opencv-contrib-Python编译module 'cv2.cv2' has no attribute 'xfeatures2d'

引言

opencv-contrib-Python编译出现错误：
sift = cv.xfeatures2d.SIFT_create()
cv2.error: OpenCV(3.4.3) C:\projects\opencv-python\opencv_contrib\modules\xfeatures2d\src\sift.cpp:1207: 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::SIFT::create’

即是sift = cv.xfeatures2d.SIFT_create()函数无法使用，也是opencv-contrib-python无法使用。解决方法为降低opencv版本，因为当初安装时是默认版本安装的，版本比较高，但是也不清楚为什么版本高了不支持，反正TensorFlow里面对版本对应要求比较严格。

解决步骤一

打开Anaconda卸载opencv。

pip uninstall opencv-python
pip install opencv-contrib-python

解决步骤二

重装opencv-python、opencv-contrib-python。指定版本安装。

 pip install opencv_python==3.4.2.16 
 pip install opencv-contrib-python==3.4.2.16

最后打开Pycharm运行sift算法测试：

import numpy as np
import cv2 as cv
img = cv.imread('test_1.jpg')
gray = cv.cvtColor(img,cv.COLOR_BGR2GRAY)
sift = cv.xfeatures2d.SIFT_create()
kp = sift.detect(gray,None)
img=cv.drawKeypoints(gray,kp,img)

cv.imshow("SIFT", img)
cv.imwrite('sift_keypoints.jpg',img)
cv.waitKey(0)
cv.destroyAllWindows()