Removing moire patterns fro line-cut images

[j_peck]

I am experiencing some problematic patterns with some line-cut images. I arrived at this site while searching for an FFT filter (new to me).

Please see the PDF and original images as an example.

Thanks for any assistance.

[Doug Nelson]

This has been a problem since the beginning of digital photography. It's becoming less so with higher-resolution imaging tech. But scanners are pretty much stuck at the same resolution they have been for a decade. Moiré is caused by the interference pattern between the image data and the size/number of photo receptors. Solving it requires changing one or both of these.

FFT only works for regular, repeating patterns. Moiré is very irregular.

Some techniques that have proven to work (sometimes) in the past:

Scanning off-axis (at an angle). This could require combining good parts of scans done at different angles.
Taking a photo instead of scanning. This is probably the most successful technique. Some modern cameras have effectively more resolution than the best flatbed. Conversely, if these are from a camera, try a flatbed scanner. You're basically looking for the highest DPI imager you have available.

Anyone else, feel free to post your own tips and advice here.

[unimatrix001]

This might help a little



Scan_RemoveMoiréPattern 09-2010 cc
REMOVE MOIRÉ PATTERNS FROM SCANS
Moiré patterns can occur when scanning photos from printed matter
such as books. They are crosshatched or dotted patterns resulting
from interference between the printed image and the scanner. This
does not occur with original photographs and slides.

Some moiré can be prevented during the scan by using the
“descreen” option. If this does not work or was not done, moiré
patterns can be reduced in PhotoShop:
1. Scan the image at a resolution one to two times higher than what you need
2. Open the scanned image in PhotoShop
3. Set view to 100% magnification
4. Go to Filter > Blur > Gaussian blur. This may be sufficient for some images
depending on the type of moiré pattern.
5. Go to Filter > Noise > Median - use a radius between one and three. The higher the
quality of the source, the lower the radius can be used. One often works for books,
two for magazines, and three for newspapers.
6. Go to Image > Image Size > reduce to the desired image size and resolution
7. Go to Filter > Sharpen > Unsharp Mask – begin with settings of:
Amount: 50 – 100%
Radius: 1
Threshold: 1
Depending upon the image resolution, these settings may need to be adjusted. Use
the Preview Window to determine the best settings for a particular image.
8. If you see glowing or a halo effect in the image after using the Unsharp Mask, go to
Edit > Fade Unsharp Mask > set Opacity to 50% and Mode to luminosity > click OK.
9. Save your changes when satisfied with the results. If not satisfied, changes can be
undone at any time in the History Tool or by using Edit > Undo.