i.fft(1grass)

💖 i.fft 💖

okay bestie, so this is i.fft from the GRASS User's Manual — and no, the "i" is not giving iPhone, it's giving imagery. Sit down, hydrate, we're doing Fourier transforms today. ✨

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🌸 NAME 🌸

i.fft — Fast Fourier Transform (FFT) for image processing. that's it, that's the whole name. iconic behavior honestly. 👑

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🏷️ KEYWORDS 🏷️

imagery, transformation, Fast Fourier Transform

iykyk 💅

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✨ SYNOPSIS ✨

i.fft
i.fft --help
i.fft input=name real=name imaginary=name  [--overwrite]  [--help]
[--verbose]  [--quiet]  [--ui]

🎀 Flags: 🎀

• --overwrite — lets your output files just steamroll right over any existing files. no asking, no warning, it's giving bold.
• --help — prints the usage summary so you can read it and go "ohhh okay". 🫶
• --verbose — verbose module output. she talks a lot and we love her for it.
• --quiet — quiet module output. shhh. the opposite vibe.
• --ui — forces the GUI dialog to launch. for the girlies who like a little window moment. 🪟

🌈 Parameters: 🌈

• input=name — [required] the name of your input raster map. mandatory bestie, you can't skip this one.
• real=name — [required] the name for output real part arrays, stored as a raster map.
• imaginary=name — [required] the name for output imaginary part arrays, stored as a raster map. yes it's really called imaginary, no that's not a typo, that's math. 😭

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💫 DESCRIPTION 💫

so i.fft is an image processing program, and it's built on the FFT algorithm given by Frigo et al. (1998) — shoutout to them, legends fr fr. 🙌

What it does: it takes one single input raster map layer (input) and constructs the real and imaginary Fourier components in frequency space. one input, two beautiful components, main character energy all around. ✨

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📝 NOTES 📝

The real and imaginary components get stored into the real and imaginary raster map layers respectively. no cap.

And here's the layout you need to remember: in those raster map layers, the low frequency components are in the center and the high frequency components are toward the edges. center girlies = low freq, edge girlies = high freq. 💅

The input need not be square — she's flexible, she contains multitudes. Also, a color table is assigned to the resultant map layer, so it will look cute when you view it. 🎀

The current geographic region and mask settings are respected when reading the input file. periodt.

🚨 okay important warning bestie: the presence of nulls or a mask will make the resulting fast Fourier transform invalid. like fully invalid. not "a little off", not "we'll fix it in post" — INVALID. so clear your nulls and masks before you run this, or your output is gonna be a mess. 💀

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🦋 EXAMPLE 🦋

North Carolina example, our beloved:

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

look at that flow. chef's kiss. 👨‍🍳💋

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📚 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. she does it ALL. 🔥
• 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. vintage correspondence moment, we stan. 📮

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👀 SEE ALSO 👀

i.cca, g.gui.iclass, i.ifft, i.pca

(if you liked this one, girl, you're gonna want the inverse — that's i.ifft, go get her 🌸)

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👑 AUTHORS 👑

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

two absolute icons. thank you for your service. 🫡💖

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💻 SOURCE CODE 💻

Available at: i.fft 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 8.5.0 Reference Manual

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okay and that's the whole page bestie!! 🎀 this was i.fft(1grass), served to you straight from the GRASS 8.5.0 Reference Manual — © 2003-2026 GRASS Development Team, last touched Friday Jan 02 21:25:59 2026 in commit 83362df5af36c1e7d95fad93dff0a57b4dfde476. go do a Fourier transform, you're gonna slay. ✨🦋💜