OpenCV Sift/Surf/Orb : drawMatch function is not working well

Question

I use Sift/Surf and ORB but sometimes i have a problem with the drawMatch function.

Here the error :

OpenCV Error: Assertion failed (i2 >= 0 && i2 < static_cast(keypoints2.size())) in drawMatches, file /home/opencv-2.4.6.1/modules/features2d/src/draw.cpp, line 208 terminate called after throwing an instance of 'cv::Exception' what(): /home/opencv-2.4.6.1/modules/features2d/src/draw.cpp:208: error: (-215) i2 >= 0 && i2 < static_cast(keypoints2.size()) in function drawMatche

The code :

drawMatchPoints(img1,keypoints_img1,img2,keypoints_img2,matches);

I tried to invert img 1,keypoints_img1 with img2 and keypoints_img2 like that :

drawMatchPoints(img2,keypoints_img2,img1,keypoints_img1,matches);

Corresponding to my function who is doing an homography :

void drawMatchPoints(cv::Mat image1,std::vector<KeyPoint> keypoints_img1,
                                      cv::Mat image2,std::vector<KeyPoint> keypoints_img2,std::vector<cv::DMatch> matches){

    cv::Mat img_matches;
    drawMatches( image1, keypoints_img1, image2, keypoints_img2,
                         matches, img_matches, Scalar::all(-1), Scalar::all(-1),
                         vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );
            std::cout << "Number of good matching " << (int)matches.size() << "\n" << endl;



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

            for( int i = 0; i < matches.size(); i++ )
            {
              //-- Get the keypoints from the good matches
              obj.push_back( keypoints_img1[ matches[i].queryIdx ].pt );
              scene.push_back( keypoints_img2[matches[i].trainIdx ].pt );
            }

            Mat H = findHomography( obj, scene, CV_RANSAC );
            std::cout << "Size of homography " << *H.size << std::endl ;

            //-- 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( image1.cols, 0 );
            obj_corners[2] = cvPoint( image1.cols, image1.rows ); obj_corners[3] = cvPoint( 0, image1.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( image1.cols, 0), scene_corners[1] + Point2f( image1.cols, 0), Scalar(0, 255, 0), 4 );
            line( img_matches, scene_corners[1] + Point2f( image1.cols, 0), scene_corners[2] + Point2f( image1.cols, 0), Scalar( 0, 255, 0), 4 );
            line( img_matches, scene_corners[2] + Point2f( image1.cols, 0), scene_corners[3] + Point2f( image1.cols, 0), Scalar( 0, 255, 0), 4 );
            line( img_matches, scene_corners[3] + Point2f( image1.cols, 0), scene_corners[0] + Point2f( image1.cols, 0), Scalar( 0, 255, 0), 4 );

            //-- Show detected matches
            cv::imshow( "Good Matches & Object detection", img_matches );
            cv::waitKey(5000);

}

But i have still the error !

I have noticed that the error happened when the size of my keypoints_img1 is lower than the size of my keypoints_img2 :

Size keyPoint1 : 244 - Size keyPoint2 : 400

So if i invert the loading of my two picture, that works but i can't now in advance if my first picture will have more keypoints that my second picture...

My code (the most important step) in order to create the features :

init_Sift(400,5,0.04,25,1.6);
void init_Sift(int nf,int nOctaveL,double contrastThresh, double edgeThresh,double sigma){
this->nfeatureSift=nf;
this->nOctaveLayerSift=nOctaveL;
this->contrastThresholdSift=contrastThresh;
this->edgeThresholdSift=edgeThresh;
this->sigmaSift=sigma;}



 cv::FeatureDetector* detector=new SiftFeatureDetector(nfeatureSift,nOctaveLayerSift,contrastThresholdSift,edgeThresholdSift,sigmaSift);
cv::DescriptorExtractor* extractor=new SiftDescriptorExtractor

extractor->compute( image, keypoints, descriptors );

