/*

     File: PerspectiveMotionEffect.m

 Abstract: Subclass of UIMotionEffect that simulates a camera which orbits

 around a specific point of interest as the reported viewer offset changes.

  Version: 1.0

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 */

#import "PerspectiveMotionEffect.h"

@implementation PerspectiveMotionEffect

//| ----------------------------------------------------------------------------

//  This is the only method declared by UIMotionEffect and the only method that

//  subclasses of UIMotionEffect must override.  Your implementation must return

//  an NSDictionary containing key paths (relative to an instance of UIView) and

//  the amount by which to change the current value of the property.  Values

//  returned are added to (or in the case of a matrix, multiplied with) the

//  existing value at the corresponding key path of any view to which the

//  motion effect has been applied.

//

//  Remember, instances of UIMotionEffect should be stateless.  An instance of

//  UIMotionEffect knows nothing about the views it has been applied to.  Your

//  subclass should not attempt to break from this design.

//

- (NSDictionary*)keyPathsAndRelativeValuesForViewerOffset:(UIOffset)viewerOffset

{    

    CATransform3D transform = CATransform3DIdentity;

    transform.m34 = -1.0/1500;

    transform = CATransform3DTranslate(transform, 0, 0, -1504);

    // Transform the viewer offset using the function y=x^(2/3).  This makes

    // the effect more noticable at slight deviations from the center. (Try

    // plotting the function in the Grapher app on your Mac to see the

    // exact curve.)

    viewerOffset.horizontal = ((viewerOffset.horizontal < 0) ? -1 : 1) * powf(fabsf(viewerOffset.horizontal), 2.0/3.0);

    viewerOffset.vertical = ((viewerOffset.vertical < 0) ? -1 : 1) * powf(fabsf(viewerOffset.vertical), 2.0/3.0);

    // Gimble Lock is not an issue here because we are only rotating around

    // two axis at most.

    transform = CATransform3DRotate(transform, self.maximumViewingAngleX * viewerOffset.vertical, 1, 0, 0);

    transform = CATransform3DRotate(transform, self.maximumViewingAngleY * viewerOffset.horizontal, 0, 1, 0);

    return @{@"layer.sublayerTransform": [NSValue valueWithCATransform3D:transform]};

}

@end