Path: utzoo!utgpu!attcan!uunet!lll-winken!ames!killer!radar!buffington
From: buffington@radar.UUCP (Jon Buffington)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Fast image changes
Message-ID: <158@radar.UUCP>
Date: 25 Jan 89 15:42:14 GMT
References: <2350@ilium.cs.swarthmore.edu>
Organization: CDS Consulting and MacInterested, Nashville, TN
Lines: 51

From article <2350@ilium.cs.swarthmore.edu>, by jackiw@cs.swarthmore.edu (Nick Jackiw):
> In article <1453@ditsyda.oz> vincent@ditsyda.oz (David A. Vincent) writes:
>> in article <243@usage.csd.unsw.oz>, grants@usage.csd.unsw.oz (Grant Sayer) says:
>> >  	I am trying to draw two images one on an offscreen pixelmap and then
>> [...]
>> >code which swaps the images, basically a copybits operation, into the VBL 
>> >task queue seem a viable solution.  If this is the casethen is it true
>> 
>> This seems viable provided that the copybits can be completed during
>> the VBL period.  
>> 
> 
> Much easier is just to coordinate your copyBits with the VBL interval.  
> You can do this in Pascal by waiting for the system timer to tick, something
> which happens as a system-VBL task.
> 
> 	var x:longint;
> 
> 	Delay(1,X);	{Wait for next VBL interval}
> 	Copybits(...);	{Here we go, do it fast!}
> 
> Follow-ups to comp.sys.mac.programmer ONLY.  Eradicate cross-posts!

Your above advice (using tickcount to synch the raster) is sound
only if your monitor is running at 60Hz.  Since the person was
inteerested in pixmaps, I would think they are using Apple's
MAc II monitors which runn near 67Hz.  In this case you have
to use SlotVInstall() and assoc. routines to create a goAhead
Boolean which you should check before CopyBits() in the main
code.

ex: pushScreen(bMapPtr, src, dest)
BitMap  *bMapPtr;
Rect  *src, *dest;
{
	extern Boolean goAhead; /* this is modified in the VTask routine */
      
    goAhead = FALSE;
   while (!goAhead) ; /* wait for task to signal retrace interupt */
    CopyBits(...);
}

Helpful?  Hope so...

-------------------
jcb

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eexxex: pushImage2Scree