Two of the methods use convolution of the projection data with the inverse Fourier transform of |w|. The other two methods perform an Fourier transform of the projection data and multiply that by the |w| filter and then perform an inverse fourier transform.
Though multiplying by |w| gives the sharpest reconstructions, in practice, superior results are obtained by reducing the higher frequencies. This is performed by mutiplying the |w| filter by another filter that attenuates the higher frequencies. CTSim has multiple filters for this purpose.