From: Tom Seddon <tom AT sunholme DOT demon DOT co DOT uk>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Graphics - jerky scrolling
Date: Tue, 30 Dec 1997 16:13:15 +0000
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In article <m0xmdp7-000S26C AT inti DOT gov DOT ar>, "Salvador Eduardo Tropea
(SET)" <salvador AT inti DOT gov DOT ar> writes
>riker AT pipcom DOT com (Matt Riker) wrote:
>
>> Whenever I run a program that has graphics scrolling, I notice that 
>> around 2 times every second, the graphic that is scrolling jerks.  I 
>> have a 1 MB video card, which has forever ran anything I've tried with 
>> it, so I'm positive it's not my hardware.  For all I know, it could just 
>> be psychological :)  Thanks.
>Are you sure the machine isn't interrupted by other project? Are you running 
>plain DOS?
>Double check the code, perhaps you are bliting a wrong buffer from time to 
>time.

How does the scrolling work? If it is multidirectional, this is quite
hard to achieve without having to redraw the entire screen every now and
again. Perhaps this is causing the problem.

If you are waiting for the vertical sync, so that all your graphics will
be smooth, it is possible that one or two frames each second are going
over the 1/60th second frame time -- when you next wait for the vertical
sync, you will end up waiting two frames in total rather than one, which
can cause the jerks.

And as above, if you are running under Windows '95 it will probably be a
bit jerky anyway... despite what many say, I've yet to find the DOS game
that runs anything better than acceptably under Windows '95.

--Tom