def scale(k):
    """
    compute default box scale.
    In SSD-Thesis, s_k is defined as
        s_k = s_min + (s_max - s_min) * (k - 1.0) / (m - 1.0), m = 6
    I corresponded with index from zero, so replace (k - 1.0) -> k.

    Args: feature map number
    Returns: scale
    """
    s_min = 0.1
    s_max = 0.9
    m = 6.0
    s_k = s_min + (s_max - s_min) * k / (m - 1.0)
    return s_k