Minimizing Moire Patterns

[tedster]

I recently was sent a bunch of black and white scans to prepare for the web that are full of moire patterns. My old stand-by trick (rotate the canvas 25 degrees CW and then 25 degrees C-CW) isn't cleaning them up as much as I need, and the source materials are not available for a new scan.

Does anyone have any good techniques for minimzing moire patterns? (I have Photoshop 6)

[minnapple]

Ted,

Moires can be tough, each one can require a different technique. I have had to fix many types over the years. Post an example and stickymail me the url and I will take a look.

Minnapple

[mivox]

Hmm... You've tried the standard "despeckle" noise filter then? Too bad they're already scanned in, as I've found even a low-end scanner's "de-screen" setting helps with halftone dots and moire problems quite well...

[tedster]

I just got angry enough to try something new. First I went to L*a*b color and grabbed the lightness channel (the orginal is a black and white so this is all I need). Then I alternated filters: despeckle - dust&scratches - despeckle. I got it pretty good that way, especially when I reduce the size.

Minnapple, thanks for the offer. Look to your sticky mail!

[dabrisz]

Try a combination of Gaussian Blur followed by Unsharp Mask. I use the following settings the majority of the time.

Gaussian Blur:
Radius = .5

Unsharp Mask
Amount = 50%
Radius = 1
Threshold = 3

[tedster]

Thanks dabrisz. Those settings help.

What I'm struggling with now becomes visible after the typical moire becomes unnoticeable. There's a kind of striping artifact - vertical bands about 2 pixels wide, alternating slightly darker and lighter.

I may end up hand selecting the dark bands pixel by pixel and using brightness or lightness to make them come up to a near match. Tedious, but we need this image!

[mivox]

Hehe... Could you just up the overall contrast and tweak it into a kind of "retro-teevee cool" effect? ;) Sounds like intentional distortions I've seen applied to 50's clip art before.

But sometimes nothing beats those painful hours of hand-tweaking when you really need to clean up a lousy image.