How to mask a UIView

Question

I'm working on an app where I draw a UIView via code and I fill it with a UIColor. Next thing I want to load a mask.png file (no transparency, just black and white) and mask the UIView to change its visual appearance.

Any idea how to do this?

Solution 1

// Create your mask layer
CALayer* maskLayer = [CALayer layer];
maskLayer.frame = CGRectMake(0,0,yourMaskWidth ,yourMaskHeight);
maskLayer.contents = (__bridge id)[[UIImage imageNamed:@"yourMaskImage.png"] CGImage];

// Apply the mask to your uiview layer        
yourUIView.layer.mask = maskLayer;

Remember to import QuartzCore and add the framework

#import <QuartzCore/QuartzCore.h>

It is very easy but I didn't find the answer anywhere!

Solution 2

iOS 8 introduces the maskView property which allows you to use an alternate view to provide the mask image. In Swift:

if let maskImage = UIImage(named: "MaskImage") {
    myView.maskView = UIImageView(image: maskImage)
}

Note that you need to supply an image which actually has transparency; an image with white parts (for opaque) and black parts (for transparent) won't work.

Solution 3

You might be able to use CGContextClipToMask:

-(void) drawRect:(CGRect)dirty {
  CGContextRef context = UIGraphicsGetCurrentContext();
  CGContextSaveGState( context );
  CGContextClipToMask( context , [self bounds] , [self maskImage] );
  [super drawRect:dirty]; // or draw your color manually
  CGContextRestoreGState( context );
}

Solution 4

I have create a category with the code above :

UIView+ImageMask.h :

#import <UIKit/UIKit.h>

@interface UIView (ImageMask)

- (void) maskWithImage:(UIImage*) mask;
- (void) maskWithImage:(UIImage*)mask size:(CGSize)maskSize;
- (void) removeMask;

@end

UIView+ImageMask.m :

#import <QuartzCore/QuartzCore.h>

#import "UIView+ImageMask.h"

@implementation UIView (ImageMask)

- (void) maskWithImage:(UIImage*) mask {
    [self maskWithImage:mask size:mask.size];
}

- (void) maskWithImage:(UIImage*)mask size:(CGSize)maskSize {
    CALayer* maskLayer = [CALayer layer];
    maskLayer.frame = CGRectMake(0, 0, maskSize.size.width, maskSize.size.height);
    maskLayer.contents = (__bridge id)[mask CGImage];

    // Apply the mask to your uiview layer
    self.layer.mask = maskLayer;
}

- (void) removeMask {
    self.layer.mask = nil;
}

@end

Solution 5

Thanks drawnonward,

ended up with this function:

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

    UIGraphicsBeginImageContext(image.bounds.size);
    [image.layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *viewImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();


    CGImageRef maskRef = maskImage.CGImage; 

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

    CGImageRef masked = CGImageCreateWithMask([viewImage CGImage], mask);
    return [UIImage imageWithCGImage:masked];

}

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Comments

[zakdances]

Any answer that uses Core Animation instead of Core Graphics should be preferred. CA's API is much easier to understand and use.

[Vassily]

What kind of image do you have in input ? black& white or translucent and white or translucent and black ?

[mt81]

I used a black image with alpha. But I don't think this will make a difference. If you try it, let me know :).

[Arsynth]

I tried. Actually have a difference. Only alpha component will make effect

[Vassily]

yes, image needs to be PNG with alpha component. only the alpha will be masked, the rest can be white or black it doesn't matter

[Yaroslav]

Any idea why this might crash with EXC_BAD_ACCESS? (-[UIImageView _maskView]: message sent to deallocated instance)