c++ - OpenCV - BFMatcher 只检查对象特征之间的距离，而不检查场景特征之间的距离？

Question

我正在运行以下代码。目标是检测图片“卡片”是否出现在“棋盘”上，也就是可能包含该卡片的屏幕截图。

检测工作近乎完美，但当我尝试绘制它时，我注意到有些线相距太远。

虽然来自物体的点与场景的距离很近，但在“场景”中它们通常相距太远，从而给出错误的结果。

您可以从以下屏幕截图中看出。在场景中检测到对象，但许多线都偏离了位置。我也希望放弃相距太远的线条。

我认为当您比较对象的起点时，我的函数会删除任何相距太远的线。但是，对于场景中相距太远的点，这似乎不会发生。我怎么能删除那些？

bool isCardOnBoard(Mat card, string filename) {
         //-- Step 1: Detect the keypoints using SURF Detector

      vector<KeyPoint> keypoints_object;
      detector.detect( card, keypoints_object );


    //-- Step 2: Calculate descriptors (feature vectors)
      Mat descriptors_object;

      extractor.compute( card, keypoints_object, descriptors_object );


      //-- Step 3: Matching descriptor vectors using FLANN matcher
      // FlannBasedMatcher matcher;
      BFMatcher matcher(extractor.defaultNorm(), false);

      std::vector< DMatch > matches;
      matcher.match( descriptors_object, descriptors_scene, matches );

      double max_dist = 0; double min_dist = 100;
      //-- Quick calculation of max and min distances between keypoints
      for( int i = 0; i < descriptors_object.rows; i++ )
      { double dist = matches[i].distance;
        if( dist < min_dist ) min_dist = dist;
        if( dist > max_dist ) max_dist = dist;
      }

      // printf("-- Max dist : %f \n", max_dist );
      // printf("-- Min dist : %f \n", min_dist );

      //-- Draw only "good" matches (i.e. whose distance is less than 3*min_dist )
      std::vector< DMatch > good_matches;

      for( int i = 0; i < descriptors_object.rows; i++ ) { 
        if( matches[i].distance < 3*min_dist)
          good_matches.push_back( matches[i]); 
      }

      if (good_matches.size() > 100) {
        cout << filename << " NOT on the board" << endl;
        return false;
      }

      Mat img_matches;
      drawMatches( card, keypoints_object, board, keypoints_scene,
                   good_matches, img_matches, Scalar::all(-1), Scalar::all(-1),
                   // vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );
                   vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );

      //-- Localize the object
      std::vector<Point2f> obj;
      std::vector<Point2f> scene;

      // cout << good_matches.size() << endl;

      for( int i = 0; i < good_matches.size(); i++ ) {
        //-- Get the keypoints from the good matches
        obj.push_back( keypoints_object[ good_matches[i].queryIdx ].pt );
        scene.push_back( keypoints_scene[ good_matches[i].trainIdx ].pt );
      }

      Mat H = findHomography( obj, scene, RANSAC );

      //-- Get the corners from the image_1 ( the object to be "detected" )
      std::vector<Point2f> obj_corners(4);
      obj_corners[0] = cvPoint(0,0); obj_corners[1] = cvPoint( card.cols, 0 );
      obj_corners[2] = cvPoint( card.cols, card.rows ); obj_corners[3] = cvPoint( 0, card.rows );
      std::vector<Point2f> scene_corners(4);

      perspectiveTransform( obj_corners, scene_corners, H);

      //-- Draw lines between the corners (the mapped object in the scene - image_2 )
      line( img_matches, scene_corners[0] + Point2f( card.cols, 0), scene_corners[1] + Point2f( card.cols, 0), Scalar(0, 255, 0), 4 );
      line( img_matches, scene_corners[1] + Point2f( card.cols, 0), scene_corners[2] + Point2f( card.cols, 0), Scalar( 0, 255, 0), 4 );
      line( img_matches, scene_corners[2] + Point2f( card.cols, 0), scene_corners[3] + Point2f( card.cols, 0), Scalar( 0, 255, 0), 4 );
      line( img_matches, scene_corners[3] + Point2f( card.cols, 0), scene_corners[0] + Point2f( card.cols, 0), Scalar( 0, 255, 0), 4 );

      //-- Show detected matches
      imshow( "Good Matches & Object detection", img_matches );
      waitKey(0);
      return true;
    }

Answer 1

如您所说，“相距太远的线”代表异常值:来自 matcher.match( descriptors_object, descriptors_scene, matches );

当您估计单应性 H 时，您在内部使用统计方法来拒绝那些异常值。这里使用的方法称为 RANSAC . OpenCV 函数中可用的另一种方法是 LMeDS。正如 OpenCV 文档中所解释的:RANSAC 方法实际上可以处理任何比例的离群值，但它需要一个阈值来区分离群值和离群值。 LMeDS 方法不需要任何阈值，但只有在超过 50% 的内点时才能正常工作。

我建议您尝试使用不同的 RANSAC 阈值或尝试使用 LMeDS。请注意，场景中的打印字符肯定会给您异常值..

如果您只想“删除相距太远的线”(为什么？)，您可能只想从重新投影的对象中的匹配项绘制线(？)