On 19/04/15 22:11, wk_ wrote:
Faced the very same (or similar?) problem while trying to resize
photos made by digital camera. Very similar "grids" appear on
nearly-flat surfaces with a bit of noise (like on sky). Surprisingly,
faced the very same grids not only in GIMP, but in some other programs
as well (in Image Viewer in Ubuntu 14.04, for example), so there may
be a problem with underlying library (GTK?), not with GIMP itself.
But you said, you tested on Win 7 also. Was there same problem or not?

Tests were performend with gimp 2.8.10-0ubuntu1 on Ubuntu 14.04, and
with gimp 2.8.14 on Windows 7 Home.
I tried with your image too and can confirm that Moire appearead (with linear,
cubic and sinc methods).  It is serious problem, I think.

Moiré patterns are due to spatial frequency folding. They can normally be dealt with by blurring the picture before scaling down (in other words, cutting off the high frequencies that won't be in the final picture anyway). Typically you apply a Gaussian blur of X pixels when you scale down by a factor of X.

The frequency response of the various interpolation methods make them sensitive to different frequencies, so without blurring one method can give better results than others, but that doesn't mean it will work every time.

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