Looking for information on the aforementioned detectors, I noticed they are listed in the "Non-free 2D Features Algorithms" section of the OpenCV docs. See: http://docs.opencv.org/trunk/modules/xfeatures2d/doc/extra_features.html
I thought only SURF and SIFT were non-free... Could someone confirm the info? It might be only an error in the title of the section, though!