Cut a UIImage into a circle

Objective-C version

The code in the viewDidLoad or loadView:

import #include <math.h>

- (void)loadView
{
[super loadView];
UIImageView *imageView=[[UIImageView alloc]initWithFrame:CGRectMake(0, 50, 320, 320)];
[self.view addSubview:imageView];
UIImage *image=[UIImage imageNamed:@"Dubai-Photos-Images-Travel-Tourist-Images-Pictures-800x600.jpg"];
imageView.image=[self circularScaleAndCropImage:[UIImage imageNamed:@"Dubai-Photos-Images-Travel-Tourist-Images-Pictures-800x600.jpg"] frame:CGRectMake(0, 0, 320, 320)];
}

Finally the function which does the heavy lifting circularScaleAndCropImage is as defined below

- (UIImage*)circularScaleAndCropImage:(UIImage*)image frame:(CGRect)frame {
    // This function returns a newImage, based on image, that has been:
    // - scaled to fit in (CGRect) rect
    // - and cropped within a circle of radius: rectWidth/2
    
    // Create the bitmap graphics context
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(frame.size.width, frame.size.height), NO, 0.0);
    CGContextRef context = UIGraphicsGetCurrentContext();
    
    // Get the width and heights
    CGFloat imageWidth = image.size.width;
    CGFloat imageHeight = image.size.height;
    CGFloat rectWidth = frame.size.width;
    CGFloat rectHeight = frame.size.height;
    
    // Calculate the scale factor
    CGFloat scaleFactorX = rectWidth/imageWidth;
    CGFloat scaleFactorY = rectHeight/imageHeight;
    
    // Calculate the centre of the circle
    CGFloat imageCentreX = rectWidth/2;
    CGFloat imageCentreY = rectHeight/2;
    
    // Create and CLIP to a CIRCULAR Path
    // (This could be replaced with any closed path if you want a different shaped clip)
    CGFloat radius = rectWidth/2;
    CGContextBeginPath (context);
    CGContextAddArc (context, imageCentreX, imageCentreY, radius, 0, 2*M_PI, 0);
    CGContextClosePath (context);
    CGContextClip (context);
    
    // Set the SCALE factor for the graphics context
    // All future draw calls will be scaled by this factor
    CGContextScaleCTM (context, scaleFactorX, scaleFactorY);
    
    // Draw the IMAGE
    CGRect myRect = CGRectMake(0, 0, imageWidth, imageHeight);
    [image drawInRect:myRect];
    
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return newImage;
}

SWIFT 3 Example

override func viewDidLoad() {
       super.viewDidLoad()
       // Do any additional setup after loading the view, typically from a nib.
       let imageView = UIImageView(frame: CGRect(x: CGFloat(0), y: CGFloat(50), width: CGFloat(320), height: CGFloat(320)))
       view.addSubview(imageView)
       let image = UIImage(named: "Dubai-Photos-Images-Travel-Tourist-Images-Pictures-800x600.jpg")
       imageView.image = circularScaleAndCropImage(UIImage(named: "Dubai-Photos-Images-Travel-Tourist-Images-Pictures-800x600.jpg")!, frame: CGRect(x: CGFloat(0), y: CGFloat(0), width: CGFloat(100), height: CGFloat(100)))
   }

Finally the function which does the heavy lifting circularScaleAndCropImage is as defined below

func circularScaleAndCropImage(_ image: UIImage, frame: CGRect) -> UIImage{
        // This function returns a newImage, based on image, that has been:
        // - scaled to fit in (CGRect) rect
        // - and cropped within a circle of radius: rectWidth/2
        //Create the bitmap graphics context
        UIGraphicsBeginImageContextWithOptions(CGSize(width: CGFloat(frame.size.width), height: CGFloat(frame.size.height)), false, 0.0)
        let context: CGContext? = UIGraphicsGetCurrentContext()
        //Get the width and heights
        let imageWidth: CGFloat = image.size.width
        let imageHeight: CGFloat = image.size.height
        let rectWidth: CGFloat = frame.size.width
        let rectHeight: CGFloat = frame.size.height
        //Calculate the scale factor
        let scaleFactorX: CGFloat = rectWidth / imageWidth
        let scaleFactorY: CGFloat = rectHeight / imageHeight
        //Calculate the centre of the circle
        let imageCentreX: CGFloat = rectWidth / 2
        let imageCentreY: CGFloat = rectHeight / 2
        // Create and CLIP to a CIRCULAR Path
        // (This could be replaced with any closed path if you want a different shaped clip)
        let radius: CGFloat = rectWidth / 2
        context?.beginPath()
        context?.addArc(center: CGPoint(x: imageCentreX, y: imageCentreY), radius: radius, startAngle: CGFloat(0), endAngle: CGFloat(2 * Float.pi), clockwise: false)
        context?.closePath()
        context?.clip()
        //Set the SCALE factor for the graphics context
        //All future draw calls will be scaled by this factor
        context?.scaleBy(x: scaleFactorX, y: scaleFactorY)
        // Draw the IMAGE
        let myRect = CGRect(x: CGFloat(0), y: CGFloat(0), width: imageWidth, height: imageHeight)
        image.draw(in: myRect)
        let newImage: UIImage? = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return newImage!
    }