cout<<"型別1:  "<<desc1.type()<<endl;
desc1.convertTo(desc1, CV_32F);
desc2.convertTo(desc2, CV_32F);
cout<<"型別2:  "<<desc1.type()<<endl;
FlannBasedMatcher matcher;
 vector< DMatch > matches;
 vector< vector<DMatch> > m_knnMatches;

 matches.clear();
 const float minRatio=1.f / 1.5f;
 matcher.knnMatch(desc1,desc2,m_knnMatches,2);

 for (int i=0; i<m_knnMatches.size(); i++)
 {
  const DMatch& bestMatch=m_knnMatches[i][0];
  const DMatch& betterMatch=m_knnMatches[i][1];

  float distanceRatio=bestMatch.distance/betterMatch.distance;

  if (distanceRatio<minRatio)
  {
	  matches.push_back(bestMatch);
  }
 }
 

當使用KNN匹配時，注意它的接受的描述子的型別應該為CV_32F，所以需要轉換到所需的型別上。
錯誤為：OpenCV Error: Unsupported format or combination of formats (type=0 ) in buildIndex_, file /tmp/buildd/ros-fuerte-opencv2-2.4.2-1precise-20130312-1306/modules/flann/src/miniflann.cpp, line 299 terminate called after throwing an instance of ‘cv::Exception’ what(): /tmp/buildd/ros-fuerte-opencv2-2.4.2-1precise-20130312-1306/modules/flann/src/miniflann.cpp:299: error: (-210) type=0 in function buildIndex_