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function [a,M,L,N,Ngood]=gabimagepars(Ls,x,y)
%-*- texinfo -*-
%@deftypefn {Function} gabimagepars
%@verbatim
%GABIMAGEPARS Find Gabor parameters to generate image
% Usage: [a,M,L,N,Ngood]=gabimagepars(Ls,x,y);
%
% [a,M,L,N,Ngood]=GABIMAGEPARS(Ls,x,y) will compute a reasonable set of
% parameters a, M and L to produce a nice Gabor 'image' of a signal
% of length Ls. The approximate number of pixels in the time direction is
% given as x and the number of pixels in the frequency direction is given
% as y.
%
% The output parameter Ngood contains the number of time steps (columns
% in the coefficients matrix) that contains relevant information. The
% columns from Ngood until N only contains information from a
% zero-extension of the signal.
%
% If you use this function to calculate a grid size for analysis of a
% real-valued signal (using DGTREAL), please input twice of the desired
% size y. This is because DGTREAL only returns half as many
% coefficients in the frequency direction as DGT.
%
% An example: We wish to compute a Gabor image of a real valued signal f*
% of length 7500. The image should have an approximate resolution of
% 600 x800 pixels:
%
% [f,fs]=linus; f=f(4001:4000+7500);
% [a,M,L,N,Ngood] = gabimagepars(7500,800,2*600);
% c = dgtreal(f,'gauss',a,M);
% plotdgtreal(c,a,M,fs,90);
%
% The size of c is (M/2)+1 xN equal to 601 x700 pixels.
%
% For this function to work properly, the specified numbers for x and
% y must not be large prime numbers.
%
%@end verbatim
%@strong{Url}: @url{http://ltfat.github.io/doc/gabor/gabimagepars.html}
%@seealso{dgt, dgtreal, sgram}
%@end deftypefn
% Copyright (C) 2005-2016 Peter L. Soendergaard <peter@sonderport.dk>.
% This file is part of LTFAT version 2.3.1
%
% This program is free software: you can redistribute it and/or modify
% it under the terms of the GNU General Public License as published by
% the Free Software Foundation, either version 3 of the License, or
% (at your option) any later version.
%
% This program is distributed in the hope that it will be useful,
% but WITHOUT ANY WARRANTY; without even the implied warranty of
% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
% GNU General Public License for more details.
%
% You should have received a copy of the GNU General Public License
% along with this program. If not, see <http://www.gnu.org/licenses/>.
if min(x,y)>Ls
% Small values case, just do an STFT
M=Ls;
N=Ls;
a=1;
Ngood=N;
L=Ls;
else
% Set M and N to be what the user specified
M=y;
N=x;
% Determine the minimum transform size.
K=lcm(M,N);
% This L is good, but is it not the same as DGT will choose.
Llong=ceil(Ls/K)*K;
% Fix a from the long L
a=Llong/N;
% Now we have fixed a and M, so we can use the standard method of choosing L
Lsmallest=lcm(a,M);
L=ceil(Ls/Lsmallest)*Lsmallest;
% We did not get N as desired.
N=L/a;
% Number of columns to display
Ngood=ceil(Ls/a);
if M<=a
error('LTFAT:noframe',['Cannot generate a frame, the signal is too long as compared ' ...
'to the size of the image. Increase x and y.']);
end;
end;
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