def compute_polarisation_factors(args):
    """
    From Kabsch 1988 J. Appl. Cryst. 21 619

    DIVIDE the intensities by:
    <sin2 psi> = (1 - 2p) [ 1 - (n.S/|S|^2) ] + p { 1 + [S.S_0/(|S||S_0|)^2]^2}

    p = degree of polarisation (sync = 1, tube = 0.5 , mono + tube in between)
        or "probability of finding electric field vector in plane having
            normal, n"
    S = scattered vector
    S_0 = incident vector
    n = normal to polarisation plane, typically perpendicular to S_0

    In ImageD11 we normally expect to find:
    x axis along the beam
    z axis being up, and parallel to the normal n mentioned above
    """
    n = [0, 0, 1]
