Matching template imge(scaled) to Main/larger image

Question

I want to find/check subImage/template image in main image and want to know its coordinates,

I have used code given at following link to implement it,

Check presence of subimage in image in iOS

It is working fine, if the size of the template image is exactly same as size of the matching part of larger image.

But it is not giving result properly if the subimage is scaled down or scaled up than matching part of larger image.

Answer 1

Use OpenCV Feature Detection. it is more accurate than template matching..

Please try with this code..

-(void)featureDetection:(UIImage*)largerImage withImage:(UIImage*)subImage
{
    cv::Mat tempMat1 = [largerImage CVMat];
    cv::Mat tempMat2 = [subImage CVMat];

    cv::cvtColor(tempMat1, tempMat1, CV_RGB2GRAY);
    cv::cvtColor(tempMat2, tempMat2, CV_RGB2GRAY);

    if( !tempMat1.data || !tempMat2.data ) {
        return;
    }

    //-- Step 1: Detect the keypoints using SURF Detector
    int minHessian = 25;

    cv::SurfFeatureDetector detector( minHessian ); // More Accurate bt take more time..
    //cv::FastFeatureDetector detector( minHessian ); //Less Accurate bt take less time..

    std::vector<cv::KeyPoint> keypoints_1, keypoints_2;

    detector.detect( tempMat1, keypoints_1 );
    detector.detect( tempMat2, keypoints_2 );

    //-- Step 2: Calculate descriptors (feature vectors)
    cv::SurfDescriptorExtractor extractor;

    cv::Mat descriptors_1, descriptors_2;

    extractor.compute( tempMat1, keypoints_1, descriptors_1 );
    extractor.compute( tempMat2, keypoints_2, descriptors_2 );

    std::vector<cv::Point2f> obj_corners(4);

    //Get the corners from the object
    obj_corners[0] = (cvPoint(0,0));
    obj_corners[1] = (cvPoint(tempMat2.cols,0));
    obj_corners[2] = (cvPoint(tempMat2.cols,tempMat2.rows));
    obj_corners[3] = (cvPoint(0, tempMat2.rows));

    //-- Step 3: Matching descriptor vectors with a brute force matcher
    //cv::BruteForceMatcher < cv::L2<float> > matcher;
    cv::FlannBasedMatcher matcher;
    //std::vector< cv::DMatch > matches;
    std::vector<cv::vector<cv::DMatch > > matches;

    std::vector<cv::DMatch > good_matches;
    std::vector<cv::Point2f> obj;
    std::vector<cv::Point2f> scene;
    std::vector<cv::Point2f> scene_corners(4);
    cv::Mat H;

    matcher.knnMatch( descriptors_2, descriptors_1, matches,2);

    for(int i = 0; i < cv::min(tempMat1.rows-1,(int) matches.size()); i++)  {

        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]);
        }
    }
    cv::Mat img_matches;
    drawMatches( tempMat2, keypoints_2, tempMat1, keypoints_1, good_matches, img_matches );

    NSLog(@"good matches %lu",good_matches.size());

    if (good_matches.size() >= 4)  {

         for( int i = 0; i < good_matches.size(); i++ ) {
             //Get the keypoints from the good matches
             obj.push_back( keypoints_2[ good_matches[i].queryIdx ].pt );
             scene.push_back( keypoints_1[ good_matches[i].trainIdx ].pt );
         }

         H = findHomography( obj, scene, CV_RANSAC );

         perspectiveTransform( obj_corners, scene_corners, H);

         NSLog(@"%f %f",scene_corners[0].x,scene_corners[0].y);
         NSLog(@"%f %f",scene_corners[1].x,scene_corners[1].y);
         NSLog(@"%f %f",scene_corners[2].x,scene_corners[2].y);
         NSLog(@"%f %f",scene_corners[3].x,scene_corners[3].y);


         //Draw lines between the corners (the mapped object in the scene image )
         line( tempMat1, scene_corners[0], scene_corners[1], cvScalar(0, 255, 0), 4 );

         line( tempMat1, scene_corners[1], scene_corners[2], cvScalar( 0, 255, 0), 4 );

         line( tempMat1, scene_corners[2], scene_corners[3], cvScalar( 0, 255, 0), 4 );

         line( tempMat1, scene_corners[3], scene_corners[0], cvScalar( 0, 255, 0), 4 );
     }

     // View matching..

     UIImage *resultimage = [UIImage imageWithCVMat:img_matches];
     UIImageView *imageview = [[UIImageView alloc] initWithImage:resultimage];
     imageview.frame = CGRectMake(0, 0, 320, 240);
     [self.view addSubview:imageview];

     // View Result

     UIImage *resultimage2 = [UIImage imageWithCVMat:tempMat1];
     UIImageView *imageview2 = [[UIImageView alloc] initWithImage:resultimage2];
     imageview2.frame = CGRectMake(0, 240, 320, 240);
     [self.view addSubview:imageview2];
}