CODEWITHSUNDEEP
c++opencv
wireless_lab
wireless_lab

Reputation: 187

displacement between two images using opencv surf

I am working on image processing with OPENCV. I want to find the x,y and the rotational displacement between two images in OPENCV. I have found the features of the images using SURF and the features have been matched. Now i want to find the displacement between the images. How do I do that? Can RANSAC be useful here?

regards, shiksha

Upvotes: 0

Views: 1807

Answers (1)

Vlad
Vlad

Reputation: 4525

Rotation and two translations are three unknowns so your min number of matches is two (since each match delivers two equations or constraints). Indeed imagine a line segment between two points in one image and the corresponding (matched) line segment in another image. The difference between segments' orientations gives you a rotation angle. After you rotated just use any of the matched points to find translation. Thus this is 3DOF problem that requires two points. It is called Euclidean transformation or rigid body transformation or orthogonal Procrustes.

Using Homography (that is 8DOF problem ) that has no close form solution and relies on non-linear optimization is a bad idea. It is slow (in RANSAC case) and inaccurate since it adds 5 extra DOF. RANSAC is only needed if you have outliers. In the case of pure noise and overdetrmined system (more than 2 points) your optimal solution that minimizes the sum of squares of geometric distance between matched points is given in a close form by:

Problem statement: min([R*P+t-Q]2), R-rotation, t-translation
Solution: R = VUT, t = R*Pmean-Qmean
where X=P-Pmean; Y=Q-Qmean and we take SVD to get X*YT=ULVT; all matrices have data points as columns. For a gentle intro into rigid transformations see this

Upvotes: 1

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