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Operators and Keywords

Function List:

C++ API

: method = fftw ("planner")
: fftw ("planner", method)
: wisdom = fftw ("dwisdom")
: fftw ("dwisdom", wisdom)
: fftw ("threads", nthreads)
: nthreads = fftw ("threads")

Manage FFTW wisdom data.

Wisdom data can be used to significantly accelerate the calculation of the FFTs, but implies an initial cost in its calculation. When the FFTW libraries are initialized, they read a system wide wisdom file (typically in /etc/fftw/wisdom), allowing wisdom to be shared between applications other than Octave. Alternatively, the fftw function can be used to import wisdom. For example, 

wisdom = fftw ("dwisdom")

will save the existing wisdom used by Octave to the string wisdom. This string can then be saved to a file and restored using the save and load commands respectively. This existing wisdom can be re-imported as follows 

fftw ("dwisdom", wisdom)

If wisdom is an empty string, then the wisdom used is cleared.

During the calculation of Fourier transforms further wisdom is generated. The fashion in which this wisdom is generated is also controlled by the fftw function. There are five different manners in which the wisdom can be treated:

"estimate"

Specifies that no run-time measurement of the optimal means of calculating a particular is performed, and a simple heuristic is used to pick a (probably sub-optimal) plan. The advantage of this method is that there is little or no overhead in the generation of the plan, which is appropriate for a Fourier transform that will be calculated once.

"measure"

In this case a range of algorithms to perform the transform is considered and the best is selected based on their execution time.

"patient"

Similar to "measure", but a wider range of algorithms is considered.

"exhaustive"

Like "measure", but all possible algorithms that may be used to treat the transform are considered.

"hybrid"

As run-time measurement of the algorithm can be expensive, this is a compromise where "measure" is used for transforms up to the size of 8192 and beyond that the "estimate" method is used.

The default method is "estimate". The current method can be queried with 

method = fftw ("planner")

or set by using 

fftw ("planner", method)

Note that calculated wisdom will be lost when restarting Octave. However, the wisdom data can be reloaded if it is saved to a file as described above. Saved wisdom files should not be used on different platforms since they will not be efficient and the point of calculating the wisdom is lost.

The number of threads used for computing the plans and executing the transforms can be set with 

fftw ("threads", NTHREADS)

Note that octave must be compiled with multi-threaded FFTW support for this feature. The number of processors available to the current process is used per default.

See also: fft, ifft, fft2, ifft2, fftn, ifftn.

Package: octave