iOS:获取图片Alpha图片

-(void)createImages
{
    // Load the alpha image, which is just the same Ship.png image used in the clipping demo
    NSString *imagePath = [[NSBundle mainBundle] pathForResource:@"Ship.png" ofType:nil];
    UIImage *img = [UIImage imageWithContentsOfFile:imagePath];
    _alphaImage = CGImageRetain(img.CGImage);

    // To show the difference with an image mask, we take the above image and process it to extract
    // the alpha channel as a mask.
    // Allocate data
    NSMutableData *data = [NSMutableData dataWithLength:90 * 90 * 1];
    // Create a bitmap context
    CGContextRef context = CGBitmapContextCreate([data mutableBytes], 90, 90, 8, 90, NULL, (CGBitmapInfo)kCGImageAlphaOnly);
    // Set the blend mode to copy to avoid any alteration of the source data
    CGContextSetBlendMode(context, kCGBlendModeCopy);
    // Draw the image to extract the alpha channel
    CGContextDrawImage(context, CGRectMake(0.0, 0.0, 90.0, 90.0), _alphaImage);
    // Now the alpha channel has been copied into our NSData object above, so discard the context and lets make an image mask.
    CGContextRelease(context);
    // Create a data provider for our data object (NSMutableData is tollfree bridged to CFMutableDataRef, which is compatible with CFDataRef)
    CGDataProviderRef dataProvider = CGDataProviderCreateWithCFData((__bridge CFMutableDataRef)data);
    // Create our new mask image with the same size as the original image
    _maskingImage = CGImageMaskCreate(90, 90, 8, 8, 90, dataProvider, NULL, YES);
    // And release the provider.
    CGDataProviderRelease(dataProvider);
}