def _flip_top_bottom_boundingbox(img, boxes):
    """Flip top bottom only bounding box.

    Args:
        img: np array image.
        boxes(np.ndarray): bounding boxes. shape is [num_boxes, 5(x, y, w, h, class_id)]
    """
    height = img.shape[0]
    if len(boxes) > 0:
        boxes[:, 1] = height - boxes[:, 1] - boxes[:, 3]

    return boxes