Why does applying a bezierPathWithRoundedRect mask yield a different result from setting the layer's cornerRadius?

Question

I typically use the cornerRadius property to apply rounded corners to view layers. When the views only need rounded top or bottom corners, I apply a mask with a bezier path instead (using bezierPathWithRoundedRect:byRoundingCorners:cornerRadii:). When both approaches are combined in the same view hierarchy, the rounded corners are not properly aligned, as illustrated below:

This simplified example can be reproduced with the following code:

@interface ViewController : UIViewController

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.view.backgroundColor = [UIColor blackColor];

    CGRect frame = CGRectMake(50.0, 50.0, 100.0, 100.0);
    CGFloat radius = 20.0;

    // Apply cornerRadius to green backdrop view.
    UIView *backdropView = [[UIView alloc] initWithFrame:frame];
    backdropView.backgroundColor = [UIColor greenColor];
    backdropView.layer.cornerRadius = radius;
    [self.view addSubview:backdropView];

    // Apply bezier path mask to black front view.
    UIView *frontView = [[UIView alloc] initWithFrame:frame];
    frontView.backgroundColor = [UIColor blackColor];
    CAShapeLayer * maskLayer = [CAShapeLayer layer];
    maskLayer.path = [UIBezierPath bezierPathWithRoundedRect:frontView.bounds
                                           byRoundingCorners:UIRectCornerAllCorners
                                                 cornerRadii:CGSizeMake(radius, radius)].CGPath;
    frontView.layer.mask = maskLayer;
    [self.view addSubview:frontView];
}

@end

Setting the shouldRasterize property of the different layers involved did not solve the issue. I would like to understand why this happens. A possible workaround would be to always apply the bezier path mask rather than simply setting the corner radius but that feels like overkill.

Answer 1

This website does a pretty good job explaining: http://www.paintcodeapp.com/blogpost/code-for-ios-7-rounded-rectangles (in short, it's an iOS7 only thing)

For an extra illustration, see: http://www.mani.de/backstage/?p=483