of Microprocessors

356 MUSICAL ApPLICATIONS OF MICROPROCESSORS
is drawn. With a pixel display, additional hardware is generally necessary to
determine the X and Y positions of the beam when the pulse occurs. In either
case, one can easily point to an object on the screen and the program can
ascertain which object it is or its location.
Drawing on the screen where nothing currently exists is a little more
difficult. In practice, a tracking pattern is displayed, which the user can point
to and move around. The path followed by the pattern then becomes the
drawn line. Various types of tracking patterns are used, but the circle type is
easiest to understand. A circle ofdots is displayed that has a diameter slightly
larger than the acceptance area of the light pen. As long as the pen points to
the center region, no light is seen and the pattern remains stationary. Pen
movement in any direction, however, will put one of the dots in sight of the
pen. The tracking program in the computer will respond by moving the
entire pattern such that the new pen position is in the center. Besides
drawing lines, interactive graphics software can provide for "attaching" the
tracking pattern to an object, which can then be positioned as desired.
Unfortunately, a vector display or extremely fast computer is just about
mandatory for this degree of interaction.
Light pens do have their problems, however. In general, they must be
carefully tuned to the display used such as by adjusting the focus length to
match CRT faceplate thickness. For tracking applications, the CRT brightness
needs to be tightly controlled to avoid the effects of light scattering
within the CRT faceplate and ambient light. Short persistence phosphors are
necessary for good pulse resolution, although long persistence visible short
persistence infrared types exist. Obtaining the necessary speed at low light
levels is a design problem that increases cost. Nevertheless, a properly functioning
light pen is a joy to use and probably the best interactive graphics
input device available short of mind reading.
Finally, most interactive graphics setups use a standard typewriter
keyboard and a special function keyboard. Each function key is set up to
instruct the interactive program to perform a specific function each time the
key is pressed. The actual function of a particular key is entirely dependent
on the particular interactive software in use at the time. For example, six
keys may be reserved to control the display. One might expand the image
5% when pressed and a second could shrink it 5%. Four more could move
the entire display left, right, up, or down. Other often-used interactive
functions may be assigned to other keys. A fu·nction keyboard works just like
an alphanumeric keyboard but is usually limited to 16 to 32 keY5 and
constructed so that the key legends may be easily changed. In fact, legend
overlay sheets are commonly used and one would typically have a sheet for
each interactive program available. In sophisticated systems the sheets may
actually be punched or othetwise coded so that the program can verify that
the operator has actually inserted the correct one!