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bwfilterf
Butterworth filter for vector/scalar fields

Description ```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. Example v = loadvec('*.vc7'); showf(bwfilterf(v,3,8)); See Also filterf, interpf, addnoisef, truncf, extractf. Published output in the Help browser showdemo bwfilterf ```
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