def calculate_dimensions(image_size, desired_size):
        """Return the Tuple with the arguments to pass to Image.crop.

        If the image is smaller than than the desired_size Don't do
        anything. Otherwise, first calculate the (truncated) center and then
        take half the width and height (truncated again) for x and y.

        x0, y0: the center coordinates
        """

        current_x, current_y = image_size
        target_x, target_y = desired_size

        if current_x < target_x and current_y < target_y:
            return None

        if current_x > target_x:
            new_x0 = floor(current_x / 2)
            new_x = new_x0 - ceil(target_x / 2)
            new_width = target_x
        else:
            new_x = 0
            new_width = current_x

        if current_y > target_y:
            new_y0 = floor(current_y / 2)
            new_y = new_y0 - ceil(target_y / 2)
            new_height = target_y
        else:
            new_y = 0
            new_height = current_y

        return (int(new_x), int(new_y), new_width, new_height)