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FFT Raster Removal


Have you tried scanning something from a newspaper, and realising that the raster dots from the print process were quite horrible to look at? I've come up with a method on how to remove them using an FFT Plugin for Photoshop. Update: The free software Gimp now also has an FFT filter that works well.

Original scanned image:

original scan

After this you go through a rather abstract sequence of noise identification and removal within the fourier transformed space. Some of the involved steps will be explained below.

Resulting image for comparison:

original scan

The Process

  1. Transform the image into 2D FFT space. This gives you a frequency representation of the image, where the presence of large shapes appear as bright areas closer to the center, and smaller shapes gives more stuff towards the border of the image.

    step 1

  2. The image itself will typically show up as a bright star exactly in the center of the image. The raster pattern from the newspaper print shows up as weird dots next to the center. Now you remove the weird "star" in order to not affect the image itself.

    step 2

  3. After this you highpass and level correct the FFT data to retain only the stray dots that represent the raster pattern.

    step 3

  4. When these stray dots are isolated, you can subtract them from the original FFT data to get rid of the raster pattern frequencies.

    step 4

  5. After this, you do the inverse FFT to convert the strange FFT data back into a normal image.

    step 5

The real chain of events is a little more complex than this. One problem is that a greyscale image will turn into a color image when you run the FFT filter. This is a necessary step because the FFT'ed data splits up a greyscale image into "phase" and "amplitude", which need to be kept separately. Thus, the process can only handle grayscale pictures or separate color channels.

So to denoise a color image (in RGB mode), i had to go through this process three times: One for red, one for green and one for blue. To make this entire process less painful, i've created this action for Photoshop. The action requires this FFT Plugin.

More examples

Once again i've tried doing the procss in color. This is somewhat troublesome compared to processing a greyscale image, as you have to repeat the process three times. This trick is now included in the action.

Original scanned image:

original scan

Blurred to remove rasters (this is what people often do):

guassian blur

Using my new FFT approach:

rasters removed

Credits to Alex Chirokov for writing this very cool plugin.

Website by Joachim Michaelis