IOS: Get image Alpha

-(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);}