Merge branch '3.4' into merge-3.4

modules/features2d/src/keypoint.cpp

@@ -77,7 +77,7 @@ void KeyPointsFilter::retainBest(std::vector<KeyPoint>& keypoints, int n_points)
             return;
         }
         //first use nth element to partition the keypoints into the best and worst.
-        std::nth_element(keypoints.begin(), keypoints.begin() + n_points, keypoints.end(), KeypointResponseGreater());
+        std::nth_element(keypoints.begin(), keypoints.begin() + n_points - 1, keypoints.end(), KeypointResponseGreater());
         //this is the boundary response, and in the case of FAST may be ambiguous
         float ambiguous_response = keypoints[n_points - 1].response;
         //use std::partition to grab all of the keypoints with the boundary response.