pro conv_ft
;Example of comparison between convolution and FFT
A=findgen(5,5); create array A
;A=replicate(1.0,5,5)
h=replicate(1.0/9,3,3) ;create mask
Ap=zeropad(A,9,9,/center);zero pad to size NA+2*Nh-2,MA+2*Mh-2

;first we'll convolve them
g_conv=Convol(Ap,h)
g_conv=g_conv(1:7,1:7)
print,g_conv

;then using FFT
Ap=zeropad(A,7,7,/center)
Af=FFT(Ap) ;forward FFT

hp=zeropad(h,7,7,/center)
hf=FFT(hp)
Gf=Af*hf
g_inv=FFT(Gf,/inverse); calculate the inverse FFT
g_real=shift(float(g_inv),4,4)*49

print
print,g_real
end