iOS opencv无法使用Alpha填充ConvexPoly

Question

我刚开始学习在IOS应用程序中使用openCV将正方形绘制成imageView。

这是我的代码：

class StyledTextField: UITextField { 
    let insetConstant = UIEdgeInsets(top: 4, left: 10, bottom: 4, right: 10)

    override func textRect(forBounds bounds: CGRect) -> CGRect {
        return UIEdgeInsetsInsetRect(bounds, insetConstant)
    }

    override func editingRect(forBounds bounds: CGRect) -> CGRect {
        return UIEdgeInsetsInsetRect(bounds, insetConstant)
    }

    override func placeholderRect(forBounds bounds: CGRect) -> CGRect {
        return UIEdgeInsetsInsetRect(bounds, insetConstant)
    }

    override init(frame: CGRect) {
        super.init(frame: frame)

        self.layer.cornerRadius = 5
        self.layer.borderColor = UIColor(white: 2/3, alpha: 0.5).cgColor
        self.layer.borderWidth = 1
        self.clearButtonMode = .whileEditing
        self.keyboardType = UIKeyboardType.decimalPad
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

上面的代码成功地将矩形绘制到UIImage，但这不能将alpha应用于此矩形。

有人可以告诉我如何解决吗？