FFT Denoise

About the FFT Denoise filter

Periodic noise is encountered on scanned images when the scanner has been subject to electrical or electromagnetic interference. By using a greyscale Fourier spectrum generated from the image, periodic noise peaks can be edited out of the spectrum by using a brush.

Fourier spectrum for FFT Denoise — Paired noise peaks on Fourier spectrum mirrored across both horizontal and vertical lines.

This filter can be found in the Filters menu, in the Noise category.

Settings

The following settings can be adjusted in the Fourier spectrum:

Width—the brush (stroke) size in pixels. Type directly in the text box or drag the pop-up slider to set the value.
Opacity—how see through the brush is. 100% opacity erases pixels completely on the first pass. A lower opacity only partially erases the peaks.
Flow—how fast the brush effect is applied (1% is very slow, 100% is immediate). Type directly in the text box or drag the pop-up slider to set the value.
Hardness—how hard the edges of the brush are. The brush appears softer as the percentage decreases. Type directly in the text box or drag the pop-up slider to set the value.
More—click to display the Brushes dialog to access advanced brush settings.

To remove periodic noise:

From the greyscale Fourier spectrum, select brush properties from the top toolbar. Take care to set a brush width that encompasses the noise peaks entirely.
Paint out the frequency noise pairings on both horizontal and vertical lines.

About the FFT Denoise filter

Settings

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