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

Process

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 );
   

Sample