python+cv2+SIFT对图像进行局部特征提取

在用SIFT时一直报以下错误，带解决中。。。。

代码：

# -*- coding: utf-8 -*-
# !/usr/bin/env python
# @Time    : 2018/10/31 16:26
# @Author  : xhh
# @Desc    : SITF检测特征点
# @File    : opencv_SITF.py
# @Software: PyCharm
import numpy as np
import cv2

# 读取图片
input_file = '../cat1/cat.1.jpg'
img = cv2.imread(input_file)
img_gray = cv2.cvtColor(img, cv2.IMREAD_GRAYSCALE)
# 提取特征点
sift = cv2.xfeatures2d.SURF_create()
keypoints = sift.detect(img_gray, None)
img_sift = np.copy(img_gray)

cv2.drawKeypoints(img, keypoints, img_sift, flags=cv2.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS)
cv2.imshow('input_image',img_gray)
cv2.imshow('img_sift', img_sift)
cv2.waitKey(0)

报以下错误：

D:\python36\python.exe E:/pycharm/image_opencv/opencv_test/opencv_SITF.py
Traceback (most recent call last):
  File "E:/pycharm/image_opencv/opencv_test/opencv_SITF.py", line 16, in <module>
    sift = cv2.xfeatures2d.SURF_create()
cv2.error: OpenCV(3.4.3) C:\projects\opencv-python\opencv_contrib\modules\xfeatures2d\src\surf.cpp:1016: error: (-213:The function/feature is not implemented) This algorithm is patented and is excluded in this configuration; Set OPENCV_ENABLE_NONFREE CMake option and rebuild the library in function 'cv::xfeatures2d::SURF::create'