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fft2db

Performs a two-dimensional discrete backward Fourier transform (Fourier synthesis).

Prototype

	function fft2db (
		coef [2][*][*] : numeric   
	)

Arguments

coef

A three dimensional array containing the real and imaginary parts. Generally, this would be the array created by fft2df.

            input: coef(2,:,:)
where
        real part: coef(0,:,:)
   imaginary part: coef(1,:,:)

Description

This function computes the discrete backward Fourier transform of a real periodic array. This transform is also known as Fourier synthesis, transforming from spectral to physical space.

A call to fft2df followed by a call to fft2db (or vice-versa) reproduces the original array within roundoff error.

Missing values are not allowed. For efficiency reasons, no checking for missing values [i.e., _FillValue] is performed. Hence, if missing values are present, the results will be erroneous.

An old but but very readable reference is:

        Peter Bloomfield
        Fourier analysis of time series : An introduction
        New York : John Wiley and Sons  , 1976

NCL uses FFTPACK5 developed by Paul Swarztrauber and Richard Valent. The specific Fortran subroutines used are:

       fft2df:
       http://www2.cisl.ucar.edu/resources/legacy/fft5/documentation#rfft2f.html

       fft2db:
       http://www2.cisl.ucar.edu/resources/legacy/fft5/documentation#rfft2b.html

Please note that Fortran is column major while NCL (like C/C++) is row-major.

See Also

fft2df, ezfftf, ezfftb, taper, dtrend, specx_anal, specxy_anal

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