Easy UIInterpolatingMotionEffect. Learn more: http://hack.swic.name/easy-uiinterpolatingmotioneffect

NaturalMotion.swift

//
//  NaturalMotion.swift
//
//  Created by Maciej Swic on 2014-06-06.
//  Released under the MIT license.
//

import UIKit

extension UIView {
    func addNaturalOnTopEffect(maximumRelativeValue : Float = 20.0) {
        //Horizontal motion
        var motionEffect = UIInterpolatingMotionEffect(keyPath: "center.x", type: .TiltAlongHorizontalAxis);
        motionEffect.minimumRelativeValue = maximumRelativeValue;
        motionEffect.maximumRelativeValue = -maximumRelativeValue;
        addMotionEffect(motionEffect);
        
        //Vertical motion
        motionEffect = UIInterpolatingMotionEffect(keyPath: "center.y", type: .TiltAlongVerticalAxis);
        motionEffect.minimumRelativeValue = maximumRelativeValue;
        motionEffect.maximumRelativeValue = -maximumRelativeValue;
        addMotionEffect(motionEffect);
    }
    
    func addNaturalBelowEffect(maximumRelativeValue : Float = 20.0) {
        addNaturalOnTopEffect(maximumRelativeValue: -maximumRelativeValue)
    }
}

UIView+NaturalMotion.h

//
//  UIView+NaturalMotion.h
//
//  Created by Maciej Swic on 30/04/14.
//  Released under the MIT license.
//

#import <UIKit/UIKit.h>

@interface UIView (NaturalMotion)

+ (void)addNaturalOnTopEffectWithMaximumRelativeValue:(CGFloat)maximumRealtiveValue;
+ (void)addNaturalBelowEffectWithMaximumRelativeValue:(CGFloat)maximumRealtiveValue;

@end

UIView+NaturalMotion.m

//
//  UIView+NaturalMotion.m
//
//  Created by Maciej Swic on 30/04/14.
//  Released under the MIT license.
//

#import "UIView+NaturalMotion.h"

@implementation UIView (NaturalMotion)

+ (void)addNaturalOnTopEffectWithMaximumRelativeValue:(CGFloat)maximumRealtiveValue {
    UIInterpolatingMotionEffect* motionEffect = [[UIInterpolatingMotionEffect alloc] initWithKeyPath:@"center.x" type:UIInterpolatingMotionEffectTypeTiltAlongHorizontalAxis];
    motionEffect.minimumRelativeValue = @(maximumRealtiveValue);
    motionEffect.maximumRelativeValue = @(-maximumRealtiveValue);
    [self addMotionEffect:motionEffect];
    motionEffect = [[UIInterpolatingMotionEffect alloc] initWithKeyPath:@"center.y" type:UIInterpolatingMotionEffectTypeTiltAlongVerticalAxis];
    motionEffect.minimumRelativeValue = @(maximumRealtiveValue);
    motionEffect.maximumRelativeValue = @(-maximumRealtiveValue);
    [self addMotionEffect:motionEffect];
}

+ (void)addNaturalBelowEffectWithMaximumRelativeValue:(CGFloat)maximumRealtiveValue {
    UIInterpolatingMotionEffect* motionEffect = [[UIInterpolatingMotionEffect alloc] initWithKeyPath:@"center.x" type:UIInterpolatingMotionEffectTypeTiltAlongHorizontalAxis];
    motionEffect.minimumRelativeValue = @(-maximumRealtiveValue);
    motionEffect.maximumRelativeValue = @(maximumRealtiveValue);
    [self addMotionEffect:motionEffect];
    motionEffect = [[UIInterpolatingMotionEffect alloc] initWithKeyPath:@"center.y" type:UIInterpolatingMotionEffectTypeTiltAlongVerticalAxis];
    motionEffect.minimumRelativeValue = @(-maximumRealtiveValue);
    motionEffect.maximumRelativeValue = @(maximumRealtiveValue);
    [self addMotionEffect:motionEffect];
}

@end

Why not just make this a category on UIView? That would reduce the API down to something like:

@interface UIView (NaturalMotion)

- (void)addNaturalMotionEffectAboveWithMaximumRelativeValue:(CGFloat)maximumRelativeValue;
- (void)addNaturalMotionEffectBelowWithMaximumRelativeValue:(CGFloat)maximumRelativeValue;

@end

Good idea! I also added a Swift version.

Remove Effects-

func removeMotionEffects() {
    let motionEffects = self.motionEffects as [UIMotionEffect]
    for effect in motionEffects {
        self.removeMotionEffect(effect)
    }
}

- (void)removeMotionEffects {
    NSArray *motionEffects = [NSArray arrayWithArray:self.motionEffects];
    [motionEffects enumerateObjectsUsingBlock:^(UIMotionEffect *motionEffect, NSUInteger idx, BOOL *stop) {
        [self removeMotionEffect:motionEffect];
    }];

}

Should group the horizontal and vertical effects into a UIMotionEffectGroup for performance so they are evaluated at the same time.

Left-right and top-bottom are inverted. The image should move from right-to-left if in the background.

Typo: realtive instead of relative.