Forum Moderators: not2easy
This picture is a scan from a magazine, at 300dpi, with all neutral settings.
As you should see, the picture have a lot of little squares, they are less visible at 100% zoom, but I still see it and they are annoying.
The scanner (HP G4050) software has nice option "Descreen" which helped to get ride of this noise like by 50%. (The sample posted was not scanned using that feature).
In Photoshop, the Blur tool seems to make a good job if I manually retouch the picture, but that's a very painful process, I'm gonna scan a lot of picture and it would take a lot of time.
In Corel PaintShopPro, there's a sort filter called "Salt and Peper" remover that together with "Skin" smoothing, somehow fixed it, but not enough to my sight.
How do you recommend me to fix this?
BTW, these squares are not visible on the magazine at all, just if you see it with a magnifying glass.
<short tutor>
In order to avoid moire in process printing, the angles of the 4 colors must be at least 45 degrees apart. Simple math will tell you there's not enough room to do this, so the angles are cast (most often) at 45 degrees for black, 75 for cyan, 105 for magenta, and 90 for yellow. Obviously the axes of yellow are not 45 degrees from C and M, so there is moire between yellow and the opposing angles - it's just rarely visible. If an image relies heavily on the yellow against another color, this is fixed by switching one of the angles. We won't get into the complications of desktop publishing, but the fact that is is virtually impossible to cast a *perfect* 75 and 105 degree angle in Postscript makes this issue even worse. :-)
</short tutor>
Now lay it in your scanner, which scans on a 90 degree grid - no matter what you do you are going to get a moire pattern in your scanned image because the axes of your scanning grid can't possibly be 45 degrees apart from every angle in the printed copy. What we used to do old-school on drum scanners still applies - put it back in the scanner on a slight angle, not square to the scan frame. If it's worse, twist it a few more degrees. Keep twisting it a little at a time until you get the one with the *least* amount of moire pattern.
The "descreen" filter of most scanners has an effect similar to what we also used to do, knock the image slightly out of focus. This will help, a little. Also it helps to scan the image at a much greater resolution than you need, so in subsequent operations of rotation and reduction the moire is deminished.
Take the best one, rotate and scale it down in Photoshop, this will minimize what's left of the moire. Sometimes you can modify the channels individually to knock out the most offensive areas. With enough tweaking you can almost completely eliminate moire pattern from a scanned image.
[edited by: rocknbil at 2:05 am (utc) on April 1, 2007]
Edit:
put it back in the scanner on a slight angle, not square to the scan frame. If it's worse, twist it a few more degrees. Keep twisting it a little at a time until you get the one with the *least* amount of moire pattern.
I've only ever had to scan images like this once but that's a great tip considering your explantion as to why it happens. Bewtween that an neat image you should be able to remove it completey, I did a quick test on his sample and it just about removed all of it.
Once this has been done to your satisfaction, you can then apply a small amount of the unsharp mask filter to sharpen the image up again. Alternatively, reducing the image size is a good option.
If you have a batch of similarly moired images, you can record an action based on the above and process a whole folder at a time.
