def lmPointToParam(xp, yp):
    """ Return the line parameters of a landmark from its coordinates.

    Wall landmarks are characterized by the point corresponding to the
    intersection of the wall line and its perpendicular passing through the
    origin (0, 0). The wall line is characterized by a vector (a, c) such as
    its equation is given by y = ax + c.

    xp and yp are the landmark coordinate.
    """

    a = -xp/yp
    b = yp*(1+a**2)

    return [a, b]