of Microprocessors

CONTROL SEQUENCE DISPLAY AND EDITING 355
in an interactive graphics application. First, of course, is the computer and
graphic display device. Second is the array of input devices used by the
operator to communicate with the system. Last is the software necessary to
tie it all together. In a well-implemented interactive graphics application,
the user sits with eyes glued to the screen, hands manipulating input devices,
and mind closing the feedback loop. The effect is as if the image on the
screen is an extension of the user's consciousness. To meet this goal, a highly
effective human interface, both input devices and software, is required.
Many of the music input devices described previously are suitable for
and in fact some were orginally developed for interactive graphics work. A
joystick, for example, is typically a two-dimensional device, thus mating
perfectly with a two-dimensional display. When connected to the computer
through an ADC and suitable programming, one can point to objects or
literally draw curves on the face of the display. As a practical matter, however,
it is fairly difficult to control a joystick precisely; thus, intended
straight lines are crooked and pointing is somewhat of a trial-and-error
procedure.
Graphic Input Techniques
A mouse is a related device that overcomes the awkwardness of a joystick.
Typically, it is about the size and shape of an orange half and is
equipped with orthogonal wheels that allow it to roll around easily on a table
top. The wheels are connected to transducers and provide X and Y outputs.
Essentially, the position of the mouse corresponds to the joystick's handle,
but a much larger movement range allows more precise control. Another
device called a trackball consists of a smooth ball about the size of a baseball
sitting nearly frictionless in a stationary socket. The user can roll the ball in
any direction and thus provide X and Y outputs. Since the range may
encompass one or more complete revolutions, control is more precise. Its
major advantage is conservation of table space.
The graphic digitizer mentioned earlier is the most precise of all. Since
the pen or stylus is completely frictionless, one may input to the system as
accurately as drawing on paper can be done. In fact, because of their precision
and repeatability, a screen outline may be taped to the digitizer surface and
used as a guide in the interaction process. If real ink is used in the digitizer
pen, then a permanent record of the input is produced as a by-product.
It would seem though that the ideal situation would be drawing directly
on the display screen itself and in fact devices called light pens actually
accomplish this-almost. A light pen consists of a high-speed photocell and
a lens system that makes its angle of acceptance very small and localized. If
placed on the surface of the CRT directly above an illuminated portion of the
image, a pulse is generated whenever the beam refreshes that part of the
image. With the software-refreshed vector display described previously, the
pulse can be connected to interrupt the CPU while the line "seen" by the pen