of Microprocessors

CONTROL SEQUENCE DISPLAY AND EDITING 341
JONE
RASTER
lUNIT
(A)
(S)
Fig. 11-1. Methods of displaying a line. (A) Line-plotting display. (8) Pointplotting
display.
lies the difference between addressable points, which are the product of X and
Y raster unit counts, and resolvable points, which refer to the quantity that
can theoretically be displayed without merging into a continuous sheet of
light.
Graphic Display Classifications
Graphic displays are classified by whether the fundamental display
element is a line or a dot (point). A line drawing display is capable of drawing
a smooth continuous line from any X,Y coordinate (X and Yare integers in
terms of raster units) to any other on command. A point display is capable of
illuminating any combination of dots at X and Y raster unit intersections.
Figure 11-1 shows the difference in appearance between the two presentations
in which all other factors such as number of raster units and beam size
are equal. Clearly, the line display provides a superior image. This is because
only the endpoints are constrained to be at grid points in the line display.
Line drawing displays are usually called vector displays because the lines
satisfy the mathematical definition of a vector. Such displays have a number
of important advantages besides better-looking images. The most important
is that they are easy to program. The display accepts data in essentially the
same form that it is likely to be manipulated and stored in. Another advantage
is that interactive editing of the image is fairly easy to implement and
large data buffers in memory are generally not required. Resolution of the
image (number of horizontal and vertical raster units) can be made quite high
for a moderate increase in cost and memory usage.
There are, however, disadvantages that prevent vector from being the
dominant CRT display technology. Perhaps most serious is the required
CRT monitor, which consists of the tube, high-voltage power supply, and X
and Y deflection amplifiers. It must be capable of random (on command) X