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From: djvelleman@amherst.bitnet
Newsgroups: comp.sys.mac.programmer
Subject: Re: Animation
Message-ID: <11971.27b2e2ae@amherst.bitnet>
Date: 8 Feb 91 17:41:02 GMT
References: <9102030634.AA09598@enuxha.eas.asu.edu> <1991Feb7.093610.12594@ecmwf.co.uk> <1991Feb7.233344.7781@umiami.ir.miami.edu>
Lines: 44

In article <1991Feb7.233344.7781@umiami.ir.miami.edu>, dweisman@umiami.ir.miami.edu (Ordinary Man) writes:
> In article <1991Feb7.093610.12594@ecmwf.co.uk>, mab@ecmwf.co.uk (Baudoin Raoult) writes:
              [stuff deleted]
>> To avoid flickery animation, you must synchronise your CopyBits with the video spot.
>> The simplest way is to use TickCount, as the tick counter is updated every 60th of
>> second, each time the video spot starts to update  new screen. So just write:
>> 
>> 	tmp := TickCount;
>> 	while tmp=Tickcount do { nothing } ;
>> 	
>> 	CopyBits(.....)
> 
> This is fine except for a minor detail. As it says in IM Vol. I...
> 
> "...don't rely on the tick count being incremented to a certain value, such
> as testing whether it has become equal to its old value plus 1; check for
> 'greater than or equal to' (since an interrupt task may keep control for
> more than one tick)."
> 
> So instead say:
> 
> 	tmp:=TickCount;
> 	repeat until TickCount>=(tmp+1);
> 
> 	CopyBits.....
> 
> ==Dan

  Sorry Dan, but you've missed the point of the warning in IM.  IM says not to
rely on the tick count being INCREMENTED TO a certain value, because it might
skip over values if an interrupt task keeps control for more than one tick.
But the proposed code doesn't rely on TickCount being incremented to the value
stored in tmp.  TickCount already has that value, and the code relies on it
eventually being incremented to a different value, which is just fine.  Your
code is OK too.  What IM was warning against was something like this:
       tmp:=TickCount;
       repeat until TickCount=tmp+1;
       CopyBits...
This could turn into an infinite loop if TickCount goes right from tmp to
tmp+2.
  By the way, the THINK Pascal procedure Synch will take care of this for
you.

  -Dan Velleman