On Wed, 2015-04-22 at 21:11 +0200, wk_ wrote:
> Liam wrote:
> > If it is a scan of a printed document is is to be expected that
> > there may be moiré patterns, even if the scanner is set to apply a
> > "descreen filter".
> I feel that there is something lost from my original post. Like it
> never reached
> to the list. I see still it in archive, I point it for reference:
> Have you looked my original scan I linked there?
Yes, I looked at it in detail.
> Yes, it is scan of printed
> document. But this is not problem. I have more than 20 years
> experience of
> scanning printed docs.
Please remember that we don't know your background.
> Problem is, when I use same settings in Gimp 2.6 and 2.8
> for scaling down the same sample I got completely different results.
This is probably because the default downscaling method changed.
The new method is better for some things and worse for others.
> And unfortunately, 2.8 is so much worse. So we must use additional
> processing before
> scaling. I like to have full workflow foolproof and simple, so I can
> delegate it
> whomever I need. Adding additional levels of processing is bad
> practice in my
You can change the default downscaling method.
> I tried with two different computers (both Gimp 2.8) and got same
> ugly result.
> It seemed unbelievable, that new version may have such comedown and
> no one has
> noticed such behaviour, so I asked here for others experience.
> > If that's the case, the "grid" is liable to appear at any time on
> > scaling down the image, or possibly with other image editing
> > operations, both in gimp and in other programs, especially with 8-
> > bit per channel colour. It's a function of the image, not of the
> > software.
> So, if I take same image and scale it down with 3 different tools
> (Gimp 2.8,
> Gimp 2.6 and ImageMagick), I got 3 different results and it is
> function of
> image??? How?
It is a combination of the image and the tools, of course.
> > I deal with these often in processing scans. You can use a
> > frequency decomposition to remove them, or you can do a guassian
> > blur as I think others have suggested, before scaling down.
> I do, if needed. In 2.6 it was not necessary, generally. In 2.8 it
> is not
> avoidable, that's my problem.
> > It depends on the interpolation method that's used - in a recent
> > gimp 2.9 snapshot I found the pattern appeard with one
> > interpolation method but not another. The default interpolation
> > method I think changes from time to time in different GIMP
> > releases.
> I explicitly used Sinc interpolation in both cases, with 2.6 and
> with 2.8.
Well, you didn't say that before. However, I do believe the code
changed. I think in 2.6 the dialogue was misleading as Cubic was used
for downscaling in that case.
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