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Butterworth filter for vector/scalar fields

FF = bwfilterf(F,FSIZE,ORDER) applies a spatial Butterworth filter to 
the vector/scalar fields F,  with a cutoff size FSIZE (in grid units) 
and order ORDER. A Butterworth filter is a low-pass (for ORDER>0) or 
a high-pass (for ORDER<0) filter in the Fourier space. 
bwfilterf first Fourier-transforms the field(s), applies a low/high- 
pass transfer function T(k) defined as 
    T(k) = 1/(1+(k/kc)^(ORDER/2)), 
and inverse Fourier transforms back in the physical space. Here 
k = (kx^2+ky^2)^(1/2) is the wave number, and kc is the cutoff 
wave number of the filter, such that kc = L/FSIZE, with L the size the 
fields. If ORDER>0, the filter is low-pass (i.e., it filters out the 
small scales). If ORDER<0, the filter is high-pass (i.e., it filters 
out the large scales). The X and Y dimensions of the fields must be 
even. If one of the dimension is odd, the last column/row is discarded. 
Typical values for FSIZE are around 1-10, and typical values for ORDER 
are in the range 2-10 (positive for a low-pass filter, negative for a 
high-pass filter). Large values of ORDER correspond to a sharper 
filter. Values too large (say, |ORDER| > 10) may produce oscillations 
in the physical space. 
If FSIZE and ORDER are scalar, the same cutoff and order is applied 
to each field F.  IF FSIZE and/or ORDER are arrays, a different cutoff 
and/or order is applied to each field (their dimensions must match the 
dimension of the field F). 
FF = bwfilterf(F,FSIZE,ORDER,'opt',...) specifies the options: 
  'low', 'high':   applies a lowpass (by default) or highpass filter. 
                   option 'high' simply changes the sign of ORDER. 
  'trunc':         truncates the borders of width FSIZE, which are 
                   affected by the filtering. 
If no output argument, the result is displayed by showf. 
Note: If there are missing data in the field, it is better to first 
interpolate the data, using interpf. 
   v = loadvec('*.vc7'); 
See Also
filterf, interpf, addnoisef, truncf, extractf. 
Published output in the Help browser 
   showdemo bwfilterf 

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