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: Re: Algorithm to orbit?
Date: 3 Aug 1997 21:53:10 GMT
Organization: Justice Center
Lines: 70
Message-ID: <01bca04f$83feb6a0$8bf135ce@jjustice.iquest.net>
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Reply-To: "John Emmer" <jjsutice@cs.bsu.edu>
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John Emmer <jjustice@cs.bsu.edu> wrote in article
<01bca043$beb64300$8bf135ce@jjustice.iquest.net>...
> 
> what I need to do is change the center of rotation for the turret sprite
> when the tank rotates.  That is, when the turret is rotating with the
tank,
> I rotate it around the center of the tank, and when it rotates on its
own,
> I use its normal center of rotation.  Doh!

	Here are the routines I'm using for this (there are, of course, equivalent
routines for rotation to the right):

// some notes:
// FULLROTATION = 1470464 (360 degrees of size 4096)
// the macros ending in '_OFF' store the offsets for the
//	 center of rotation of their respective objects

void RotateTankLeft(Tank * rotate)
{	
	short tempMX, tempMY;
	
	rotate->tankSprite.rotate -= ONE;
	if (rotate->tankSprite.rotate < 0)
		rotate->tankSprite.rotate += FULLROTATION;

	// store current center of rotation for turret
	tempMX = rotate->turret.turretSprite.mx;
	tempMY = rotate->turret.turretSprite.my;

	// move center of rotation of turret to center of rotation for tank
	rotate->turret.turretSprite.mx += 
		(rotate->turret.turretSprite.x+TURRET_XROT_OFF
		 - rotate->tankSprite.x+TANK_XROT_OFF);
	rotate->turret.turretSprite.my +=
		(rotate->turret.turretSprite.y+TURRET_YROT_OFF
		 - rotate->tankSprite.y+TANK_YROT_OFF);

	// rotate turret around tank center
	RotateTurretLeft(rotate);

	// reset turret center of rotation
	rotate->turret.turretSprite.mx = tempMX;
	rotate->turret.turretSprite.my = tempMY;	
}

	And the turret routine is simply:

void RotateTurretLeft(Tank * rotate)
{
	rotate->turret.turretSprite.rotate -= ONE;
	if (rotate->turret.turretSprite.rotate < 0)
		rotate->turret.turretSprite.rotate += FULLROTATION;
}

	Now, I thought that what I needed to do was temporarily change the center
of rotation for the turret by the current difference between its screen
location and that of the center of rotation for the tank.  But the turret
still swims around as the tank rotates.  Where did my thinking - or my
programming - go wrong?

-- 
  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