Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!cs.utexas.edu!uunet!sco!jamesm
From: jamesm@sco.COM (James M. Moore)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Saving area under dialog boxes (Re: WingZ)
Message-ID: <2776@viscous.sco.COM>
Date: 1 May 89 19:15:44 GMT
References: <2993@tank.uchicago.edu> <29855@apple.Apple.COM>
Reply-To: jamesm@sco.COM (James M. Moore)
Distribution: na
Organization: The Santa Cruz Operation, Inc.
Lines: 29

In article <29855@apple.Apple.COM> keith@Apple.COM (Keith Rollin) writes:
>>I refresh my image) for permanent storage when the drawn object is done (i.e,
>>on MouseUp).  Wouldn't there be times when you need to CopyBits from screen
>>memory to your offscreen buffer?  I suppose one _could_ draw to your offscreen
>>bugger and CopyBits it on screen constantly, but that sounds fairly awful in
>>terms of speed, etc.
>
>I'm afraid that I don't see the problem here. Either you draw to the screen
>and copy it to an offscreen buffer, or you draw to an offscreen buffer and
>copy it to the screen.

But if/when context switching becomes preemptive, you can't be sure
that something like this won't happen:

	draw to screen
	** context switch - someone else puts another window on the screen,
		which erases part of your window
	** context switch - back to paint program
	update event generated for application window
	copy screenbits to buffer - we've just copied a blank screen.

In the case of the paint program, the speed hit for drawing offscreen
first probably wouldn't be that large.  Just pick a small section of
the drawing to copy back and forth.
-- 
** James Moore **
** Internet:  jamesm@sco.com **
** uucp:  {decvax!microsoft | uunet | ucbvax!ucscc | amd}!sco!jamesm **
** Nil clu no suim ar bith ag SCO ceard a bhfuil me ag scriobh anois. **