Re: [Gimp-user] Gimp 2.8 pixelation on resize

["Liam R. E. Quin" <liam holoweb net>]

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:
>
> http://www.mail-archive.com/gimp-user-list%40gnome.org/msg08063.html
>
> 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
> environment.

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.

> Wbr,
>
> Gunnar