how to compare size of kepoints matching in opencv - c++

Question

I'm going compare images with SURF detector in opencv. for this work,I need size and Orientation of keypoints that must be compare them. for example, I have to extract keypoints matching of second image larger than keypoints matching of first image. (keypoint 1.size > keypoint 2.size).
question:
how to extract size of keypoints matching in opencv?

Answer 1

I'm not sure if I quite understand your question.
What I understood was that:
You will compare images using keypoints
You want to compare the size of matched keypoints
First u want at least 2 images:
Mat image1; //imread stuff here
Mat image2; //imread stuff here
Then detect keypoints in the two images using SURF:
vector<KeyPoint> keypoints1, keypoints2; //store the keypoints
Ptr<FeatureDetector> detector = new SURF();
detector->detect(image1, keypoints1); //detect keypoints in 'image1' and store them in 'keypoints1'
detector->detect(image2, keypoints2); //detect keypoints in 'image2' and store them in 'keypoints2'
After that compute the descriptors for detected keypoints:
Mat descriptors1, descriptors2;
Ptr<DescriptorExtractor> extractor = new SURF();
extractor->compute(image1, keypoints1, descriptors1);
extractor->compute(image2, keypoints2, descriptors2);
Then match the descriptors of keypoints using for example BruteForce with L2 norm:
BFMatcher matcher(NORM_L2);
vector<DMatch> matches;
matcher.match(descriptors1, descriptors2, matches);
After these steps matched keypoints were stored in the vector 'matches'
You can get the index of matched keypoints as follows:
//being idx any number between '0' and 'matches.size()'
int keypoint1idx = matches[idx].query; //keypoint of the first image 'image1'
int keypoint2idx = matches[idx].train; //keypoint of the second image 'image2'
Read this for further information:
http://docs.opencv.org/modules/features2d/doc/common_interfaces_of_descriptor_matchers.html
Finally, to know the size of the matched keypoints u can do the following:
int size1 = keypoints1[ keypoint1idx ].size; //size of keypoint in the image1
int size2 = keypoints2[ keypoint2idx ].size; //size of keypoint in the image2
Further info: http://docs.opencv.org/modules/features2d/doc/common_interfaces_of_feature_detectors.html
And that's it! Hope this helps