Derivatives along a line in an image

no_derivs.py

    def no_derivs(img, pt1,pt2,thresh=10):
        '''
        Test whether there are no image derivatives > thresh between pt1 and pt2
        :param img:     BGR opencv numpy image
        :param pt1:     first (x,y) point
        :param pt2:     second (x,y) point
        :param thresh:  no image differences > than this amount allowed
        :return:        True if no derivs, else False
        '''
        coords = np.around(np.linspace(pt1, pt2, np.linalg.norm(pt1 - pt2))).astype(int)
        vals = np.empty([len(coords), 3], dtype=int)
        for i, p in zip(range(len(coords)),coords):
            vals[i] = img[p[1],p[0]]
        maxd = np.max(np.fabs(vals[0:-1] - vals[1:]))
        if maxd < thresh:
            return True
        else:
            return False