masking a UIImage

I'm working on an app that can change the borders or a rectangular UIImage. The borders will 开发者_开发技巧vary, but will look like the UIImage was cut out with scissors, or something to that affect.

What is the best way to do this?

My first thought is to prep a bunch of transparent PNGs with the correct border effect I'm looking for, and then somehow use that as a mask for my UIImage. Is this the right path? Or is there a more flexible programmatic way to do this?

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Here are the Core Graphics calls that you can use to mask the image:

//Mask Image
UIImage *inputImage = [UIImage imageNamed:@"inputImage.png"];
CGImageRef maskRef = [UIImage imageNamed:@"mask.png"].CGImage; 

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

CGImageRef masked = CGImageCreateWithMask([inputImage CGImage], mask);
CGImageRelease(mask);

UIImage *maskedImage = [UIImage imageWithCGImage:masked];

CGImageRelease(masked);

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You need image masking for that, I wrote a tutorial on how to use it and how I've used it in my own application.

The following example code should help you get started, it needs the original image and a mask image as input and it returns the masked image as output.

- (UIImage*) maskImage:(UIImage *) image withMask:(UIImage *) mask
{
    CGImageRef imageReference = image.CGImage;
    CGImageRef maskReference = mask.CGImage;

    CGImageRef imageMask = CGImageMaskCreate(CGImageGetWidth(maskReference),
                                             CGImageGetHeight(maskReference),
                                             CGImageGetBitsPerComponent(maskReference),
                                             CGImageGetBitsPerPixel(maskReference),
                                             CGImageGetBytesPerRow(maskReference),
                                             CGImageGetDataProvider(maskReference),
                                             NULL, // Decode is null
                                             YES // Should interpolate
                                             );

    CGImageRef maskedReference = CGImageCreateWithMask(imageReference, imageMask);
    CGImageRelease(imageMask);

    UIImage *maskedImage = [UIImage imageWithCGImage:maskedReference];
    CGImageRelease(maskedReference);

    return maskedImage;
}

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Yep, that's pretty much what you need to do. The best approach is to use Core Graphics; the Quartz 2D Programming guide has an article describing how to mask images with other images. You can obtain a CGImageRef (which these Core Graphics methods use) from a UIImage object via its CGImage property, and then get a UIImage back from your altered (masked) CGImageRef using the UIImage class method +imageWithCGImage:.

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There is another way of doing this with a path. I've posted code that does it with an circle and could easily be edited to work with any closed path.

How to crop UIImage on oval shape or circle shape?