i.fft(1)

📦
version 8.4.2 · 8 versions
, in fedora 44

💖 i.fft — the manual, but make it make sense 💖

okay bestie, this is the GRASS GIS User's Manual page for i.fft(1), and I'm gonna walk you through it like we're texting at 1am. no cap, it's actually so good. 🦋

✨ NAME ✨

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

like. it takes your image and yeets it into frequency space. iconic behavior honestly. 💅

🏷️ 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 — bestie, this lets your output files just bulldoze whatever was already there. Allow output files to overwrite existing files. use responsibly, ok? 😭
  • --help — prints the usage summary so you're not out here guessing. Print usage summary.
  • --verbose — the module gets chatty and tells you everything. Verbose module output.
  • --quiet — the opposite. it shuts up. Quiet module output.
  • --ui — forces the GUI dialog to pop off. Force launching GUI dialog.

📥 Parameters:

  • input=name [required] — Name of input raster map. this is the thing you're transforming, periodt.
  • real=name [required] — Name for output real part arrays stored as raster map. yes you have to name it yourself, I don't make the rules.
  • imaginary=name [required] — Name for output imaginary part arrays stored as raster map. your imaginary bestie needs a name too. 💜

🌸 DESCRIPTION 🌸

so i.fft is an image processing program, and it's based on the FFT algorithm given by Frigo et al. (1998). it takes a single input raster map layer (input) and constructs the real and imaginary Fourier components in frequency space. that's the whole vibe. lowkey elegant.

💀 NOTES 💀

okay so listen up, this part matters:

  • 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. remember this. it's giving "map layout lore."
  • The input need not be square. no pressure to be a perfect square, we love you as you are. 🫶
  • 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. so like, don't do that. fr fr. 😭

🔥 EXAMPLE 🔥

North Carolina example, a classic:

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

see that comment in the middle? yeah — you gotta reset the region to the resulting FFT output map, because the FFT lives in a whole new coordinate space. main character energy, honestly. ✨

📚 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

(go read i.ifft next, it's the reverse girly. the way they're a matched set 😭💖)

👑 AUTHORS 👑

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

💻 SOURCE CODE 💻

Available at: i.fft source code (history)

Accessed: Saturday Jan 17 05:41:45 2026

Main index | Imagery index | Topics index | Keywords index | Graphical index | Full index

and that's the whole page bestie!! 🎀 this was GRASS GIS 8.4.2 Reference Manual, © 2003-2025 GRASS Development Team, and the version vibes are GRASS 8.4.2, accessed Saturday Jan 17 05:41:45 2026. go slay your Fourier transform. no cap. 💅🌈⭐

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