Re: [Gimp-user] Gimp 2.8 pixelation on resize



On 04/19/2015 07:34 AM, Patrick Shanahan wrote:
* renat <forums 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 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.
[...]
Attachments:
* http://www.gimpusers.com/system/attachments/195/original/gimp-ubuntu-scaled.png

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:

http://pilobilus.net/xfer/gimp-ubuntu-scaled-detail.png

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:

https://en.wikipedia.org/wiki/File:Moir%C3%A9_grid.svg

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.

Example:

http://pilobilus.net/xfer/sample-before.png

http://pilobilus.net/xfer/sample-after.png

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.

:o)

Steve







[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]