DMatch.queryIdx giving out of bounds error against keypoints?!?

[Richard]

So I have a block of code to detect an instance of an image in List refImages in a screencapture.

I extract each reference images keypoints into a List<List<KeyPoint>> refImagesKey like so

for(int i=0; i<refImages.size(); i++){
   List<KeyPoint> imgKeyPoints = new ArrayList<KeyPoint>();
   featureDetector.detect(refImages.get(i), imgKeyPoints);
   refImagesKey.add(imgKeyPoints);
   obj_descriptor_mats.add(new Mat());
}
descriptorExtractor.compute(refImages, refImagesKey, obj_descriptor_mats);
descriptorMatcher.add(obj_descriptor_mats);

Then I test screen cap against images like so

descriptorMatcher.match(sceneDescriptors, matches);

I filter out good matches...

          double max_dist = 0;
          double min_dist = 100;
            int rowCount = matches.size();
            for(int i=0; i<rowCount; i++){
              double dist = matches.get(i).distance;
              if(dist < min_dist) min_dist=dist;
              if(dist > max_dist) max_dist=dist;
            }
           
            List<DMatch> good_matches = new ArrayList<DMatch>();
              double good_dist = 3*min_dist;
              for(int i=0; i<rowCount; i++){
                if(matches.get(i).distance < good_dist){
                  good_matches.add(matches.get(i));
                }
              }

All that works, although I'm starting to question if it does correctly because when I go to get the points associated with each DMatch like so

List<Point> obj = new ArrayList<Point>();
List<Point> scene = new ArrayList<Point>();
int good_matches_size = good_matches.size();
           
for(int i=0; i<good_matches_size; i++){
  DMatch imatch = good_matches.get(i);
  obj.add(refImagesKey.get(imatch.imgIdx).get(imatch.queryIdx).pt);
  scene.add(sceneKeyPoints.get(imatch.trainIdx).pt);
}

I get a Index out of bounds error for the queryIdx.

I cant help but feel I'm just using these properties incorrectly but there seems to be very little documentation on them.

Any advice would be greatly appreciated.

Thanks,

[Richard]

DMatch.imgIdx is always part of the train descriptors. so I had to swap queryIdx and trainIdx.

[Rui Marques]

Yes that is it, it is a common mistake i guess.