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

   [c language=”c++”]
   cvtColor(frame, img_scene, CV_BGR2GRAY);
   [/c]

2. Detect the keypoints

   [c language=”c++”]
   detector_Surf = getSurfFeatureDetector();
   detector_Surf.detect( img_object, keypoints_object );
   detector_Surf.detect( img_scene, keypoints_scene );
   [/c]

3. Compute local descriptors

   [c language=”c++”]
   extractor_freak.compute( img_object, keypoints_object, descriptors_object );
   extractor_freak.compute( img_scene, keypoints_scene, descriptors_scene );
   [/c]

4. Matching local descriptors

   [c language=”c++”]
   BruteForceMatcher<Hamming> matcher;
   matcher.match( descriptors_object, descriptors_scene, matches );
   [/c]

5. Draw good matches to frame

   [c language=”c++”]
   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 );
   [/c]

6. Finding homography and drawing frame of video

   [c language=”c++”]
   Mat H = findHomography( obj, scene, CV_RANSAC );
   perspectiveTransform( obj_corners, scene_corners, H);
   imshow( "Object detection on video", img_matches );
   [/c]

Sample