How to Determine perspective transform between two images

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I have transformed an image by using affine transform by using the function getAffineTransform() by manually passing 3 points for each image. (i.e. I know the affine transform)

Now, I would like to obtain a homogenous matrix of the transform. How is it possible ? The reason, I am doing it is that I am trying to determine the false positives by extracting the features using SURF and matching using FLANN based Matcher . So, if I can get the homogenous transform between the two images, I can just compare the output of warpperspective on original keypoints and the matched keypoints.