How to clip image on drag in ios

Question

I have an application having functionality for image clipping. The Functionality is I have three images :

• Main image - set as Background
• Overlay image - image set on Main image
• Pointer image - image that will move on touch

I want to remove the Overlay image on touch as i drag the Pointer image. After clipping Overlay image Main image should get visible.

- (void) touchesBegan:(NSSet*)touches withEvent:(UIEvent*)event {
    // Retrieve the touch point
    CGPoint pt = [[touches anyObject] locationInView:imgPointer];
    startLocation = pt;
    [[imgPointer superview] bringSubviewToFront:imgPointer];
}
- (void) touchesMoved:(NSSet*)touches withEvent:(UIEvent*)event {
    // Move relative to the original touch point
    CGPoint pt = [[touches anyObject] locationInView:imgPointer];
    CGRect frame = [imgPointer frame];
    frame.origin.x += pt.x - startLocation.x;
    frame.origin.y += pt.y - startLocation.y;
    [imgPointer setFrame:frame];

    UIGraphicsBeginImageContext(imgPointer.frame.size);

    CGContextRef currentContext = UIGraphicsGetCurrentContext();
    CGRect clippedRect = CGRectMake(0, 0, imgPointer.frame.size.width, imgPointer.frame.size.height);
    CGContextClipToRect( currentContext, clippedRect);
    CGRect drawRect = CGRectMake(imgPointer.frame.origin.x * -1,
                                 imgPointer.frame.origin.y * -1,
                                 imgOverlay.frame.size.width,
                                 imgOverlay.frame.size.height);

    //draw the image to our clipped context using our offset rect

    CGContextDrawImage(currentContext, drawRect, imgOverlay.image.CGImage);

    //pull the image from our cropped context

    UIImage *cropped = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

}

From using above code i am not able to clip Overlay image.

Thank you.