How to use the Fourier Transform plug-in
Once you copied the plug-in to your host's plug-in folder you will find it in the Filter menu (Photoshop), under "Forensic Filters".
Your source image should be in RGB mode, because eventhough the Fourier Transform only works on the intensity in the image, you will need the RGB channels to store the Fourier transformed image. The power spectrum is in the Red channel, and the phase information in the Green channel.
The image below shows a fingerprint on a printed document, and its Fourier transform (the power spectrum in the Red channel).
Download this testimage in full quality, if you want to try the following for yourself.
After Fourier transforming the image we choose the Red channel (see below) to edit the power spectrum alone.
We can then remove the diagonal pattern in the source image by getting rid of the lower frequencies associated with it (near the center of the image):
N.B.: Don't forget to select all channels before you transform back (by clicking on RGB):
In the resulting image after applying the inverse FFT, the diagonal pattern has been suppressed:
Move your mouse over the above image to see the original and observe the improvement.
We can also remove the screening pattern in the Fourier transformed image (using for example the healing brush of Photoshop):
Move your mouse over the above image to see the original and observe the improvement.
Download the Fourier Transform plug-in so you can use it with your host application.