Path: chuka.playstation.co.uk!scea!greg_labrec@interactive.sony.com
From: "John Emmer" <jjustice@cs.bsu.edu>
Newsgroups: scea.yaroze.programming.2d_graphics
Subject: Sprite/OT trouble
Date: 2 Aug 1997 19:10:52 GMT
Organization: Justice Center
Lines: 64
Message-ID: <01bc9f6f$da300e60$37f135ce@jjustice.iquest.net>
Reply-To: "John Emmer" <jjsutice@cs.bsu.edu>
NNTP-Posting-Host: and-000-23.iquest.net
X-Newsreader: Microsoft Internet News 4.70.1161

Here's my main display loop:

while(!(padValue&PAD1_SELECT))
{
	padValue = PadRead();

	activeBuff = GsGetActiveBuff();
		
	GsSetWorkBase((PACKET*)GpuPacketArea[activeBuff]);
		
	GsClearOt(0,0,&WorldOT[activeBuff]);
		
	// register sprites in OT (just players for now)
	for (i = 0; i < 2; i++)
	{
		GsSortSprite(&(player[i].tank.turret.turretSprite),
			&WorldOT[activeBuff],0);
		GsSortSprite(&(player[i].tank.tankSprite),
			&WorldOT[activeBuff],1);
	}

	// add a dummy tank because last sprite is not displaying
	GsSortSprite(&(computer[0].tank.tankSprite),&WorldOT[activeBuff],0);
		
	DrawSync(0);

	VSync(0);

	GsSwapDispBuff();

	GsSortClear(255,255,255,&WorldOT[activeBuff]);

	GsDrawOt(&WorldOT[activeBuff]);
}

And here are my questions:

	A couple of weird things are happening.  I want the turrets to display
over the tanks, so I set the priority of the tanks to 1 and the turrets to
0.  (My OT length is currently 4). For some reason, if I swap the order of
the two lines inside the for loop, the screen gets garbage.  In their
current order it works.  I know "Doctor, it hurts when I put them in this
order" "If it hurts, don't do it!" - but I want to understand _why_ this
makes a difference.
	Secondly, as you see from the comment after the for loop, the fourth
sprite doesn't display (actually, it flickers on for a fraction of a second
every now and then).  I figured it had something to do with the timing, so
I added a fifth sprite that I don't currently care about, so that the fifth
one wouldn't display instead of the fourth one that I do want.  But why
isn't the fourth sprite displaying?
	Finally, both of these problems go away if I run the program stepwise
through the CodeWarrior Debugger.  This suggests to me even more strongly
that the problem has to do with timing.  But I have my DrawSync(0) and
VSync(0) in there, and the rest of my code looks like the examples I've
seen (thanks Shadow and Mario!) as far as I can tell.  What's up?
	Thanks!

-- 
  John Emmer     Video Game Enthusiast, Philosopher, Fledgling Programmer
     jjustice@cs.bsu.edu  jjustice@iquest.net  jpemmer@bsuvc.bsu.edu  
 
    Vectrex; 7800, Supercharger, Lynx, Jaguar; NES, SNES, Virtual Boy;
  Turbo Duo & Express; SG, SGCD, 32X, Saturn; 3DO; PlayStation, Yaroze;
C64, A600, A1200, P100; Arcade Centipede, Spy Hunter, Neo-Geo; BA, MA, ABD