@jni : Ideally, they are learnt over a large number of keypoints in different images (Section 4.2 in the paper). Taken from the source code by the authors(https://github.com/kikohs/freak/blob/master/src/freak.cpp#L76). They updated the same in OpenCV's implementation(https://github.com/Itseez/opencv/blob/master/modules/features2d/src/freak.cpp#L57).