def pull_out_L_channel(img_lab):
    """
    The grayscale image can be thought as the L-channel of the image in the LAB color space (lack of A/B channels).
    We take L-channel as the model input.
    :param img_lab: Frame in LAB color space
    :return: Lightness channel
    """
    img_l = img_lab[:, :, 0]
    return img_l