Which methods to use to make use of dense SIFT?

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there is no more cv2.FeatureDetector_create() and no more a Dense detector in current opencv3, those are all from 2.4

to do this with SIFT, you'd need opencv_contrib, create a SIFT detector like this:

f2d = cv2.xfeatures2d.SIFT_create()

and then setup your own list of grid-spaced keypoints instead of calling detect()