---
name: "i.fft"
section: "1grass"
platform: "linux"
package: "grass-doc"
version: "8.4.1-1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/i.fft"
---

# i.fft(1grass)

Fast Fourier Transform (FFT) for image processing.

## NAME

**i.fft**  - Fast Fourier Transform (FFT) for image processing.

## KEYWORDS

imagery, transformation, Fast Fourier Transform

## SYNOPSIS

**i.fft**\
 **i.fft --help**\
 **i.fft** **input**=*name* **real**=*name* **imaginary**=*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:

- **input**=*name* **\[required\]**

  Name of input raster map

- **real**=*name* **\[required\]**

  Name for output real part arrays stored as raster map

- **imaginary**=*name* **\[required\]**

  Name for output imaginary part arrays stored as raster map

## DESCRIPTION

*i.fft* is an image processing program based on the FFT algorithm given by Frigo et al. (1998), that processes a single input raster map layer (**input**) and constructs the real and imaginary Fourier components in frequency space.

## NOTES

The real and imaginary components are stored into the **real** and **imaginary** raster map layers.  In these raster map layers the low frequency components are in the center and the high frequency components are toward the edges.  The **input** need not be square. A color table is assigned to the resultant map layer.

The current geographic region and mask settings are respected when reading the input file. The presence of nulls or a mask will make the resulting fast Fourier transform invalid.

## EXAMPLE

North Carolina example:\

```
g.region raster=lsat7_2002_70
i.fft input=lsat7_2002_70 real=lsat7_2002_70.real imaginary=lsat7_2002_70.imag
# set region to resulting FFT output map (due to new FFT coordinate space):
g.region raster=lsat7_2002_70.real -p
d.mon x0
d.rast lsat7_2002_70.real
d.rast lsat7_2002_70.imag
```

## 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.

- •

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

- •

  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.ifft,* *i.pca*

## AUTHORS

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

## SOURCE CODE

Available at: i.fft source code (history)

Accessed: Monday Feb 24 17:51:40 2025

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