title: How to use ridge detection filter in opencv
body: <p>I am trying to use ridge/valley filter with opencv-python. And I just checked the document in openCV's official <a href="https://docs.opencv.org/trunk/d4/d36/classcv_1_1ximgproc_1_1RidgeDetectionFilter.html" rel="noreferrer">website</a>, which tells me to use</p>
  
  <p><code>out = cv.ximgproc_RidgeDetectionFilter.getRidgeFilteredImage(  _img[, out] )</code>.</p>
  
  <p>However, after trying, this function seems do not exist in cv2(python). Is there an alternative way to do this with openCV or any other available approach?</p>