How does drawMatches work? (with respect to DMatch)

Hey,

So I have been working on a tracker based on SIFT descriptor. I saw this example of flannMatcher. http://docs.opencv.org/2.4.2/doc/tutorials/features2d/feature_flann_matcher/feature_flann_matcher.html

I tried to implement it and worked really well.

Now, my question is how does drawMatches work? As it is mentioned here,

http://docs.opencv.org/modules/features2d/doc/drawing_function_of_keypoints_and_matches.html?highlight=drawmatch#void%20drawMatches%28const%20Mat&%20img1,%20const%20vector%3CKeyPoint%3E&%20keypoints1,%20const%20Mat&%20img2,%20const%20vector%3CKeyPoint%3E&%20keypoints2,%20const%20vector%3CDMatch%3E&%20matches1to2,%20Mat&%20outImg,%20const%20Scalar&%20matchColor,%20const%20Scalar&%20singlePointColor,%20const%20vector%3Cchar%3E&%20matchesMask,%20int%20flags%29

matches1to2 is a vector of DMatch and it matches from the first image to the second one, which means that keypoints1[i] has a corresponding point in keypoints2[matches[i]]. Now, how does this work?

DMatch has

float distance; 
int queryIdx; // query descriptor index
int trainIdx; // train descriptor index
int imgIdx;   // train image index

So, using these parameters, how can I (manually) check if keypoints1[i] has a corresponding point in keypoints2[matches[i]] ?

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answered Sep 22 '12

Vladislav Vinogradov

4775 ●4 ●24 ●62

keypoints1[matches[i].queryIdx] has a corresponding point in keypoints2[matches[i].trainIdx]

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Thanks. Actually I was trying something similar and found that out. :)

jayrambhia (Sep 22 '12)edit

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answered Mar 21 '13

Marwen Z
81 ●1 ●3 ●10

updated Mar 21 '13

Hi, I want to get good matches from the ORB feature detection algorithm, so i did like that but i don't know haw push good matching in matches JAVA

    MatOfDMatch matches, matches12, matches21;
    matcher.match( descriptors1, descriptors2, matches12 );
    matcher.match( descriptors2, descriptors1, matches21 );
 // iterate matches12
        DMatch forward = matches12[i];  // error 
        DMatch backward = matches21[forward.trainIdx]; 
        if( backward.trainIdx == forward.queryIdx ) 
         matches.push_back( forward );  // error

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I think you should open a new question. This is the place to answers, not questions.

Alejandro Silvestri (Aug 4 '14)edit

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How does drawMatches work? (with respect to DMatch)

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