OpenCV 2.4.1 - computing SURF descriptors in Python -

i'm trying update code use cv2.surf() opposed cv2.featuredetector_create("surf") , cv2.descriptorextractor_create("surf"). i'm having trouble getting descriptors after detecting keypoints. what's correct way call surf.detect?

i tried following opencv documentation, i'm little confused. says in documentation.

python: cv2.surf.detect(img, mask) → keypoints¶ python: cv2.surf.detect(img, mask[, descriptors[, useprovidedkeypoints]]) → keypoints, descriptors 

how pass keypoints in when making second call surf.detect?

i not sure whether understand questions correctly. if looking sample of matching surf keypoints, simple , basic 1 below, similar template matching:

import cv2 import numpy np  # load images img =cv2.imread('messi4.jpg')  # convert them grayscale imgg =cv2.cvtcolor(img,cv2.color_bgr2gray)  # surf extraction surf = cv2.surf() kp, descritors = surf.detect(imgg,none,useprovidedkeypoints = false)  # setting samples , responses knn samples = np.array(descritors) responses = np.arange(len(kp),dtype = np.float32)  # knn training knn = cv2.knearest() knn.train(samples,responses)  # loading template image , searching similar keypoints template = cv2.imread('template.jpg') templateg= cv2.cvtcolor(template,cv2.color_bgr2gray) keys,desc = surf.detect(templateg,none,useprovidedkeypoints = false)  h,des in enumerate(desc):     des = np.array(des,np.float32).reshape((1,128))     retval, results, neigh_resp, dists = knn.find_nearest(des,1)     res,dist =  int(results[0][0]),dists[0][0]      if dist<0.1: # draw matched keypoints in red color         color = (0,0,255)     else:  # draw unmatched in blue color         print dist         color = (255,0,0)      #draw matched key points on original image     x,y = kp[res].pt     center = (int(x),int(y))     cv2.circle(img,center,2,color,-1)      #draw matched key points on template image     x,y = keys[h].pt     center = (int(x),int(y))     cv2.circle(template,center,2,color,-1)  cv2.imshow('img',img) cv2.imshow('tm',template) cv2.waitkey(0) cv2.destroyallwindows() 

below results got (copy pasted template image on original image using paint):

as can see, there small mistakes. startup, hope ok.