Determining Scale of query image against larger target image

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So let me summarize. You read in your two images into 2 cv::Mats, generate key points and descriptors (SIFT SURF) for your two images, but once I have my key points, how would I compare those? (I also found this link which might address the matching http://stackoverflow.com/questions/13...).

Look at this tutorial for keypoints matching: AKAZE local features matching.

The affine transformation matrix estimation with a RANSAC method can also discard some outliers (false matching). Basically, the RANSAC is:

• pick randomly the minimal number of points allowing to compute the affine transformation (3 points for affine transformation)
• check if the majority of the matches agree, fit with the model previously estimated
• if no, pick another minimal set of points
• if yes, the inliers are the matches whose the model error is below a threshold, refines the model with only the inliers

It works only if there is a majority of inliers.

PS: for SIFT or SURF keypoints, the appropriate distance is the L2 distance.

Also, affine transformation describes 6 degrees of freedom: translation + rotation + scale + aspect ratio + shear.