Scrolling Large Background Image

[manuMenzella]

Hi. First of all, let me introduce myself. My name is Manuel, I am a college student from Argentina, and I am quite a beginner in iPhone programming. I have been benefiting from this forum for a few weeks, and I think that I am ready to make my first "real" iPhone app. I have come across a problem that I cannot solve. Could you guys take a look at it?

I am building a game which has a vertically scrolling background image. This image is quite large, about 2000 pixels high. If I load it using a simple UIImageView and then move its center around, the animation is far from smooth. So what I did was slice the image and ended up with 8 slices, called BG2.u@2x.jpg, with u being 1,2, ... , 8. What I am currently doing is loading them up dynamically as I need them (when a slice goes offscreen, I change the center and the image of that UIImageView to reuse it). The problem is that I still can't make it smooth. It looks jerky, and stops for a few instants every few seconds. I know that this can be done on the iPhone because I have seen it on countless other games, but for some reason I can't get it to work. Can you guys take a look at the code that I have come up with?

This is the .h file:

#import <UIKit/UIKit.h>
#import <QuartzCore/QuartzCore.h>

@interface ScrollBG2ViewController : UIViewController {
    IBOutlet NSMutableArray *bgArray;
    int bgIndex;
}

@property (nonatomic, retain) IBOutlet NSMutableArray *bgArray;
@property int bgIndex;

@end

This is the relevant part of the .m file:

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    if (!self.bgArray) bgArray = [[NSMutableArray alloc] init];
    
    bgIndex = 8;
    int numOfSlices = ceil((self.view.bounds.size.height / kBGSliceHeight)) + 2;
    for (int i = 1; i <= numOfSlices; i++) {
        NSString *bgImageName = [NSString stringWithFormat:@"BG2.%d.jpg", bgIndex];
        UIImage *bgImage = [UIImage imageNamed:bgImageName];
        UIImageView *bgImageView = [[UIImageView alloc] initWithImage:bgImage];
        
        CGPoint bgFrameOrigin = CGPointMake(0.0f, self.view.bounds.size.height - i * kBGSliceHeight);
        CGPoint bgCenter = CGPointMake(bgFrameOrigin.x + bgImageView.frame.size.width/2.0f,
                                       bgFrameOrigin.y + bgImageView.frame.size.height/2.0f);
        bgImageView.center = bgCenter;
        
        [self.bgArray addObject:bgImageView];
        [self.view addSubview:bgImageView];
        
        [bgImageView release];
        
        bgIndex -= 1;
        if (bgIndex < 1) bgIndex = 8;
    }
    
    CADisplayLink *aDisplayLink = [[UIScreen mainScreen] displayLinkWithTarget:self selector:@selector(gameLoop)];
    [aDisplayLink setFrameInterval:2];
    [aDisplayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
}

- (void)gameLoop
{
    for (UIImageView *bgImageView in self.bgArray) {
        CGPoint bgImageCenter = CGPointMake(bgImageView.center.x, bgImageView.center.y + 2.0f);
        bgImageView.center = bgImageCenter;
        
        if (bgImageCenter.y - kBGSliceHeight/2.0f > self.view.bounds.size.height) {
            [self.bgArray removeObject:bgImageView];
            
            UIImageView *bgImageViewLast = [self.bgArray lastObject];
            bgImageCenter = CGPointMake(bgImageViewLast.center.x, bgImageViewLast.center.y - kBGSliceHeight);
            bgImageView.center = bgImageCenter;
            
            NSString *bgImageName = [NSString stringWithFormat:@"BG2.%d.jpg", bgIndex];
            UIImage *bgImage = [UIImage imageNamed:bgImageName];
            bgImageView.image = bgImage;
            
            bgIndex -= 1;
            if (bgIndex < 1) bgIndex = 8;
            
            [self.bgArray addObject:bgImageView];
        }
    }
}

I know that you guys are busy with your stuff, and I appreciate you taking your time to take a look at the code and help me out. If you need any other piece of information, just let me know, and I will upload it as soon as possible.

BTW, sorry about my English. Spanish is my first language.

Thanks again for your help,
Manuel

[manuMenzella]

Still couldn't solve this. It seriously is driving me crazy. Do you guys have any idea?
Thanks again.