def _signed_area_tri(points):
    """
    points : the coordinates of the triangle vertices -- (3x2) float array
    returns signed area of the triangle.

    """
    x1, y1 = points[0]
    x2, y2 = points[1]
    x3, y3 = points[2]
    return ((x1 - x3) * (y2 - y3) - (x2 - x3) * (y1 - y3)) / 2
