def tform(self, points, sharedTransforms=[]):
        """transform a set of points through this transformation
        Parameters
        ----------
        points : numpy.array
            a Nx2 array of x,y points
        Returns
        -------
        numpy.array
            a Nx2 array of x,y points after transformation
        """
        dst = []

        refids = np.array([r.transformId for r in sharedTransforms])

        for ab in [self.a, self.b]:
            dst.append(np.copy(points))
            for tf in ab.tforms:
                if isinstance(tf, ReferenceTransform):
                    rind = np.argwhere(refids == tf.refId).flatten()
                    if rind.size != 1:
                        raise RenderError("Could not find unique "
                                          "sharedTransform for refId %s" %
                                          tf.refId)
                    tf = sharedTransforms[rind[0]]
                dst[-1] = tf.tform(dst[-1])

        return (1.0 - self.lambda_) * dst[0] + self.lambda_ * dst[1]