[softwarelist] Re: Descreening halftone images

Anthony Hilton wrote:

In message <4915E0B2.6080109@xxxxxxxx>
          Clive Bonsall <cbonsall@xxxxxxx> wrote:


I have a series of images scanned at 600 dpi from B&W "photos" in a
book, which I want to import into an OPW document. Of course, the
original halftone screen is visible and I want to descreen the images.



Can anyone suggest a technique for doing this, or a piece of software
David Pilling's TWAIN driver has an option for Superscaling which I think is aimed at this situation. Does your scanning software offer any options?
I don't have DP's TWAIN driver for this particular scanner, so I tend to do scanning via my Apple Mac. But now you mention it, I see that the EpsonScan software has a descreening setting, which does improve things.
How will you be using the photographs? The answer may suggest that 600dpi is excessively detailed and a lower scan resolution may mask the half-tone screen to some extent.
These are to be published in a book. I have to provide camera-ready copy as high quality PDF. Probably 300 dpi for images would be sufficient, but I tend to scan at higher resolution then reduce the dpi in software, if necessary.
I tend to control the scanner via Photoshop. Apparently, there is a descreening plugin for PS, but it seems to only be for the PC version (not Mac). But, after I sent out the email, I found a potential solution on the Internet ... which is (for those who have PS):
1. Scan the halftoned image at a resolution to yield approximately 4 pixels per halftone cell. (IOW, if the image is screened at 150 lpi, scan at 600 dpi).
2. Run the Median filter with a pixel radius of 3 (IOW, smaller than the individual halftone cell).
3. Run Unsharp Mask at 250% (or more), with a radius of 1.5, and a threshold of between 12 and 18 levels. 

4. Play with the above settings to achieve optimum results.

++++++++++++++++++++
It was interesting to compare the results of the above PS method (steps 1-3) with the Epson descreening scan. EpsonScan produced a sharper image, but you can still see a fine halftone screen effect (tho' it's much improved over a basic B&W scan). With the PS method the halftone screen effect is no longer evident, but the image is very slightly blurred compared to the original -- the effect of using the Median filter (but not fully restored to sharpness by the USM settings), I guess. 

Thanks for the help.
--
C.B.
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