Use Euclidean distance in SURF

edit

Hi, I may b asking a silly question, but i still need to solve this. In my code im filtering the good images based on the nearest neighbor distance ratio, as follows.

 for(int i = 0; i < min(des_image.rows-1,(int) matches.size()); i++) //THIS LOOP IS SENSITIVE TO SEGFAULTS
        {
            if((matches[i][0].distance < 0.6*(matches[i][1].distance)) && ((int) matches[i].size()<=2 && (int) matches[i].size()>0))
            {
                good_matches.push_back(matches[i][0]);
            }
}

Since im filtering the good images based on the nearest neighbor distance ratio, do i need to still do Euclidean distance calculation.

and i want to know when i use the knnMatch method in FlannBasedMatcher, inside the method do they use the Euclidean distance to match the keypoints.