OpenCV Python : No drawMatchesknn function

Question

When I tried to use drawMatchesKnn function as mentioned in this tutorial for FLANN feature matching, I get the following error

AttributeError: 'module' object has no attribute 'drawMatchesKnn'

I checked with other resources that drawMatchesKnn method is present in opencv.

Why am I getting this error?

Thanks in advance

IMP - This tutorial is meant for OpenCV 3x version. Not OpenCV 2x, it says clearly on the README page. You didn't read that? — bad_keypoints
@bad_keypoints actually the tutorials don't work in 2x or 3x. The tutorials are filled with calls like this cv2.SIFT()that don't exist in 3x (needs to be compiled with contrilbs and callled like cv2.xfeatures2d.SIFT_create(), so the tutorials are just a mess of old and new and few really work without changes. — tiagosilva

ryanmeasel ryanmeasel · Accepted Answer · 2016-02-24T23:03:14

The functions cv2.drawMatches and cv2.drawMatchesKnn are not available in newer versions of OpenCV 2.4. @rayryeng provided a lightweight alternative which works as is for the output of DescriptorMatcher.match. The difference with DescriptorMatcher.knnMatch is that the matches are returned as a list of lists. To use the @rayryeng alternative, the matches must be extracted into a 1-D list.

For example, the Brute-Force Matching with SIFT Descriptors and Ratio Test tutorial could be amended as such:

# BFMatcher with default params
bf = cv2.BFMatcher()
matches = bf.knnMatch(des1,des2, k=2)

# Apply ratio test
good = []
for m,n in matches:
    if m.distance < 0.75*n.distance:
       # Removed the brackets around m 
       good.append(m)

# Invoke @rayryeng's drawMatches alternative, note it requires grayscale images
gray1 = cv2.cvtColor(img1,cv2.COLOR_BGR2GRAY)
gray2 = cv2.cvtColor(img2,cv2.COLOR_BGR2GRAY)
drawMatches(gray1,kp1,gray2,kp2,good)

OpenCV Python : No drawMatchesknn function

3 Answers