iOS Capture screen, then crop and mask the result image

Question

In my application, I want to do the following steps:

1 - Capture the screen, this part is no problem for me, I'm using the following code:

- (UIImage *)captureScreen {
    UIGraphicsBeginImageContextWithOptions(self.view.frame.size, YES, 0.0f);
    [self.view.layer renderInContext:UIGraphicsGetCurrentContext()];

    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return image;
}

2 - I cropped the image with this function

- (UIImage *)cropImage(UIImage *)image inRect:(CGRect)rect  {
    CGImageRef imageRef = CGImageCreateWithImageInRect(image.CGImage, rect);
    UIImage *resultImage = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);

    return resultImage;
}

3 - Then I mask the cropped image with a pure black and white 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 maskedRef = CGImageCreateWithMask([image CGImage], mask);
    UIImage *resultImage = [UIImage imageWithCGImage:maskedRef];
    CGImageRelease(mask);
    CGImageRelease(maskedRef);

    return resultImage;
}

However, the result image I got is that outside the shape of the mask, the image is in black color instead of transparent. Can anybody help me?

Answer 1

I solve my problem, it is due to the alpha channel of the image to be masked. So before masking, I create another UIImage with alpha channel and continue my steps.

This is the code for creating a UIImage with alpha

- (UIImage *)imageWithAlpha  {
    CGImageRef imageRef = self.CGImage;
    CGFloat width = CGImageGetWidth(imageRef);
    CGFloat height = CGImageGetHeight(imageRef);

    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context =  CGBitmapContextCreate(nil, width, height, 8, 4 * width, colorSpace, kCGImageAlphaPremultipliedFirst);
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);

    CGImageRef resultImageRef = CGBitmapContextCreateImage(context);
    UIImage *resultImage = [UIImage imageWithCGImage:resultImageRef scale:self.scale orientation:self.imageOrientation];

    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);
    CGImageRelease(resultImageRef);

    return resultImage;
}