Andriy Myronenko

To candelabro
1) As for the zero-padding, modifying fft(x,n) is incorrect.
Just pad the input 3D vector x before taking the dct.
For instance, use :

newsize=[M N K]; % Define padded size of the 3D array;
tmp=zeros([M N K]); % padded data buffer
cutsize=min([size(x); newsize]);
tmp(1:cutsize(1),1:cutsize(2),1:cutsize(3))=x(1:cutsize(1),1:cutsize(2),1:cutsize(3));
x=tmp;
y=mirt_dctn(x); take 3D DCT (padded or reduced to the M N K size)

2) The positive and negative frequencies is only appropriate for fft. If you really want to have the analogy with fft, then remember that DCT gives an output for positive frequencies. And negative frequencies can be obtained by mirror flip.

Thanks! Works great for large 3D data sets.

Great job! (I couldn't get to work CUDA versions.) Thank you!

IS THIS Discrete Cosine Transform TECHNIQUE GOOD FOR HIDING TEXT IN TEXT