[Gimp-user] Gimp 2.8 pixelation on resize

[wk_ <forums gimpusers com>]

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, it is scan of printed
document. But this is not problem. I have more than 20 years experience of
scanning printed docs. 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. 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.

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?

> 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. 

Wbr,

Gunnar

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wk_ (via www.gimpusers.com/forums)