Note: A new GRASS GIS stable version has been released: GRASS GIS 7, available here. And see the new manual page here

## NAME

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

imagery, FFT
## SYNOPSIS

**i.ifft**

**i.ifft help**

**i.ifft** **real_image**=*name* **imaginary_image**=*name* **output_image**=*name* [--**overwrite**] [--**verbose**] [--**quiet**]
### Flags:

**--overwrite**
- Allow output files to overwrite existing files
**--verbose**
- Verbose module output
**--quiet**
- Quiet module output

### Parameters:

**real_image**=*name*
- Name of input raster map (image fft, real part)
**imaginary_image**=*name*
- Name of input raster map (image fft, imaginary part
**output_image**=*name*
- 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 GRASS program *r.digit* 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*.
The real and imaginary components are read from arrays of
doubles in the *cell_misc* directory (produced by
the forward transform program,
*i.fft*),
and the reconstructed image will preserve the cell value
scaling of the original image processed by
*i.fft*. No color
table is assigned to the output map; one should be created
before viewing the *output_image*.

## SEE ALSO

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.
**Remote Sensing Digital Image Analysis**,
by John A. Richards,
Springer-Verlag, 1986.

Personal communication,
between program author and Ali R. Vali, Space Research Center,
University of Texas, Austin, 1990.

*i.cca*,
*i.class*,
*i.fft*,
*i.pca*,
*r.circle*

## AUTHOR

David Satnik, GIS Laboratory,
Central Washington University

Glynn Clements (FFTW support)
*Last changed: $Date: 2011-11-08 01:42:51 -0800 (Tue, 08 Nov 2011) $*

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