Accelerometer and calibration - iPhone SDK

Question

I need to use the functionality of an accelerometer in my iPhone game. I just have to move an image by tilting the device. However most videos on YouTube just show the tilt feature that is somehow inverted and forget to include the calibration.

I want the user to calibrate their device to whatever position they're in. Does anyone know how I should get started on this?

Answer 1

I made an app like this once. I'll post it here, but it's for iOS 4...

To calibrate:

int tapCount = 0;
- (void)cancelDoubleTap {
    if (tapCount < 2) {
        tapCount = 0;
    }
}
- (void)reset {
    [[NSUserDefaults standardUserDefaults] setFloat:0 forKey:@"X-Calibrate"];
    [[NSUserDefaults standardUserDefaults] setFloat:0 forKey:@"Y-Calibrate"];
    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Reset!" message:@"Calibration reset." delegate:nil cancelButtonTitle:@"Okay." otherButtonTitles:nil];
    [alert show];
    [alert release];
}
- (void)calibrate {
    if (tapCount == 3) {
        tapCount = 0;
        return;
    }
    [[NSUserDefaults standardUserDefaults] setFloat:accelX forKey:@"X-Calibrate"];
    [[NSUserDefaults standardUserDefaults] setFloat:accelY forKey:@"Y-Calibrate"];
    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Calibrated!" message:[NSString stringWithFormat:@"Calibrated to: %.2f %.2f.", accelX, accelY] delegate:nil cancelButtonTitle:@"Okay." otherButtonTitles:nil];
    [alert show];
    [alert release];
    tapCount = 0;
}
- (BOOL)canBecomeFirstResponder {
    return YES;
}
- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(cancelDoubleTap) userInfo:nil repeats:NO];
    tapCount ++;
    if (tapCount == 3) {
        [timer invalidate];
        [self reset];
        return;
    }
    if (tapCount == 2) {
        [timer invalidate];
        timer = [NSTimer scheduledTimerWithTimeInterval:0.5 target:self selector:@selector(calibrate) userInfo:nil repeats:NO];
    }  
}

And...

 - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions { 
    but = [[UIButton alloc] initWithFrame:window.frame];
    [but addTarget:self action:@selector(touchesEnded:withEvent:) forControlEvents:UIControlEventTouchUpInside];
    [window addSubview:but];
     // Override point for customization after application launch.
    box = [[UIView alloc] initWithFrame:CGRectMake(self.window.center.x - 50, self.window.center.y - 50, 100, 100)];
  [window makeKeyAndVisible];
    box.backgroundColor = [UIColor colorWithRed:1.0 green:0.0 blue:0.0 alpha:0.75];
    [window addSubview:box];
    [[UIAccelerometer sharedAccelerometer] setDelegate:self];
    [[UIAccelerometer sharedAccelerometer] setUpdateInterval:(1/100.0)];
    slider = [[UISlider alloc] initWithFrame:CGRectMake(20, 438, 280, 22)];
    [slider setMinimumValue:2.0];
    [slider setMaximumValue:50.0];
    [window addSubview:slider];
    return YES;
 }

.h file:

#import <UIKit/UIKit.h>

@interface TilterAppDelegate : NSObject <UIApplicationDelegate, UIAccelerometerDelegate> {
    UIWindow *window;
    UIView *box;
    UISlider *slider;
    UIButton *but;
}

@property (nonatomic, retain) IBOutlet UIButton *but;
@property (nonatomic, retain) IBOutlet UISlider *slider;
@property (nonatomic, retain) IBOutlet UIView *box;
@property (nonatomic, retain) IBOutlet UIWindow *window;

@end

Move code:

 - (void)moveBoxByX:(float)x byY:(float)y {
    float newX = box.center.x + x;
    float newY = box.center.y + y;
    if (newX < 50)
        newX = 50;
    if (newX > 270)
        newX = 270;
    if (newY < 50)
        newY = 50;
    if (newY > 430)
        newY = 430;
    box.center = CGPointMake(newX, newY);
}

Should work most of the time.