The matching part :

    std::cout << "Type of matcher : " << type_of_matcher << std::endl;
if (type_of_matcher=="FLANN" || type_of_matcher=="BF"){
    std::vector<KeyPoint> keypoints_img1 = keyfeatures.compute_Keypoints(img1);
    std::vector<KeyPoint> keypoints_img2 = keyfeatures.compute_Keypoints(img2);

    cv::Mat descriptor_img1 = keyfeatures.compute_Descriptors(img1);
    cv::Mat descriptor_img2 = keyfeatures.compute_Descriptors(img2);

    std::cout << "Size keyPoint1 " << keypoints_img1.size() << "\n" << std::endl;
    std::cout << "Size keyPoint2 " << keypoints_img2.size() << "\n" << std::endl;

    //Flann with sift or surf
    if (type_of_matcher=="FLANN"){
        Debug::info("USING Matcher FLANN");
        fLmatcher.match(descriptor_img1,descriptor_img2,matches);

        double max_dist = 0; double min_dist = 100;

        //-- Quick calculation of max and min distances between keypoints
        for( int i = 0; i < descriptor_img1.rows; i++ ){
            double dist = matches[i].distance;
            if( dist < min_dist ) min_dist = dist;
            if( dist > max_dist ) max_dist = dist;
         }

        std::vector< DMatch > good_matches;

          for( int i = 0; i < descriptor_img1.rows; i++ )
          { if( matches[i].distance <= max(2*min_dist, 0.02) )
            { good_matches.push_back( matches[i]); }
          }

          std::cout << "Size of good match : " <<  (int)good_matches.size() << std::endl;
          //-- Draw only "good" matches
          if (!good_matches.empty()){
              drawMatchPoints(img1,keypoints_img1,img2,keypoints_img2,good_matches);

          }
          else {
              Debug::error("Flann Matcher : Pas de match");
              cv::Mat img_matches;
              drawMatches( img1, keypoints_img1, img2, keypoints_img2,
                                matches, img_matches, Scalar::all(-1), Scalar::all(-1),
                                vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );
              cv::imshow( "No match", img_matches );
              cv::waitKey(5000);
          }

    }
    //BruteForce with sift or surf
    else if (type_of_matcher=="BF"){
        Debug::info("USING Matcher Brute Force");

        bFmatcher.match(descriptor_img1,descriptor_img2,matches);
        if (!matches.empty()){
            std::nth_element(matches.begin(),//Initial position
                             matches.begin()+24, //Position  of the sorted element
                             matches.end());//End position
            matches.erase(matches.begin()+25,matches.end());

            drawMatchPoints(img1,keypoints_img1,img2,keypoints_img2,matches);
            //drawMatchPoints(img2,keypoints_img2,img1,keypoints_img1,matches);
        }
        else {
            Debug::error("Brute Force matcher  : Pas de match");
            cv::Mat img_matches;
            drawMatches( img1, keypoints_img1, img2, keypoints_img2,
                              matches, img_matches, Scalar::all(-1), Scalar::all(-1),
                              vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );
            cv::imshow( "No match", img_matches );
            cv::waitKey(5000);

        }

}

Do you have any suggestions or advises ?

EDIT : i solved my problem. I had a c++ problem, because i had two classes . One about matching and another about finding keyFeature. I have wrote on my .h std::vector and the same for descriptors.

class keyFeatures{

public:
...   
std::vector<keyPoint> keypoints;
...

I delete this attribute and i did a function that take in argument std::vector keypoints

cv::Mat descriptor_img1 = keyfeatures.compute_Descriptors(img1,keypoints_img1);

instead of

cv::Mat descriptor_img1 = keyfeatures.compute_Descriptors(img1);

I think there was a conflict when i did the matching... But i don't know why i shouldn't have to write it on my .h and do a local parameter on my function.

Thank you !

Answer 1

For somebody like me who searched for this but couldn't find the solution.

Assertion failed (i2 >= 0 && i2 < static_cast(keypoints2.size()))

This means that assertion failed due to i2 being less than 0 or i2 being less than the keypoints2 size. But what is i2?

From the link that rbaleksandar provided in a comment

int i2 = matches1to2[m].trainIdx;

trainIdx here is an index in keypoints2. The check i2 < static_cast(keypoints2.size()) makes sure that the index is less than keypoints2.size().

For me it occured because I discarded some keypoints before calling drawMatches but after the descriptors were computed i.e. DescriptorExtractor#compute was called. This meant that the drawMatches referred to old keypoints through the descriptors while I changed those keypoints. The end result was that some keypoints had large idx but the keypoints size was small hence the error.