Posted on

Local Descriptors in OpenCv

Tomas Martinkovic

The project shows detection of chocolate cover from input image or frame of video. For each video or image may be chosen various combinations of detector with descriptor. For matching object of chocolate cover with input frame or image automatically is used FlannBasedMatcher or BruteForceMatcher. It depends on the chosen SurfDescriptorExtractor or FREAK algorithm.

Functions used: SurfFeatureDetector, FastFeatureDetector, SiftFeatureDetector, StarFeatureDetector, SurfDescriptorExtractor, FREAK, FlannBasedMatcher, BruteForceMatcher, findHomography


  1. Preprocessing – Conversion to grayscale
    cvtColor(frame, img_scene, CV_BGR2GRAY);
  2. Detect the keypoints
    detector_Surf = getSurfFeatureDetector();
    detector_Surf.detect( img_object, keypoints_object );
    detector_Surf.detect( img_scene, keypoints_scene );
  3. Compute local descriptors
    extractor_freak.compute( img_object, keypoints_object, descriptors_object );
    extractor_freak.compute( img_scene, keypoints_scene, descriptors_scene );
  4. Matching local descriptors
    BruteForceMatcher<Hamming> matcher;
    matcher.match( descriptors_object, descriptors_scene, matches );
  5. Draw good matches to frame
    drawMatches( img_object, keypoints_object, img_scene, keypoints_scene, good_matches, img_matches, Scalar::all(-1), Scalar::all(-1), vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );
  6. Finding homography and drawing frame of video
    Mat H = findHomography( obj, scene, CV_RANSAC );
    perspectiveTransform( obj_corners, scene_corners, H);
    imshow( "Object detection on video", img_matches );


Matching local descriptors in the image.
Matching local descriptors in the video.