@ahojnnes : According to the paper, the 49 x 49 patch needs to be sampled using 2d normal distribution with mean at the keypoint location and variance as 49x49/25. That is, I have to implement discrete sampling that too in a finite range(49 x 49). The current normal sampling functions in numpy.random(http://docs.scipy.org/doc/numpy/reference/routines.random.html) produce non-integer samples and that too from complete Real No. range. Any solutions?