On 04/19/2015 07:34 AM, Patrick Shanahan wrote:
> * renat <for...@gimpusers.com> [04-19-15 05:26]:
>>> scanned images are going pixelated
>> Faced the very same (or similar?) problem while trying to resize photos made
>> digital camera. Very similar "grids" appear on nearly-flat surfaces with a
>> 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,
>> example), so there may be a problem with underlying library (GTK?), not with
>> GIMP itself.
> DL'l your "original" and viewed on openSUSE Tumbleweed in geeqie, gimp and
> imagemagick (display) and do not see the "grids" even after pushing to
> 200%. Scaling "method" appeared to make no difference, nor did
> resolution. My installed gimp package is from openSUSE Build Serice.
I see the "grid" in Firefox and the GIMP. Here's a sample, scaled
up 300% for clarity:
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.
It looks to me like Moiré pattern noise, smoothed out by some kind
of dithering process. Example:
I would look at the path the image took from sensor to camera memory
to hard drive: It seems likely that, some step along the way, a Bad
Thing is happening. I would look into saving RAW format files in
the camera, for conversion to PNG or some other lossless format in a
tool like UFRaw. A few photos of subjects chosen to make that
checkerboard likely to appear would show whether that works.
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.
If the too-smooth blurred regions look unnatural, try running
Filters > Noise > HSV Noise against it, with the Hue setting at zero
(no color noise) and the Saturation and Value sliders tweaked as
necessary to restore a bit of grain.
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