The tutorial uses this method with two parameters: image and maskImage, these you have to set when you call the method. An example call could look like this, assuming the method is in the same class and the pictures are in your bundle:

Note - amazingly the images do not even have to be the same size.

...
UIImage *image = [UIImage imageNamed:@"dogs.png"];
UIImage *mask = [UIImage imageNamed:@"mask.png"];


// result of the masking method
UIImage *maskedImage = [self maskImage:image withMask:mask];


...


- (UIImage*) maskImage:(UIImage *)image withMask:(UIImage *)maskImage {


CGImageRef maskRef = maskImage.CGImage;


CGImageRef mask = CGImageMaskCreate(CGImageGetWidth(maskRef),
CGImageGetHeight(maskRef),
CGImageGetBitsPerComponent(maskRef),
CGImageGetBitsPerPixel(maskRef),
CGImageGetBytesPerRow(maskRef),
CGImageGetDataProvider(maskRef), NULL, false);


CGImageRef maskedImageRef = CGImageCreateWithMask([image CGImage], mask);
UIImage *maskedImage = [UIImage imageWithCGImage:maskedImageRef];


CGImageRelease(mask);
CGImageRelease(maskedImageRef);


// returns new image with mask applied
return maskedImage;
}

After you provided your code I have added some numbers as comments to it for reference. You still have two options. This whole thing is a method, which you are calling somewhere. You don't need to create the images inside it: this reduces the reusability of the method to zero.

To get your code working. Change the methods head (1.) to

- (UIImage *)maskImageMyImages {

Then change the name of the variable in 2. to

UIImage *maskImage = [UIImage imageNamed:@"mask.png"];

The method will return your masked images so you'll have to call this method in some place. Can you show us the code where you are calling your method?

There's an easier way.

#import <QuartzCore/QuartzCore.h>
// remember to include Framework as well


CALayer *mask = [CALayer layer];
mask.contents = (id)[[UIImage imageNamed:@"mask.png"] CGImage];
mask.frame = CGRectMake(0, 0, <img_width>, <img_height>);
yourImageView.layer.mask = mask;
yourImageView.layer.masksToBounds = YES;

For Swift 4 and plus follow code below

let mask = CALayer()
mask.contents =  UIImage(named: "right_challenge_bg")?.cgImage as Any
mask.frame = CGRect(x: 0, y: 0, width: leftBGImage.frame.size.width, height: leftBGImage.frame.size.height)
leftBGImage.layer.mask = mask
leftBGImage.layer.masksToBounds = true

I tried both code using either CALayer or CGImageCreateWithMask but none of them didn't work for me

But i found out that the problem is with png file format, NOT the code!!
so just to share my finding!

If you want to use

- (UIImage*) maskImage:(UIImage *)image withMask:(UIImage *)maskImage

you must use 24bit png without alpha channel

If you want to use CALayer mask, you must use (24bit or 8bit) png with alpha channel where transparent part of your png will mask the image (for smooth gradient alpha mask.. use 24bit png with alpha channel)

- (UIImage*) maskImage:(UIImage *)image withMask:(UIImage *)maskImage {
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();


CGImageRef maskImageRef = [maskImage CGImage];


// create a bitmap graphics context the size of the image
CGContextRef mainViewContentContext = CGBitmapContextCreate (NULL, maskImage.size.width, maskImage.size.height, 8, 0, colorSpace, kCGImageAlphaPremultipliedLast);
CGColorSpaceRelease(colorSpace);


if (mainViewContentContext==NULL)
return NULL;


CGFloat ratio = 0;


ratio = maskImage.size.width/ image.size.width;


if(ratio * image.size.height < maskImage.size.height) {
ratio = maskImage.size.height/ image.size.height;
}


CGRect rect1  = \{\{0, 0}, {maskImage.size.width, maskImage.size.height}};
CGRect rect2  = \{\{-((image.size.width*ratio)-maskImage.size.width)/2 , -((image.size.height*ratio)-maskImage.size.height)/2}, {image.size.width*ratio, image.size.height*ratio}};




CGContextClipToMask(mainViewContentContext, rect1, maskImageRef);
CGContextDrawImage(mainViewContentContext, rect2, image.CGImage);




// Create CGImageRef of the main view bitmap content, and then
// release that bitmap context
CGImageRef newImage = CGBitmapContextCreateImage(mainViewContentContext);
CGContextRelease(mainViewContentContext);


UIImage *theImage = [UIImage imageWithCGImage:newImage];


CGImageRelease(newImage);


// return the image
return theImage;
}

This works for me.

Swift version of the accepted solution:

func maskImage(image: UIImage, withMask maskImage: UIImage) -> UIImage {


let maskRef = maskImage.CGImage


let mask = CGImageMaskCreate(
CGImageGetWidth(maskRef),
CGImageGetHeight(maskRef),
CGImageGetBitsPerComponent(maskRef),
CGImageGetBitsPerPixel(maskRef),
CGImageGetBytesPerRow(maskRef),
CGImageGetDataProvider(maskRef),
nil,
false)


let masked = CGImageCreateWithMask(image.CGImage, mask)
let maskedImage = UIImage(CGImage: masked)!


// No need to release. Core Foundation objects are automatically memory managed.


return maskedImage


}

Just in case someone need it.

It`s working for me flawlessly.

we found, that correct answer is not working now and implemented little drop-in class, which helps to mask any image in UIImageView.

It's available here: https://github.com/Werbary/WBMaskedImageView

Example:

WBMaskedImageView *imgView = [[WBMaskedImageView alloc] initWithFrame:frame];
imgView.originalImage = [UIImage imageNamed:@"original_image.png"];
imgView.maskImage = [UIImage imageNamed:@"mask_image.png"];

SWIFT 3 XCODE 8.1

func maskImage(image: UIImage, withMask maskImage: UIImage) -> UIImage {


let maskRef = maskImage.cgImage


let mask = CGImage(
maskWidth: maskRef!.width,
height: maskRef!.height,
bitsPerComponent: maskRef!.bitsPerComponent,
bitsPerPixel: maskRef!.bitsPerPixel,
bytesPerRow: maskRef!.bytesPerRow,
provider: maskRef!.dataProvider!,
decode: nil,
shouldInterpolate: false)


let masked = image.cgImage!.masking(mask!)
let maskedImage = UIImage(cgImage: masked!)


// No need to release. Core Foundation objects are automatically memory managed.


return maskedImage


}

// for use

testImage.image = maskImage(image: UIImage(named: "OriginalImage")!, withMask: UIImage(named: "maskImage")!)

Swift 3 - Simplest Solution I Found

I created an @IBDesignable for this so you could see the effect right away on your storyboard.

import UIKit


@IBDesignable
class UIImageViewWithMask: UIImageView {
var maskImageView = UIImageView()


@IBInspectable
var maskImage: UIImage? {
didSet {
maskImageView.image = maskImage
updateView()
}
}


// This updates mask size when changing device orientation (portrait/landscape)
override func layoutSubviews() {
super.layoutSubviews()
updateView()
}


func updateView() {
if maskImageView.image != nil {
maskImageView.frame = bounds
mask = maskImageView
}
}
}

How to use

1. Create a new file and paste in that code above.
2. Add UIImageView to your storyboard (assign an image if you want).
3. On Identity Inspector: Change the custom class to "UIImageViewWithMask" (custom class name above).
4. On Attributes Inspector: Select mask image you want to use.

Example

Notes

• Your mask image should have a transparent background (PNG) for the non-black part.