## 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_image) and constructs the real and imaginary Fourier components in frequency space.

## NOTES

The real and imaginary components are stored into the real_image and imaginary_image 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_image 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 rast=lsat7_2002_70
    i.fft input_image=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 rast=lsat7_2002_70.real -p
    d.mon x0
    d.rast lsat7_2002_70.real
    d.rast lsat7_2002_70.imag

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

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

i.cca
i.class
i.ifft
i.pca

## AUTHOR

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

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