i.ifft - GRASS manual
Author: GRASS Development Team
Description: i.ifft: Inverse Fast Fourier Transform (IFFT) for image processing.
Keywords: imagery, transformation, Fast Fourier Transform

## NAME

i.ifft - Inverse Fast Fourier Transform (IFFT) for image processing.

## KEYWORDS

imagery, transformation, Fast Fourier Transform

## SYNOPSIS

i.ifft
i.ifft --help
i.ifft real=name imaginary=name output=name [--overwrite] [--help] [--verbose] [--quiet] [--ui]

### Flags:

--overwrite
  Allow output files to overwrite existing files
--help
  Print usage summary
--verbose
  Verbose module output
--quiet
  Quiet module output
--ui
  Force launching GUI dialog

### Parameters:

real=name [required]
  Name of input raster map (image fft, real part)
imaginary=name [required]
  Name of input raster map (image fft, imaginary part
output=name [required]
  Name for output raster map

## DESCRIPTION

i.ifft is an image processing program based on the algorithm given by Frigo et al. (1998), that converts real and imaginary frequency space images (produced by i.fft) into a normal image.

## NOTES

The current mask is respected when reading the real and imaginary component files; thus, creating a mask is a primary step for selecting the portion of the frequency space data to be included in the inverse transform. The module wxGUI vector digitizer can be used to create masks while viewing the real or imaginary component image. Alternatively r.circle can be used to generate high-, low- and donut filters specifying the DC point as circle/ring center. When i.ifft is executed, it (automatically) uses the same GRASS region definition setting that was used during the original transformation done with i.fft.

## REFERENCES

- M. Frigo and S. G. Johnson (1998): "FFTW: An Adaptive Software Architecture for the FFT". See www.fftw.org: FFTW is a C subroutine library for computing the Discrete Fourier Transform (DFT) in one or more dimensions, of both real and complex data, and of arbitrary input size.
- Richards, J.A (1986): Remote Sensing Digital Image Analysis, Springer-Verlag, 1986.
- Personal communication, between program author and Ali R. Vali, Space Research Center, University of Texas, Austin, 1990.

## SEE ALSO

i.cca, g.gui.iclass, i.fft, i.pca, r.circle, wxGUI vector digitizer

## AUTHORS

David Satnik, GIS Laboratory, Central Washington University
Glynn Clements (FFTW support)

## SOURCE CODE

Available at: i.ifft source code (history)

Latest change: Friday Jan 02 21:25:59 2026 in commit: 83362df5af36c1e7d95fad93dff0a57b4dfde476

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© 2003-2026 GRASS Development Team, GRASS 8.6.0dev Reference Manual