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I got the feature points from two consecutive by using different detectors in features2d framework:

In the first frame, the feature points are plotted in red enter image description here

In the next frame, the feature points are plotted in blue enter image description here

I want to draw a line between these red and blue (matched) points inside the first frame (the image with red dots). drawmatches function in opencv doesn't help as it shows a window with two consecutive frames next to each other for matching. Is it possible in OpenCV?

Thanks in advance

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sy456
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2 Answers2

2

I guess that you want to visualize how each keypoint moves between two frames. According to my knowledge, there is no built-in function in OpenCV meeting your requirement.

However, as you have called the drawMatches() function, you already have the two keypoint sets (take C++ code as example) vector<KeyPoint>& keypoints1, keypoints2 and the matches vector<DMatch>& matches1to2. Then you can get the pixel coordinate of each keypoint from Point keypoints1.pt and draw lines between keypoints by calling the line() function.

You should be careful that since you want to draw keypoints2 in the first frame, the pixel coordinate may exceed the size of img1.

There is a quick way to get a sense of the keypoints' motion. Below is the result shown by imshowpair() in Matlab: enter image description here

rookiepig
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  • that is exactly what i need! thanks a lot. When I decrease the number of features, the motions will be more visible. – sy456 Feb 25 '14 at 08:58
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After I found good matches, I draw the lines by using this code.

(...some code...)

//draw good matches
for( int i = 0; i < (int)good_matches.size(); i++ )
{ 

   printf( "-- Good Match [%d] Keypoint 1: %d  -- Keypoint 2: %d  \n", i, good_matches[i].queryIdx,  good_matches[i].trainIdx ); 

   //query image is the first frame
   Point2f point_old = keypoints_1[good_matches[i].queryIdx].pt;

   //train  image is the next frame that we want to find matched keypoints
   Point2f point_new = keypoints_2[good_matches[i].trainIdx].pt;

   //keypoint color for frame 1: RED
   circle(img_1, point_old, 3, Scalar(0, 0, 255), 1);  
   circle(img_2, point_old, 3, Scalar(0, 0, 255), 1);  

   //keypoint color for frame 2: BLUE
   circle(img_1, point_new, 3, Scalar(255, 0, 0), 1);  
   circle(img_2, point_new, 3, Scalar(255, 0, 0), 1); 

   //draw a line between keypoints
   line(img_1, point_old, point_new, Scalar(0, 255, 0), 2, 8, 0);
   line(img_2, point_old, point_new, Scalar(0, 255, 0), 2, 8, 0); 
}
  imwrite("directory/image1.jpg",img_1);
  imwrite("directory/image2.jpg",img_2);

(...some code...)

I saved the results to the first (img_1) and next frame (img_2). As you see, I have different results, but the line shape is same. In video homography sample of OpenCV, keypoint tracking seems accurate. They follow this approach:detect keypoints-->compute keypoints-->warp keypoints--> match--> find homography-->draw matches. However, I apply detect keypoints-->compute keypoints-->match-->draw matches. I am confused if I have to consider homography and perspective (or other things) to see the keypoint movements accurately.

my results for first frame (img_1) enter image description here

and next frame (img_2) enter image description here

sy456
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