The final aspect of graphics covered in this chapter is the
input of information to the system. Most graphic-input devices
require no programming, so their interfaces are quite simple.
Two-dimensional devices such as tablets, joysticks, and mice
provide two-dimensional values; One-dimensional devices such as
knobs and thumbwheels provide single values. The only possible
software operations are scaling and relative-motion tracking.
This section will therefore concentrate on the physical workings
of various graphic-input devices to provide a feel for their
use. One-dimensional input devices, such as knobs and
thumbwheels, are called valuators. Typically, knobs are
aggregated in large sets, each one having a specific function
associated with it. Thumbwheels are large diameter knobs turned
on their side and sunk into a panel so that the exposed edge can
be rolled like a sliding surface. Typically there are two
thumbwheels placed at right angles so that x- and y-axis
specification can be done. Valuators can have f