How do I remove print artifacts from a scan?
April 18, 2011 8:55 AM   Subscribe

Removing colour print dithering artifacts from scanned image/map? How?

I've scanned in a map/image from an old magazine. The scanner has picked up all the print dithering artifacts so I can't do anything in photoshop.

Nothing has a smooth edge and a solid colour seem to be comprised of thousands of different colours.

Is there a filter or method to remove all the "newspaper" type print dithering? You know it's like everything is made up of clumped up dots and nothing has a smooth edge or smooth single colour!
posted by flexiverse to Computers & Internet (8 answers total) 1 user marked this as a favorite
there's no way to stop this from happening since you're scanning a printed halftone. i always get best results by:

1) scanning at about 4X size it'll be reprinted
2) opening file in photoshop and adding a touch of gaussian blur
3) hit "image size," scale the thing down. the downsampling will obscure that there were ever dots (hopefully).
posted by patricking at 9:12 AM on April 18, 2011

I like using some combination of median and gaussian blur in Photoshop. Unfortunately there's no magic bulltet that I know of. It's always a tradeoff between smooth colors and preservation of detail.
posted by echo target at 9:19 AM on April 18, 2011

The technical term for this is "descreening". Photoshop doesn't have any useful tools to descreen, other than the "median" filter, which only works in odd cases. Try it with different radii.

You can also convert the image to LAB and blur the A and B channels.

I'd look for some standalone descreening software. Descreening is typically accomplished by converting the image into frequency space, eliminating peaks there, and converting back out. AFAIK Photoshop does not give you access to the tools you need to accomplish this.
posted by fake at 11:18 AM on April 18, 2011

Typically, descreening is a feature of the scanning software. Check your settings in the little piece of software that you use to do the scanning.
posted by rhizome at 11:53 AM on April 18, 2011

Depending on your scanner your scanner driver/interface may actually have a descreening function. Some of the CanoScan scanners for example offer descreening at the time you scan your images.
posted by Hairy Lobster at 12:57 PM on April 18, 2011

One thing that has worked for me was to scan at a really high resolution and then to resize it back to a sane one. Something about the way the resize algorithm worked fixed a lot of this. Or exactly what patricking said...

Things to try: scan in CMYK mode (if that's even possible?) at a high resolution and then resize or blur or [whatever] each color plane separately.
posted by gjc at 6:32 AM on April 19, 2011

If you have the GIMP you can use the wavelet denoise filter. I've had good results with this.
posted by blue funk at 6:44 AM on April 19, 2011

Just blurring is not really an effective method for what I'm doing.

This seems interesting:

Removing Moire Effectively
posted by flexiverse at 4:28 AM on April 26, 2011

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