Bandpass filter bank
Im1 = input image (single channel)
dF = stepsize in radial frequency space
NF = number of filters applied
NF=1 means no filter
NF=2 means low pass and high pass about dF
NF>2 bandpass filters in steps of dF
Im2 = spectrally decomposed output images from Im1
The optional parameter 'beta' sets the sharpness of the frequency cutoff
between the bands, higher numbers are sharper cutoffs -- smoother cutoffs
reduce ringing, but have mixed fequency regions with other bands. The
default value is beta=0.5.
The decomposed images have the property that 'sum(Im2,3)=Im1' (to a good approximation).