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Removing moire with FFT filter in Photoshop

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Removing moire with FFT filter in Photoshop

Postby MrTinkertrain » Fri Mar 25, 2016 12:09 pm

Hello everyone,

When i'm working on artwork and I want to get rid of scan moire I sometimes fall back on the FFT-filter to help me get rid of it.
I have to say that my results applying the FFT-filter are mixed, sometimes the results are pretty good and sometimes it manages to get rid of some of the moire, but it then brings back some horizontal lines in my images.
Whenever that happens, I tend to just throw away everything and look for another source file to work with.

I have a question regarding working with the FFT-filter :

From what I have learned the moire pattern should be visible in the red channel after running the filter.
However I have run into some covers with absolutely nothing being visible in the red channel.
All I saw was a perfectly red square/rectangle with no "contamination" whatsoever.
Would that mean that such a cover would be impossible to fix with the FFT-filter and I need to apply a different technique for that particular image or look for an alternative source alltogether ?

Any input would be highly apprecciated ;)
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Re: Removing moire with FFT filter in Photoshop

Postby MrTinkertrain » Tue Mar 29, 2016 3:27 pm

I found out that the largest image size the FFT-filter can handle is 2980x2822 pixels.
A lot of times I'm working with sourcefiles that are (much) bigger than that.
So apparently the way to go here is to first resize the source file and then run the filter ;)

Problem solved :)
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Re: Removing moire with FFT filter in Photoshop

Postby ZincRider » Tue Mar 29, 2016 3:52 pm

This one? http://www.djjoofa.com/download

Works for larger images (over 5000x7000px) for me. Unfortunately it's not color accurate (descreened images come out slightly darker for me) and I sometimes get weird artefacts (black or white pixels).
On the plus side, the stars in the transformed image are better visible than in the Gimp plugin.

For many images a heavy handed approach works well for me. I make a circular selection around the center star, invert the selection, apply a heavy gaussian blur, deselect and then apply the reverse FFT. If the artefacts are too intense, I undo the last step and make a larger selection.
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Re: Removing moire with FFT filter in Photoshop

Postby MrTinkertrain » Tue Mar 29, 2016 6:13 pm

No, I'm using a different one
But the one you showed looks promising, too bad it's only available for the Mac as of right now.
However on the bottom of the site it says that a Windows version is coming soon, so i'll bookmark this one to check back later ;)

I have to say that's an interesting approach you take with the circular selection around the center star and the gaussian blur.
I will definitely be trying that out.
Thanks :)
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Re: Removing moire with FFT filter in Photoshop

Postby ZincRider » Tue Mar 29, 2016 9:36 pm

It may not always be the best approach, but it can be quite effective.

Sometimes I get the best results by descreening twice, once with a heavy handed approach, once by carefully painting out the stars, and then using the best parts of both results.

The Joofa plugin is the only FFT plugin for current PS versions on Mac I could find..
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