How to mask a square image into an image with round corners in iOS?

I realize this is old news but just to boil it down a bit:

There are two possible questions here: (1) how do I apply rounded corners to a UIView (such as a UIImageView), which will be displayed on screen, and (2) how do I mask a square image (that is, a UIImage) to produce a new image with rounded corners.

For (1), the easiest course is to use CoreAnimation and set the view.layer.cornerRadius property

 // Because we're using CoreAnimation, we must include QuartzCore.h
 // and link QuartzCore.framework in a build phases 
 #import <QuartzCore/QuartzCore.h> 

 // start with an image 
 UIImage * fooImage = [UIImage imageNamed:@"foo.png"];
 // put it in a UIImageView
 UIView * view = [UIImageView alloc] initWithImage:fooImage];
 // round its corners. This mask now applies to the view's layer's *background*
 view.layer.cornerRadius = 10.f
 // enable masksToBounds, so the mask applies to its foreground, the image
 view.layer.masksToBounds = YES;

For (2), the best way is to use the UIKit graphics operations:

// start with an image 
UIImage * fooImage = [UIImage imageNamed:@"foo.png"];
CGRect imageRect = CGRectMake(0, 0, fooImage.size.width, fooImage.size.height);
// set the implicit graphics context ("canvas") to a bitmap context for images
UIGraphicsBeginImageContextWithOptions(imageRect.size,NO,0.0);
// create a bezier path defining rounded corners
UIBezierPath * path = [UIBezierPath bezierPathWithRoundedRect:imageRect cornerRadius:10.f];
// use this path for clipping in the implicit context
[path addClip];
// draw the image into the implicit context
[fooImage drawInRect:imageRect];
// save the clipped image from the implicit context into an image 
UIImage *maskedImage = UIGraphicsGetImageFromCurrentImageContext();
// cleanup
UIGraphicsEndImageContext();

What's tricky about problem (2) is that you might think you could do the whole operation using the view.layer.mask property in CoreAnimation. But you can't because the CALayer renderInContext: method, which you'd use to generate a UIImage from the masked layer, seems to ignore the mask. Worse, the documentation for renderInContext: doesn't mention this, and only alludes to the behavior for OSX 10.5.

Some further context: the above approach to (2) is using UIKit's wrappers around more basic CoreGraphics functionality. You can do the same thing using the CoreGraphics calls directly – that is what the chosen answer is doing -- but then you need build the rounded rect bezier path manually from curves and lines and you also need to compensate for the fact that CoreGraphics uses a drawing coordinate system which is flipped with respect to UIKit's.

You can use CoreGraphics to create a path for a round rectangle with this code snippet:

static void addRoundedRectToPath(CGContextRef context, CGRect rect, float ovalWidth, float ovalHeight)
{
    float fw, fh;
    if (ovalWidth == 0 || ovalHeight == 0) {
        CGContextAddRect(context, rect);
        return;
    }
    CGContextSaveGState(context);
    CGContextTranslateCTM (context, CGRectGetMinX(rect), CGRectGetMinY(rect));
    CGContextScaleCTM (context, ovalWidth, ovalHeight);
    fw = CGRectGetWidth (rect) / ovalWidth;
    fh = CGRectGetHeight (rect) / ovalHeight;
    CGContextMoveToPoint(context, fw, fh/2);
    CGContextAddArcToPoint(context, fw, fh, fw/2, fh, 1);
    CGContextAddArcToPoint(context, 0, fh, 0, fh/2, 1);
    CGContextAddArcToPoint(context, 0, 0, fw/2, 0, 1);
    CGContextAddArcToPoint(context, fw, 0, fw, fh/2, 1);
    CGContextClosePath(context);
    CGContextRestoreGState(context);
}

And then call CGContextClip(context); to clip it to the rectangle path. Now any drawing done, including drawing an image, will be clipped to the round rectangle shape.

As an example, assuming "image" is a UIImage, and this is in a drawRect: method:

CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSaveGState(context);
addRoundedRectToPath(context, self.frame, 10, 10);
CGContextClip(context);
[image drawInRect:self.frame];
CGContextRestoreGState(context);

Here is an even easier method that is available in iPhone 3.0 and up. Every View-based object has an associated layer. Each layer can have a corner radius set, this will give you just what you want:

UIImageView * roundedView = [[UIImageView alloc] initWithImage: [UIImage imageNamed:@"wood.jpg"]];
// Get the Layer of any view
CALayer * layer = [roundedView layer];
[layer setMasksToBounds:YES];
[layer setCornerRadius:10.0];

// You can even add a border
[layer setBorderWidth:4.0];
[layer setBorderColor:[[UIColor blueColor] CGColor]];

To use these methods you might need to add:

#import <QuartzCore/QuartzCore.h>