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## N-dimensional Fourier interpolation

version 1.5.0.0 (2.35 KB) by
Performs N-D FFT interpolation with upsampling, downsampling, or mixed up- and downsampling

Updated 15 Dec 2010

Performs N-D FFT interpolation on any data for which fftn works. Will upsample by zero-filling, downsample by truncating high frequencies, or combine both up- and downsampling by dimension to allow arbitrary reshaping.

### Cite As

Matthias Schabel (2020). N-dimensional Fourier interpolation (https://www.mathworks.com/matlabcentral/fileexchange/22665-n-dimensional-fourier-interpolation), MATLAB Central File Exchange. Retrieved .

Harald Penasso

Sandra Ortega

Michael Hawks

Kevin - I think the following one-line function (added to the bottom of the code) should do the trick.

function i = odd(x)
i = (floor(x/2)~=(x/2));

Kevin Sharp

This fails with ...

??? Undefined function or method 'odd' for input arguments of type 'double'.

Error in ==> fftInterpolate at 61
centeru = floor(newsz/2)+1+odd(newsz);

Matthias

Dear author

rng = [ceil(newsz/2)-ceil(sz/2)+1;ceil(newsz/2)-ceil(sz/2)+sz]';

and replace it with
rng = [ceil(newsz/2)-floor(sz/2)+1;ceil(newsz/2)-floor(sz/2)+sz]';

then the code also works with odd dimesions.

Matthias Wurm

Matthias

code works excellent for data with even dimensions. In case of odd dimensions the code fails.

Benjamin

Matthias, I cannot get this to work. The example image posted seems to interpolate the peaks dataset by a factor of two. But when I run the following code, I don't get the same result:
>>> figure; subplot(1,2,1); imagesc(peaks);
>>> subplot(1,2,2); imagesc(fftInterpolate(peaks,2));
what's going on?

Matthias Schabel

This code is needed as well (append on to end of fftInterpolate.m file):

function subData = subRange(data,rng)

if (isempty(rng))
subData = data;
return;
end;

sz = size(data);
dim = length(sz);

lo = rng(:,1)';
hi = rng(:,2)';

if (length(lo) ~= dim || length(hi) ~= dim)
error('subRange :: dimension mismatch');
end;

% replace zeros with lower/upper limit
for i=1:dim
if (lo(i) == 0) lo(i) = 1; end;
if (hi(i) == 0) hi(i) = sz(i); end;
end;

if (any(lo<1) | any(hi > sz))
error('subRange :: sub-range out of bounds');
end;

switch (dim)
case 1, subData = data(lo(1):hi(1));
case 2, subData = data(lo(1):hi(1),...
lo(2):hi(2));
case 3, subData = data(lo(1):hi(1),...
lo(2):hi(2),...
lo(3):hi(3));
case 4, subData = data(lo(1):hi(1),...
lo(2):hi(2),...
lo(3):hi(3),...
lo(4):hi(4));
case 5, subData = data(lo(1):hi(1),...
lo(2):hi(2),...[]
lo(3):hi(3),...
lo(4):hi(4),...
lo(5):hi(5));
otherwise,
% generate string and use eval
str = 'subData = data(';
for i=1:dim
str = [str 'lo(' num2str(i) '):hi(' num2str(i) ')'];
if (i~=dim)
str = [str ','];
else
str = [str ');'];
end;
end;
eval(str);
end;

return;