On Sun, 2015-04-19 at 22:20 +0200, wk_ wrote:
> Steve wrote:
> > I have never seen anything exactly like this, so I am confident 
> > that it is not an artifact introduced by scaling an image in the 
> > GIMP.
> I don't understand your way to this conclusion. In my original post I
> have
> source scan,

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 

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.

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.

> which I scaled down with 3 different tools (Gimp 2.8, Gimp 2.6 and
> ImageMаgick). With first one Moire pattern appears, but not with 
> others. How it
> could be not realated to Gimp 2.8? The process is easily 
> reproducible.

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.


> Wbr,
> Gunnar
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