the screen.

Chapter 2 The computer Eyegaze Page 67

The eyegaze system consists of a small matchbox-sized unit mounted on a headband that is worn over the user's head. Sitting in front of the eye, a low-power laser is shone into the eye and is reflected off the retina. The reflection changes as the angle of the eye alters, and by tracking the reflected beam the eyegaze system can determine the direction in which the eye is looking. This can then be used to move the screen cursor. The eyegaze is a very fast and accurate device, but is also expensive. It is fine for selection but not for drawing since the eye does not move in smooth lines. Such systems have been used in military applications, notably for guiding air-to-air missiles to their targets, but are starting to find more peaceable uses, again for disabled users and for workers in environments where it is impossible for them to use their hands. The rarity of the eyegaze is due partly to its novelty and partly to its expense, and it is usually found only in certain domain-specific applications.

Chapter 2 The computer 2.3.3 Keyboard-based positioning devices Page 67

The various ways of controlling a pointer on the screen are numerous, as shown by the examples discussed. There are many others as well. We are not attempting to provide a full categorization and list all the available devices, but to give a flavour of the range of devices available and the effect that the device chosen has on the form of the interaction. A number of devices that have specific characteristics can be grouped under the heading of keyboard-based input, in that they are either special keys placed on the keyboard, or usually placed on it but do not use keys.

Chapter 2 The computer 2.3.3 Keyboard-based positioning devices Page 68

Thumb-wheels are different in that they have two orthogonal dials to control the cursor position. Such a device is very cheap, but slow, and it is difficult to manipulate the cursor in any way other than horizontally or vertically. This limitation can sometimes be a useful constraint in the right application. For instance, in CAD the designer is almost always concerned with exact verticals and horizontals, and a device that provides such constraints is very useful, which accounts for the appearance of thumb-wheels in CAD systems. Another successful application for such a device has been in a drawing game such as Etch-a-Sketch in which straight lines can be created on a simple screen, since the predominance of straight lines in simple drawings means that the motion restrictions are an advantage rather than a handicap. However, if you were to try to write your signature using it, the limitations are all too apparent. The appropriateness of the device depends on the task to be performed.

Chapter 2 The computer 2.3.4 Classifying 2D pointing devices Page 69

Simple mappings are just transformations of user motion to screen motion; complex mappings are those where the action taken to move in a particular direction is not obviously related to movement in the direction, whilst direct mappings are those where the motion on screen is dictated by user indication on the screen. Selection refers to the process of actually selecting an icon, whilst dragging refers to the method for moving such icons.

Chapter 2 The computer Cockpit and virtual controls Page 70

In many PC games and desktop virtual reality (where the output is shown on an ordinary computer screen), the controls are themselves virtual. This may be a simulated form of the cockpit controls or more prosaic up--down left--right buttons. The user manipulates these virtual controls using an ordinary mouse (or other 2D device). Note that this means there are two levels of indirection. It is a tribute to the flexibility of the human mind that people can not only use such systems but also rapidly become proficient.

Chapter 2 The computer 2.4 Output devices Page 72

There is one predominant output device in use today: the computer screen, usually the cathode ray tube. This is highly expressive, relatively cheap and visually oriented. The vast majority of interactive computer systems would be unthinkable without screens, but many such systems do exist, though usually in specialized applications only. Considering the more general definition of the computer, systems such as cars, hi-fis and light switches all have different outputs from those expressible on a screen, but in the personal computer and workstation market, screens are pervasive.

Chapter 2 The computer 2.4 Output devices Page 72

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.

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