This is illustration for sift feature extraction:

import cv2

sift = cv2.xfeatures2d.SIFT_create()

def compute_one(im):

 return sift.detectAndCompute(im, None)

def compute_sift(frames):

 print('get sift features')

 sift_features = [(None, None) for _ in frames]

 for frame_idx, im in enumerate(frames):

  if im is None or frame_idx % 3 != 0:

   continue

  print('... sift {}/{}'.format(frame_idx, len(frames)))

  keypoints, descs = compute_one(im)

  sift_features[frame_idx] = (keypoints, descs)

 return sift_features