Moving
(Scrolling)
Playfields
If
you
want
a background display
that
moves, you can design a play
field
larger than the
display window and scroll it.
If
you are using dual playfields, you can scroll them
separately.
In vertical scrolling, the playfield appears
to
move smoothly up
or
down on the screen.
All you need do for vertical scrolling is progressively increase
or
decrease the starting
address for the bit-plane pointers by the size of a horizontal line
in
the playfield. This
has the effect of showing a lower or higher
part
of the picture each
field
time.
In horizontal scrolling the playfield appears
to
move from right to left or left to right on
the screen. Horizontal scrolling works differently from vertical scrolling - you must
. arrange
to
fetch one more word of
data
for each display line and delay the display of
this data.
For
either type of scrolling, resetting of pointers or data-fetch registers can be handled
by the Copper during the vertical blanking interval.
VERTICAL
SCROLLING
You can scroll a playfield upward or downward
in
the window. Each time you display
the playfield, the bit-plane pointers
start
at
a progressively higher
or
lower place
in
the
big picture
in
memory. As the value of the pointer increases, more of the lower
part
of
the picture
is shown and the picture appears to scroll upward. As the value of the
pointer decreases, more of the upper
part
is shown and the picture scrolls downward.
If
your picture has 200 vertical lines, each step can be as little as 1/200th of the screen. In
interlaced mode each step could be 1/400th of the screen
if
clever manipulation of the
pointers is used,
but
it
is
recommended
that
scrolling be done two lines
at
a time
to
maintain the odd/even
field
relationship.
Playfield Hardware
73
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