2-D FIR filter using frequency transformation
h = ftrans2(b, t)
h = ftrans2(b)
h = ftrans2(b, t) produces the two-dimensional FIR filter h that corresponds to the one-dimensional FIR filter b using the transform t. (ftrans2 returns h as a computational molecule, which is the appropriate form to use with filter2.) b must be a one-dimensional, Type I (even symmetric, odd-length) filter such as can be returned by fir1, fir2, or remez in the Signal Processing Toolbox software. The transform matrix t contains coefficients that define the frequency transformation to use. If t is m-by-n and b has length Q, then h is size ((m-1)*(Q-1)/2+1)-by-((n-1)*(Q-1)/2+1).
t = [1 2 1; 2 -4 2; 1 2 1]/8;
All inputs and outputs should be of class double.
Use ftrans2 to design an approximately circularly symmetric two-dimensional bandpass filter with passband between 0.1 and 0.6 (normalized frequency, where 1.0 corresponds to half the sampling frequency, or π radians):
Since ftrans2 transforms a one-dimensional FIR filter to create a two-dimensional filter, first design a one-dimensional FIR bandpass filter using the Signal Processing Toolbox function remez.
colormap(jet(64)) b = remez(10,[0 0.05 0.15 0.55 0.65 1],[0 0 1 1 0 0]); [H,w] = freqz(b,1,128,'whole'); plot(w/pi-1,fftshift(abs(H)))
Use ftrans2 with the default McClellan transformation to create the desired approximately circularly symmetric filter.
h = ftrans2(b); freqz2(h)
The transformation below defines the frequency response of the two-dimensional filter returned by ftrans2.
where B(ω) is the Fourier transform of the one-dimensional filter b:
and T(ω1,ω2) is the Fourier transform of the transformation matrix t:
The returned filter h is the inverse Fourier transform of H(ω1,ω2):
 Lim, Jae S., Two-Dimensional Signal and Image Processing, Englewood Cliffs, NJ, Prentice Hall, 1990, pp. 218-237.