ballgame question

[gbaldwin9]

Hi all,

I'm playing around with a ballgame. What I want for now is a ball that starts out bouncing around the screen, but gradually comes to a stop - as if experiencing friction. Here is my code so far:

#define kFriction 0.2

#define kBallSpeedX 12
#define kBallSpeedY 12

@implementation BallGameViewController

@synthesize ball;
@synthesize ballVelocity;
@synthesize friction;

...

- (void)viewDidLoad {
    [super viewDidLoad];
	ballVelocity = CGPointMake(kBallSpeedX,kBallSpeedY);
	friction = CGPointMake(kFriction, kFriction);
	
	[NSTimer scheduledTimerWithTimeInterval:1.0/60 target:self selector:@selector(gameLoop) userInfo:nil repeats:YES];
}

-(void)gameLoop  {
	
	ball.center = CGPointMake(ball.center.x +ballVelocity.x, ball.center.y +ballVelocity.y);
	
	if(ballVelocity.x > 0 ) {
		ballVelocity.x -= friction.x;
		
	}
	if (ballVelocity.y > 0) {
	
		ballVelocity.y -= friction.y;
		
	}
	
	if (ball.center.x >self.view.bounds.size.width  || ball.center.x < 0) {
		
		ballVelocity.x = -ballVelocity.x;	
	}
	if (ball.center.y >self.view.bounds.size.height || ball.center.y < 0 ) {
		
		ballVelocity.y = - ballVelocity.y;
	}
	
	

}

What I get, however, is a ball that bounces around, gradually slows to a near stop, and then drifts really slowly into the top left corner of the screen.

Can anyone see what I'm doing wrong?

Thanks!

[Kalimba]

Imagine that the velocity at the start of your game loop is (0.1, 0.1). What happens to the velocity during the game loop? Then imagine how your game loop behaves every subsequent time it's called.

[gbaldwin9]

Originally Posted by Kalimba

Imagine that the velocity at the start of your game loop is (0.1, 0.1). What happens to the velocity during the game loop? Then imagine how your game loop behaves every subsequent time it's called.

OK, since I was decrementing (is that a word?) it by .2, when friction is applied to a speed of .1 it becomes negative on both the x and y coordinates and then, being below zero, no more friction is applied and it just drifts up and to the left.

Thanks.

Still, what I've written -while it now comes to a complete stop - isn't as realistic as I'd like, particularly near the end. What happens now is that as the ball slows it sort of flattens out on the x axis . That is, it seems that it seems that it reaches 0 y speed before it reaches 0 x speed and the result is that it always end traveling exactly horizontally right at the end rather than ending on a diagonal path.

Is there a better way to apply friction?