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Stolen Data 3
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Stolen Data 3.adf
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MAG-TEXT
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CODE7a
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1989-01-01
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31 lines
have produced a sine-scrolling intro in
their time, and the idea behind them is
quite simple. I wont go into all the
gory details, after all you'll never
learn to code if you don't put in some
effort, so you'll have to figure out
the technical details for yourselves.
The main idea behind these things
is to use BLTAFWM and BLTALWM to mask
out a portion of a normal blitter
scroller, be it a single pixel width
line or more, and then OR it onto a
blank screen, where the x position is
the same but the Y position varies
according to a sine-table. If you
follow DEL's source, you'll see that he
has basically started at one side of
the scroller and then moved a pixel at
a time across it until he reaches the
far side. There is a large table of
long word values for BLTAFWM which
start at hex 80008000 and end with hex
00010001 rotated to the right one each
time. Then the sinetable is
premultiplied by the width of the
screen to give a y coordinate offset.
This technique is certainly easier
to follow but not particularly fast, to
optimise the routine try the following
ideas.