def get_abs_segment_point_coords(segment_vertices, eta, xsi):
    """
    Compute the absolute coordinates for a point on a segment

    Warning:
        * The function does only work for UNDEFORMED segments!

    Args:
        :segment_vertices: (dict) segment vertices
        :rel_coords: (dict) relative coordinates (eta, xsi)

    Returns:
        :p: (ndarray) absolute coordinates
    """

    a = segment_vertices['a']
    b = segment_vertices['b']
    c = segment_vertices['c']
    d = segment_vertices['d']

    a_eta = a + eta*(b - a)
    d_eta = d + eta*(c - d)

    return a_eta + xsi*(d_eta - a_eta)