def centroids(self, window_size=5):
        self.sortByMass()
        mzs = _cffi_buffer(self.masses, 'd')
        intensities = _cffi_buffer(self.intensities, 'f')
        n = self.size
        p = ims.spectrum_new_from_raw(n, mzs.ptr, intensities.ptr, int(window_size))
        return _new_spectrum(CentroidedSpectrum, p)