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1 ## Copyright (C) 2001 Paul Kienzle |
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2 ## |
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3 ## This program is free software; you can redistribute it and/or modify |
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4 ## it under the terms of the GNU General Public License as published by |
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5 ## the Free Software Foundation; either version 2 of the License, or |
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6 ## (at your option) any later version. |
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7 ## |
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8 ## This program is distributed in the hope that it will be useful, |
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9 ## but WITHOUT ANY WARRANTY; without even the implied warranty of |
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10 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
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11 ## GNU General Public License for more details. |
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12 ## |
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13 ## You should have received a copy of the GNU General Public License |
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14 ## along with this program; if not, write to the Free Software |
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15 ## Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA |
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16 |
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17 ## y = interpft (x, n) |
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18 ## Resample x with n points using Fourier interpolation. |
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19 ## The data x must be equally spaced and n >= length(x). |
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20 |
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21 ## Author: Paul Kienzle |
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22 ## 2001-02-11 |
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23 ## * initial version |
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24 |
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25 function z = interpft (x, n) |
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26 if (nargin < 2 || nargin > 3) |
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27 usage ("y=interpft (x, n)"); |
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28 endif |
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29 |
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30 transpose = ( rows (x) == 1 ); |
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31 if (transpose) x = x (:); endif |
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32 [nr, nc] = size (x); |
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33 if n > nr |
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34 y = fft (x) / nr; |
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35 k = floor (nr / 2); |
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36 z = n * ifft ( [ y(1:k,:); zeros(n-nr,nc); y(k+1:nr) ] ); |
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37 if isreal (x), z = real (z); endif |
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38 elseif n < nr |
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39 error ("interpft: n must be bigger than x"); |
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40 ## XXX FIXME XXX the following doesn't work so well |
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41 y = fft (x) / nr; |
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42 k = ceil (n / 2); |
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43 z = n * ifft ( [ y(1:k,:); y(nr-k+2:nr) ] ); |
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44 if isreal (x), z = real (z); endif |
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45 else |
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46 z = x; |
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47 endif |
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48 if transpose, z = z.'; endif |
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49 endfunction |
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50 |
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51 %!demo |
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52 %! t = 0 : 0.3 : pi; dt = t(2)-t(1); |
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53 %! n = length (t); k = 100; |
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54 %! ti = t(1) + [0 : k-1]*dt*n/k; |
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55 %! y = sin (4*t + 0.3) .* cos (3*t - 0.1); |
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56 %! yp = sin (4*ti + 0.3) .* cos (3*ti - 0.1); |
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57 %! plot (ti, yp, 'g;sin(4t+0.3)cos(3t-0.1);', ... |
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58 %! ti, interp1 (t, y, ti, 'spline'), 'b;spline;', ... |
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59 %! ti, interpft (y, k), 'c;interpft;', ... |
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60 %! t, y, 'r+;data;'); |
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61 |
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62 %!#demo |
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63 %! t = 0:0.3:pi; dt = t(2)-t(1); |
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64 %! n = length(t); k = 100; |
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65 %! ti = t(1)+[0:k-1]*dt*n/k; |
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66 %! y = sin (4*t+0.3).*cos(3*t-0.1); |
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67 %! yp = sin (4*ti+0.3).*cos(3*ti-0.1); |
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68 %! plot (ti, yp, 'g;sin(4t+0.3)cos(3t-0.1);', ... |
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69 %! t, interp1(ti,yp,t,'spline'), 'bx;spline;', ... |
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70 %! t, interpft(yp,n), 'co;interpft;', ... |
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71 %! t, y, 'r+;data;'); |
