title: sift algorithm for opencv 4.1
body: <p>I am executing statement <code>sift = cv2.xfeatures2d.SIFT_create()</code>.
  It is working fine with OpenCV 3.4 but I want to execute it with the OpenCV version 4.1.</p>
  
  <p>I am getting the following error:</p>
  
  <blockquote>
    <p>Message=OpenCV(4.1.1) D:\Build\OpenCV\opencv_contrib-4.1.1\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:</p>
  </blockquote>