Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!aplcen!uunet!mcsun!hp4nl!spcc386!stefan
From: stefan@spcc386.UUCP (Stefan Posthuma)
Newsgroups: comp.sys.atari.st
Subject: Re: Fine scrollin'
Keywords: graphics atari helpx2
Message-ID: <590@spcc386.UUCP>
Date: 16 Feb 90 14:23:57 GMT
References: <1199@elmgate.UUCP>
Reply-To: stefan@spcc386.UUCP (Stefan Posthuma)
Organization: SPC Company, Heeswijk-Dinther, the Netherlands
Lines: 27

Horizontal fine scrolling on the ST is not very easy. I have some experience
with demo/game programming on the ST and I'll tell you, nobody likes to do
it because it guzzles processor time.
Forget the blitter, you might be able to scroll 30-40% of the screen with it,
and lose all processor time.
Since the screen of the ST is just a dumb memory map, pixel-scrolling can
only be done by actually shifting the screen image. A long list of LSR of
LSL instructions will do this for you, but no way you can make this work
properly, because you simply haven't enough time. You might be able to do it
in 4-5 vbls, but then it looks awful. Add to this that the low-res screen
is divided into four planes and the horror is complete.
Now clever tricks will help you a bit, but scrolling the whole screen still
is very hard to do. Storing your images pre-shifted and blitting the right
frame is one trick, but this uses lots of memory (imagine storing everything
shifted 16 times)  Using screen buffers and clever code is also possible.
One last alternative is 'Sync scrolling'. This is a incredibly difficult
trick that uses a glitch in the Shifter/MMU to open screen borders. Mess
around properly with them and screen borders will be opened ('Overscan'),
mess around even more and the physbase will start moving around, scrolling
the screen, but this is only for the superfreaks amongst us.

Happy hacking!
-- 
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