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From: leue@galen.crd.ge.com (Bill Leue)
Newsgroups: comp.sys.mac.programmer
Subject: Animation Questions
Keywords: CopyBits, VBlank
Message-ID: <3123@crdgw1.crd.ge.com>
Date: 12 Oct 89 13:16:27 GMT
Sender: news@crdgw1.crd.ge.com
Reply-To: leue@galen.crd.ge.com (Bill Leue)
Organization: General Electric Research & Development
Lines: 37

I've got a couple of questions regarding animation on the Mac.  I know
that this material has been discussed before in this group, but I didn't 
get all of it the first time.  Please, no flames if you can restrain
yourselves :-).

I'm moving some fairly small (app. 100 pixels wide x 20 pixels high)
sprites around using what I think is the standard method:  copying the
background to an offscreen bitmap, drawing the new position of the object
in the offscreen bitmap, and CopyBits-ing the whole thing back to the
screen.  It works pretty well, and eliminates the obnoxious flickering
that characterized my first, naive efforts to do animation using XOR
drawing.

However, there are still two glitches.  First, the objects being animated
are sometimes "sheared" horizontally, so that they are torn into two
pieces which don't quite track each other as the object moves (say)
horizontally.  I'm tenatively diagnosing this problem as a lack of
synchronization with the screen refresh.  Am I right?  If so, I suspect
there's a mechanism to synchronize drawing with vertical blanking.  Does
anyone have any lore (or posssibly code) they feel like sharing on this
subject?

Second, I am using "Classic" QuickDraw to perform limited color drawing.
I use ForeColor() and BackColor() calls along with a 50% grey pattern fill
to get a useful range of colors using dithering.  In this way I can
get color objects on a color system, but don't have to make special code
for  monochrome systems.  It works fine, except that the objects which
are animated using CopyBits only display in black and white.  I guess I
thought that since I was using the same QD calls to write to the offscreen
bitmap that I used to write the "static" portions of the screen, the
objects would also show up in color.  Wrong!  Anyone have an idea what I'm
doing wrong?  Remember, I am NOT using Color QD at all -- just the 8
colors in normal QD.

Thanks!
-Bill Leue
leue@crd.ge.com