C++ - OpenCV feature detection with ORB - c++

Question

I'm trying to extract and match features with OpenCV using ORB for detecting and FLANN for matching, and i get a really weird result. After loading my 2 images and converting them to grayscale, here's my code:
// Initiate ORB detector
Ptr<FeatureDetector> detector = ORB::create();
// find the keypoints and descriptors with ORB
detector->detect(gray_image1, keypoints_object);
detector->detect(gray_image2, keypoints_scene);
Ptr<DescriptorExtractor> extractor = ORB::create();
extractor->compute(gray_image1, keypoints_object, descriptors_object );
extractor->compute(gray_image2, keypoints_scene, descriptors_scene );
// Flann needs the descriptors to be of type CV_32F
descriptors_scene.convertTo(descriptors_scene, CV_32F);
descriptors_object.convertTo(descriptors_object, CV_32F);
FlannBasedMatcher matcher;
vector<DMatch> matches;
matcher.match( descriptors_object, descriptors_scene, matches );
double max_dist = 0; double min_dist = 100;
//-- Quick calculation of max and min distances between keypoints
for( int i = 0; i < descriptors_object.rows; i++ )
{
double dist = matches[i].distance;
if( dist < min_dist ) min_dist = dist;
if( dist > max_dist ) max_dist = dist;
}
//-- Use only "good" matches (i.e. whose distance is less than 3*min_dist )
vector< DMatch > good_matches;
for( int i = 0; i < descriptors_object.rows; i++ )
{
if( matches[i].distance < 3*min_dist )
{
good_matches.push_back( matches[i]);
}
}
vector< Point2f > obj;
vector< Point2f > scene;
for( int i = 0; i < good_matches.size(); i++ )
{
//-- Get the keypoints from the good matches
obj.push_back( keypoints_object[ good_matches[i].queryIdx ].pt );
scene.push_back( keypoints_scene[ good_matches[i].trainIdx ].pt );
}
// Find the Homography Matrix
Mat H = findHomography( obj, scene, CV_RANSAC );
// Use the Homography Matrix to warp the images
cv::Mat result;
warpPerspective(image1,result,H,Size(image1.cols+image2.cols,image1.rows));
cv::Mat half(result,cv::Rect(0,0,image2.cols,image2.rows));
image2.copyTo(half);
imshow( "Result", result );
And this is a screen shot of the weird result i'm getting:
screen shot
What might be the problem?
Thanks!

Answer 1

You are experiencing the results of a bad matching: The homography which fits the data is not "realistic" and thus distorts the image.
You can debug your matching with imshow( "Good Matches", img_matches ); as done in the example.
There are multiple approaches to improve your matches:
Use the crossCheck option
Use the SIFT ratio test
Use the OutputArray mask in cv::findHompgraphy to identify totally wrong homography computations
... and so on...

Answer 2

ORB are binary feature vectors which don't work with Flann. Use Brute Force (BFMatcher) instead.