Harmonic Detective: Image FFT

A graphic equalizer for images: cut, keep, or boost the frequencies hiding under the pixels — moiré, texture, faint detail. Free to explore; unlock to export.

Harmonic Detective is a graphic equalizer for images. An audio equalizer lets you turn bands of sound up or down; this does the same for a photo or scan — except the bands are the 2D spatial frequencies hiding under the pixels, and you draw them yourself.

Load any image and see its live frequency map — a real 2D Fast Fourier Transform, the kind of tool that used to mean a MATLAB license or a Photoshop plug-in. Then reach in and act on exactly the frequencies you choose:

• CUT a band — erase periodic noise, moiré, halftone screens, or scan-line banding • KEEP a band — isolate one structure on its own, to study what it contributes • BOOST a band — amplify a frequency range to pull out faint detail

…and watch the image rebuild in real time. No presets, no "AI enhance," no guessing — you see exactly what you are changing and stay in control.

SEE A PATTERN, THEN REMOVE IT

A periodic pattern — moiré, a halftone screen, a repeating texture — shows up as a few bright dots in the frequency map. Circle those dots, turn them down, and the pattern disappears while the rest of the image stays intact (and because the math is symmetric, one region removes the pattern's mirror twin automatically). It's a hands-on instrument, not a one-tap auto-fixer: you find the frequencies and act on them yourself. That is the whole point — you stay in control.

GREAT FOR

• Cleaning periodic noise, moiré, and halftone screens out of scans and photos • Isolating and studying a texture, weave, lattice, or repeating structure • Revealing faint detail by boosting the right frequency band • Actually seeing what a 2D Fourier transform does — the most direct way to learn it

HOW IT WORKS

Four views, always in sync • Original, frequency map (FFT), result, and mask — side by side on iPad and Mac • Every pane updates live as you work

Draw exactly where you want to act • Circle, box, or ring the frequencies you want — ellipse, rectangle, and annulus tools • Move or resize by touch on iPad, or click-and-drag on Mac

Two actions on any selection • Modifier: scale a band up or down — down to zero to erase a pattern, up to emphasize one • Isolator: keep only the selected band, to see exactly what it contributes on its own

FREE TO EXPLORE

Download free and use the full instrument on screen: load any image, see its live spectrum, draw a region, and watch the result rebuild in real time — with a dozen built-in test images to try it on. Nothing is time-limited.

UNLOCK PRO — ONE-TIME $9.99

When you're ready to keep your work, a single one-time purchase (no subscription) unlocks: • Save & export your result to Photos or Files, or share it anywhere • Work with multiple frequency regions at once • Export the raw FFT mask as a binary float array for MATLAB, Python, or your own pipeline One purchase covers both iPad and Mac.

FOR THE PROS

• GPU-accelerated FFT (Metal / MPSGraph) handles any image size — about 2–3 ms at 400×400 — with a CPU (Accelerate / vDSP) path validated to match it bin-for-bin • Masks preserve full conjugate (Hermitian) symmetry, so filter gain is exact with no mirror-band artifact • Two rotation modes — keep your region fixed on screen while the result changes, or lock it to a feature as the image turns

No ads, no account, and no internet connection — every calculation happens on your device.

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