# csdms-contrib/slepian_alpha

Switch branches/tags
Nothing to show
Fetching contributors…
Cannot retrieve contributors at this time
76 lines (69 sloc) 2.45 KB
 function varargout=fftaxis(fsize,ftsize,spun) % [xfaks,yfaks,fnx,fny,xsint,ysint]=FFTAXIS(fsize,ftsize,spun) % % Makes linear frequency axis going through zero [at floor((dim+1)/2)] % -f_N < f <= f_N with f_N=1/(2Dt) % % INPUT: % % fsize is the SIZE of the field (Y X) % ftsize is the SIZE of its transform (kY kX) % spun is the physical dimension of the field (lY lX) % % OUTPUT: % % fnx,fny is the Nyquist rate in both dimensions % % SEE ALSO: % % KNUM2, FFTAXIS1D, which are actually more like Matlab does it. % So this function should start becoming OBSOLETE. % % For use after FFTSHIFT, for non-geographical data, where the "-x,-y" % or "first" is in the upper left, as opposed to geographical data, where % the "-x,+y" or "first" is in the upper left. Thus, for geographical % data, this needs to be FLIPUD. For 1-D and 2-D but see also FFTAXIS1D % and KNUM2. The advantage is that the frequencies are symmetric around % the zero-center, as they should. The magnitude of the Fourier transform % of a discrete-time signal is always an even function. Analogously, for % 2D, the center is a symmetry point. The lowest frequency resolvable % that is not the DC-component is the Rayleigh frequency, given by % 1/T=1/NDt with T the data length. % % Last modified by fjsimons-at-alum.mit.edu, 04/16/2010 % See Percival and Walden, 1993 % Figure sampling interval in both directions if nargout>3 xsint=spun(2)/(fsize(2)-1); ysint=spun(1)/(fsize(1)-1); % Calculate centered frequency axis (PW p 112) M=ftsize(1); N=ftsize(2); intvectX=linspace(-floor((N+1)/2)+1,N-floor((N+1)/2),N); intvectY=linspace(-floor((M+1)/2)+1,M-floor((M+1)/2),M); xfaks=intvectX/N/xsint; yfaks=intvectY/M/ysint; % Calculate Nyquist frequencies fnx=1/2/xsint; fny=1/2/ysint; varns={xfaks,yfaks,fnx,fny,xsint,ysint}; else xsint=spun/(max(fsize)-1); % Calculate centered frequency axis (PW p 112) N=max(ftsize); intvectX=linspace(-floor((N+1)/2)+1,N-floor((N+1)/2),N); % In other words the above is equal to % -floor((N+1)/2)+1+[0:N-1] xfaks=intvectX/N/xsint; % Calculate Nyquist frequencies fnx=1/2/xsint; varns={xfaks,fnx,xsint}; end % So if we put in % [a,b,c,d,e,f]=fftaxis([101 51],[100 111],[100 50]) % we see how the frequency goes from -1/2 to 1/2; % alternatively the angular frequency goes from -pi to pi; % we can also plot the frequency normalized by the Nyquist: % in that case, this scale should go from -1 to 1. % Prepare output varargout=varns(1:nargout);