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Chapter Five. Unification > Elementary Actions Cataloged

5-2. Elementary Actions Cataloged

As you design an interface, you should have the palette of possibilities arrayed in your mind, much as a painter has his colors organized. The spectrum of elementary actions that a user can perform is remarkably limited. From this set of elementary actions, all user interaction is built. With a keyboard, you can tap keys or press and hold them while you perform other actions. With a GID, you can move a cursor within the bounds of the display (or displays) on your system, and you can signal the computer, using the speed, direction, and acceleration of the GID, although you usually use GID speed and acceleration only as an aid to pointing. With a GID button, you can signal display locations to which you are pointing with the cursor. These elementary actions have widely varying semantics, depending on the application being run.

Pressure-sensitive graphic tablets can detect the angle at which the pen is held, which results in one or two additional numerical values being associated with each location to which the user points. Except when the user is doing freehand drawing, these parameters are rarely used. Musical keyboards may provide, as inputs to the computer, both the velocity with which the key is pressed and the pressure with which the key is being held down after being pressed. There are also joysticks and three-dimensional input devices. Nonetheless, most interaction is accomplished with a keyboard and a standard, two-dimensional GID. This section will address primarily the standard input and output devices. In many cases, it will be clear how the principles extend to more exotic physical or, eventually, mental interfaces. Having an explicit taxonomy and vocabulary of elementary actions and the elementary operations built from them is, I find, a great aid in discussing and designing interfaces.


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