DEFINING
THE
SIZE
OF
THE
DISPLAY
WINDOW
Mter
you have completely defined the playfield, you need
to
define the size of the
display window, which is the actual size of the on-screen display. Adjustment of display
window size affects the entire display area, including the border and the sprites, not
just
the playfield. You cannot display objects outside of the defined display window. Also,
the size of the border around the playfield depends on the size of the display window.
The
basic playfield described in this section is the same size as the screen display area
and also the same size as the display window. This is not always the case; often the
display window is smaller than the actual "big picture" of the playfield as defined in
memory (the raster). A display window
that
is
smaller than the play
field
allows you
to
display some segment of a large playfield
or
scroll the playfield through the window.
You can also define display windows larger than the basic playfield. These larger
playfields and different-sized display windows are described in the section below called
"Bit-Planes and Display Windows of All Sizes."
You define the size of the display window by specifying the vertical and horizontal posi-
tions
at
which the window
starts
and stops and writing these positions
to
the display
window registers. The resolution of vertical
start
and stop is one scan line. The resolu-
tion of horizontal
start
and stop is one low-resolution pixel. Each position on the screen
defines the horizontal and vertical position of some pixel, and this position is specified by
the x and y coordinates of the pixel. This document shows the x and y coordinates in
this form: (x,y). Although the coordinates begin
at
(0,0) in the upper left-hand corner of
the screen, the first horizontal position normally used is
$81
and the first vertical posi-
tion is
$20. The hardware allows you
to
specify a starting position before ($81,$20),
but
part
of the display may not be visible. The difference between the absolute starting
position of (0,0) and the normal starting position of
($81,$20) is the result of the way
many video display monitors are designed.
To
overcome the distortion
that
can occur
at
the extreme edges of the screen, the scanning beam sweeps over a larger area than the
front face of the screen can display. A starting position of
($81,$20) centers a normal
size display, leaving a border of eight low-resolution pixels around the display window.
Figure
3-9
shows the relationship between the normal display window, the visible screen
area, and the area actually covered by the scanning beam.
46
Playfield Hardware
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