public function shadow($background = 'white', $blur = 4,
                           $opacity = 60, $vector = [4,4])
    {
        $options = [
            'b' => $background,
            'l' => $blur,
            'o' => $opacity,
            'v' => $vector
        ];

        // call TransformationMixin function
        $this->setTransformUrl('shadow', $options);

        return $this;
    }