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## [Help-gsl] 2d FFTs, DCTs, etc.

**From**: |
Dimitris Papavasiliou |

**Subject**: |
[Help-gsl] 2d FFTs, DCTs, etc. |

**Date**: |
Thu, 11 May 2006 19:09:16 +0300 |

**User-agent**: |
Mail/News 1.5 (X11/20060228) |

Hello all,

`I needed to perform a 2D DCT for image processing and since speed wasn't
``such an absolutely critical issue (although welcome of course) I thought
``I'd implement it using GSL instead of FFTW. In the spirit of unifying
``all our numerical needs (which the GSL seems to be serving well enough)
``under a common interface I thought I'd implement this as a GSL package
``as well which might later be incorporated into GSL itself.
`

`Until now I have basically only radix2 DCTs (type II and inverse which
``is essentially type III) implemented for one and two dimensions. I also
``created a couple of wrappers for the FFT functions that work with
``vectors and matrices.
`
Now what I'm interested in is the following:

`a) Is there an interest in such a package (1D, 2D, DCTs, 2D FFTs maybe
``and vector/matrix interfaces for the lot) or should I just implement
``DCTs for myself? I'm asking because these things are easy enough to
``implement that I thought there might be a reason why they aren't
``provided by the GSL yet.
``b) Is there a reason why there are no vector/matrix interfaces to the
``fft functions (in constrast to the wavelets routines)?
``c) Is there a reason why there are no two-dimensional fft routines? For
``real data there would be the problem of computing them in-place I
``suppose but for complex data computation is pretty simple using
``row-column decomposition.
`

`In case there is an interest in such a package, does anyone have any
``ideas on the following:
``a) What to do with 2D real FFTs? In-place computation (without
``allocating extra memory) should be impossible ,at least using 1D FFTs to
``do the job. Should only complex 2D FFTs be implemented? Should real
``FFTs have different input and output vectors (that would break the
``symmetry in the interface though)?
``b) Does anybody know whether it is possible to compute DCTs of odd
``length using one FFT of the same length? For even lengths this is
``possible (both powers of two and not) but for odd lengths I haven't
``found anything.
``c) Would anyone be willing to help out (esp. with the vector/matrix
``wrappers and with making these work for doubles and floats through those
``GSL templates)? I'm supposed to be doing my master's thesis as well
``(what I needed the DCT for in the first place) so it might take some
``time to finish this on my own.
`
Sorry for all the questions,
Dimtiris P.

**[Help-gsl] 2d FFTs, DCTs, etc.**,
*Dimitris Papavasiliou* **<=**