Draw a mask image like Left-3 to blend a rounded corner:
func UIGraphicsDrawAntiRoundedCornerImageWithRadius(radius: CGFloat, outerSize: CGSize, innerSize: CGSize, fillColor: UIColor) -> UIImage{
UIGraphicsBeginImageContextWithOptions(outerSize, false, UIScreen.mainScreen().scale)
let currentContext = UIGraphicsGetCurrentContext()
let bezierPath = UIBezierPath()