Resize input image if necessary.

        Parameters
        ----------
        img : `PIL.Image`
            Input image.

        Raises
        ------
        ValueError
            If input image is too small.

        Returns
        -------
        `PIL.Image`
            Resized image or input image.

def input(self, img):
        """Resize input image if necessary.

        Parameters
        ----------
        img : `PIL.Image`
            Input image.

        Raises
        ------
        ValueError
            If input image is too small.

        Returns
        -------
        `PIL.Image`
            Resized image or input image.
        """
        img_width, img_height = img.size

        if self.resize:
            width, height = self.resize
            scale = min(width / img_width, height / img_height)
            img = img.resize((floor(img_width * scale),
                              floor(img_height * scale)))
            img_width, img_height = img.size

        if img_width < self.min_size or img_height < self.min_size:
            raise ValueError('input image is too small: {0} x {1} < {2} x {2}'
                             .format(img_width, img_height, self.min_size))

        return img