[softwarelist] Re: Descreening halftone images

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

> Not an OPW- or OP-specific question, but I'm hoping someone on this list
> will be able to give me the benefit of their experience ...

> 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
> (ideally for a Mac) that would serve the purpose?

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?

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.

For example:
use on a web page and they are likely to be viewed at around 90dpi;
printed at same size using a 600dpi printer using a 90lpi screen a 
scan resolution of 150dpi is plenty.

If you intend to print you either need to remove the exisiting screen 
to avoid the risk of interference fringes, or scan at 1 bit per pixel 
(pure BW, no greys) and rely on the existing half-tone screen to 
reproduce the greyscale - in this case the higher scan resolution will 
help.

Once you have the appropriate scan resolution, most image manipulation 
software (eg DPlngScan to keep some relevance to this list) will allow 
you to blur an image, add noise and other manipulations which will 
hide the original half-tone screen. Look for options for Filters or 
Effects.

I suggest you spend time playing with copies of one image applying 
different effects to each copy (single efects, combinations of 
different effects, the same effect applied multiple times...). Make 
notes of the effects applied each time so that when you fiind one you 
are happy with you can apply the same effect to the set.

Anthony

-- 
Anthony Hilton
ajh@xxxxxxxxxxxxxxxxx
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