Revision history [back]

For me, it happened because the keypoints were swapped from the expected between the matcher and the drawMatches. But it can also occur in your case because you have a loop but keypoints2 and matches are reused but not cleared. You should move the keypoints2/descriptors2/matches declarations inside of the loop and also clear those same vectors at the end of the while loop.

For both matcher and drawMatches, Query/Scene comes first, then Train/Object. In your case, descriptors1 is the trained keypoint set and descriptors2 is the query. You need to swap the order in the matcher and drawMatches functions.

Change this:

matcher.match(descriptors1, descriptors2, matches);

to this:

matcher.match(descriptors2, descriptors1, matches);

Also change this:

drawMatches( img1, keypoints1, img2, keypoints2,good_matches, img_matches, Scalar::all(-1), Scalar::all(-1),
           vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );

to this:

drawMatches( img2, keypoints2, img1, keypoints1,good_matches, img_matches, Scalar::all(-1), Scalar::all(-1),
           vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );