This is really not a problem in Octave or its packages, but rather me
trying to get help from the devs here to modify my implementation in
such a way that I get consistent results.
The challenge I am facing right now is implementing a DCT/IDCT in C
that its output matches that of Octave.
My testing input matrix (x):
1 1 1
2 2 2
3 3 3
Octave's DCT function results in (dct2(x)):
3.46410 3.46410 3.46410
-1.41421 -1.41421 -1.41421
0.00000 0.00000 0.00000
Octave's IDCT function results in (idct2(x)):
1 1 1
2 2 2
3 3 3
So far so good? Below is my implementation in C via FFTW3 (which gives
diff. results). My C code is pasted here: http://codepad.org/cHErvFMn
My DCT (via FFTW3) results are different:
72.00 0.00 0.00
-20.78 0.00 0.00
0.00 0.00 0.00
My IDCT is correct but scalled (if I divide them all by 36, it gets
OK):
36.00 36.00 36.00
72.00 72.00 72.00
108.00 108.00 108.00
Any idea how to alter my code in such a way to get results identical
to that of Octave?
Links that I found so far:
* this guy seems to have explained it (but didn't fully get it):
http://www.dsprelated.com/showmessage/93988/1.php
* FFTW3's page on DCT/IDCT
http://www.fftw.org/doc/1d-Real_002deven-DFTs-_0028DCTs_0029.html
* Matlab's page on DCT/IDCT (which should match Octave's imp.)
http://www.mathworks.com/help/toolbox/signal/ref/dct.html
* Wiki's page on
DCT/IDCThttp://en.wikipedia.org/wiki/Discrete_cosine_transform#DCT-II
To me, all of the links are presenting different equations. Which
makes me wonder what is really going on.
- mtrg
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