def concat_fm(fm):
    """
    Concatenates Directional feature maps as shown in original paper.
    This function is used for visualization purposes only.

    :param fm: 12 ltrp feature maps
    :return: list of 4 concatenated feature maps
    """

    d1 = fm[0]+fm[1]+fm[2]
    d2 = fm[3]+fm[4]+fm[5]
    d3 = fm[6]+fm[7]+fm[8]
    d4 = fm[9]+fm[10]+fm[11]

    return [d1,d2,d3,d4]