[Gimp-user] Gimp 2.8 pixelation on resize

[renat <forums gimpusers com>]

Wow, that's a huge tread. Sorry for delayed response.
I absolutely agree with Liam R. E. Quin, Steve Kinney, wk_ and Ofnuts - from my
POV it's definitely a moire.

There were many questions about real original file and it's post-processing, so
here is a RAW file:
https://drive.google.com/file/d/0B2nZFc31UFnoc2RxemYwTlVuS28/view?usp=sharing
And a TIFF file (converted in RAWTherapee 4.2, default AMAZE demosaic method),
where we don't have to deal with all this nasty JPEG artifacts. However, if
you'll try to scale it down, you'll see, that moire pattern is here.
https://drive.google.com/file/d/0B2nZFc31UFnoMTJLNk9qd2JLV0E/view?usp=sharing

> How was the image created exactly? It looks like you photographed a 
> printed image and then used rawtherapee to convert to a jpeg for gimp 
> to open?? If so, the low frequency grid is from the four-colour print 
> process.
No, it's a real landscape shoot by an old Canon 350D. I haven't faced such
patterns earlier while processing pics from that camera, so it looks like
algorithms were really different in GIMP 2.6.

> I see this pattern in gimp scaling with linear but not with cubic
Would be great, if you'll  provide exact settings you've used in GIMP - I, of
course, have tested all three interpolation methods, but the grid is easily
visible in each case.

> Meanwhile, it's not too hard to fix in existing images like the sample: Load
the image into the GIMP, make a duplicate of the base layer, and apply Gaussian
blur with a 10px radius. Add a layer mask, fill it with black, and paint on the
mask in white to "erase" the artifact by making those bits of the blurred layer
visible.

Thanks for your suggestion, but it's million times easier to prevent this grid
from appearing - I have just used graphicsmagick lib in pair with Node.js, and
result is more than fine:
https://drive.google.com/file/d/0B2nZFc31UFnoNHlTZEJFdWJpb2M/view?usp=sharing
That's why I don't think there is some problem with my camera or image
processing outside GIMP.

> But you said, you tested on Win 7 also. Was there same problem or not?

Yes, I got the very same grid on Win7. And what about your case and Windows?

> 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.

Yes, I know it, as I'm an astrophotographer, but I'm not sure it's the case for
all users of the GIMP. Wouldn't it be good to have at least one (default, of
course) interpolation method, which will blur original image itself before
scaling it down?

> That's bizarre. Scaling _MG_4282.jpg to 50% or 25% of its original size did not
produce the noise, but scaling it to a width of 1920 px did produce the pattern.
Same result with linear, cubic and sinc interpolation.

In fact, it's not that strange - you can even see similar things by a naked eye,
if you'll do some experimentation with, say, two textile nets. It's just how
moire pattern appears.

> I see a consistent rectilinear pattern in the nose, not obvious in the
unmodified original. I still think this comes from some interaction of the
sensor's noise floor and whatever algorithm the camera uses to convert sensor
data into JPG format

For me that looks like usual noise and JPEG compression artifacts (by the way,
level of compression was set to lowest in RAWTherapee). Of course that noise is
the source of the problem, but well, real images are never free from noise.


To sum up, this problem is not a problem for me, I've just used another piece of
software to downscale my images for my TV. It's more about bug reporting,
because I agree with topic starter - such behavior would be considered a
regression by many GIMP users. I'm well aware that's it's not always perfect to
blur image, but we have different interpolation methods anyway, so it would be
good to have one suitable for general public.

-- 
renat (via www.gimpusers.com/forums)