Table of Contents
Advertisement
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
Hide quick links:
Advertisement
Table of Contents
