**PURPOSE**- Computes forward Fourier transform of a 2D or 3D image, or inverse Fourier transform of a complex Fourier-formatted file. Example.

**SEE ALSO****FF**[Filter - Fourier] **FP**[Interpolate/Resize using Fourier transform ||] **FQ**[Filter - Quick Fourier, High, low pass, etc., Padded ||] **PW**[Power spectrum - Amplitudes of Fourier transform ||]

**USAGE**- .OPERATION: FT

- .INPUT FILE: PIC001

[Enter file name of data to be Fourier-transformed or file name of complex data to be inversely Fourier-transformed.].OUTPUT FILE: IMG023

[Enter file name which will contain the transformed image/volume.]

**NOTES**

- The Fourier transform is stored in a straightforward format.
Coefficient (0,0) is placed in location (1,1).
The LI operation
may be used to list values of a Fourier format file.
- Multiple threads are used (if supported by the current
SET MP setting).
- All distributed versions of SPIDER are now linked with the
FFTW3 library.
This operation (and other SPIDER Fourier operations)
works for all possible image dimensions.
Fourier operations on images with dimensions that are
large prime numbers or products of large prime numbers
may be slow. (In the unlikely case that a volume may not
fit into memory a legacy limited dimension Fourier routine
may be invoked.)

- Implemented by P. Penczek.

**SUBROUTINES**- FMRS, FMRS_2, FMRS_3, FFTW3, FMRS_2DR, FMRS_3DR, FMRS_1, FFTMCF

**CALLER**- FOUR1 , FOUR1C

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