given image index, find out full path

        Parameters:
        ----------
        index: int
            index of a specific image
        Returns:
        ----------
        full path of this image

def image_path_from_index(self, index):
        """
        given image index, find out full path

        Parameters:
        ----------
        index: int
            index of a specific image
        Returns:
        ----------
        full path of this image
        """
        assert self.image_set_index is not None, "Dataset not initialized"
        name = self.image_set_index[index]
        image_file = os.path.join(self.image_dir, 'images', name)
        assert os.path.isfile(image_file), 'Path does not exist: {}'.format(image_file)
        return image_file