給UIImage添加蒙版

http://stackoverflow.com/questions/17448102/ios-masking-an-image-keeping-retina-scale-factor-in-account

I want to mask an image by passing another image as mask. I am able to mask the image but the resulting image doesn't look good. It is jagged at borders.

I guess the problem is related to retina graphics. The scale property for the two images are different as:

1. The image from which I want to mask has a scale value 1. This image generally has a resolution greater than 1000x1000 pixels.
2. The image according to which I want the resulting image(image having black and white colors only) has scale value 2. This image is generally of resolution 300x300 pixels. 

The resulting image has a scale value of 1.

The code I am using is:

+ (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 masked = CGImageCreateWithMask([image CGImage], mask);
CGImageRelease(mask);
UIImage *maskedImage = [UIImage imageWithCGImage:masked ];
CGImageRelease(masked);
return maskedImage;
}

How can I get a masked image which follows retina scale?