AttributeError: module 'cv2' has no attribute 'SIFT'
遇到该问题时，网友多是建议补个包，即pip install opencv-contrib-python
我在补完之后又出现下面这样的错误：
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（专利保护） and is excluded in this configuration; Set OPENCV_ENABLE_NONFREE CMake option and rebuild the library in function ‘cv::xfeatures2d::SIFT::create’
将opencv版本退到3.4.2即可解决，卸载之前的包（pip uninstall opencv-python），然后
pip install opencv-python==3.4.2.16
pip install opencv-contrib-python==3.4.2.16
注意以上两条命令==左右没有空格
又报错误：
'module' object has no attribute 'xfeatures2d'
原因：opencv将SIFT等算法整合到xfeatures2d集合里面了。
siftDetector=cv2.SIFT()
变更后为
siftDetector= cv2.xfeatures2d.SIFT_create()
又报：
TypeError: Required argument 'outImage' (pos 3) not found
im=cv2.drawKeypoints(img_RGB,kp,flags=cv2.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS)
变更后为im=cv2.drawKeypoints(img_RGB,kp,img,flags=cv2.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS)
SURF：
import cv2
img = cv2.imread('cat.jpg')
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
surf = cv2.xfeatures2d.SURF_create()
kp = surf.detect(gray, None)
img = cv2.drawKeypoints(gray, kp, img)
cv2.imshow("img", img)
k = cv2.waitKey(0)
if k & 0xff == 27:
cv2.destroyAllWindows()