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[mdejong1024]

I recently needed to figure out how to implement a solution like this. Here is the code I use:

// This cropping method will render the imageToCrop into a new graphics context
// cropped by the greyscale image defined by cropToImage. The result is returned
// as a RGBA image. The result of this operation is an image where the greyscale
// pixel value is converted into the alpha channel for the pixels.

- (UIImage*)imageByCropping:(UIImage*)imageToCrop
                cropToImage:(UIImage*)cropToImage
{
  // create a context to do our clipping in

  CGImageRef imageToCropRef = imageToCrop.CGImage;
  CGImageRef cropToImageRef = cropToImage.CGImage;
  
  size_t width = CGImageGetWidth(imageToCropRef);
  size_t height = CGImageGetHeight(imageToCropRef);
  CGSize size = CGSizeMake(width, height);
  
  UIGraphicsBeginImageContext(size);
  CGContextRef currentContext = UIGraphicsGetCurrentContext();
  
  // Flip coordinate system

  CGContextTranslateCTM(currentContext, 0.0, size.height);
  CGContextScaleCTM(currentContext, 1.0, -1.0);
  
  // Create a new image that is the size of the original image.
    
  CGRect clippedRect = CGRectMake(0, 0, size.width, size.height);
  
  CGContextClipToMask( currentContext, clippedRect, cropToImageRef);
    
  CGRect drawRect = CGRectMake(0,
                               0,
                               imageToCrop.size.width,
                               imageToCrop.size.height);
  
  // draw the image to our clipped context using our offset rect
  CGContextDrawImage(currentContext, drawRect, imageToCrop.CGImage);
  
  // pull the image from our cropped context
  UIImage *cropped = UIGraphicsGetImageFromCurrentImageContext();
  
  // pop the context to get back to the default
  UIGraphicsEndImageContext();
  
  // Note: this is autoreleased
  return cropped;
}

The trick is to generate a mask image that is a grayscale image, then the white parts are transparent and the black parts can't be seen. Anything in between becomes the alpha channel for the resulting image.