A method is disclosed for designing two separable filters, LPP & HPP, that
approximates the circularly symmetric frequency response achievable using
a non-separable filter. The method selects a cut-off frequency and
designs a 1-D low pass filter. Next, the method obtains a low pass 2-D
filter and generates a 2-D contour plot therefor. The method designs a
1-D high pass filter and obtains a 2-D high pass filter and a 2-D contour
plot therefor. A 2-D filter is generated having the dimensions of the 2-D
high pass filter with the only non-zero entry of value 1 located at the
center. A matrix is created by subtracting the 2-D high pass filter from
the 2-D filter. The low pass 2-D filter is convolved with the matrix to
obtain an approximation to a desired circular symmetry.