Greetings over there.
First let me introduce myself, my name is Aliet Expósito García, I am
Cuban, Computer Science student and Free Software user, now lets get to
the point :)
I use GIMP 2.6.10 in Ubuntu 8.04 and 10.04, and 2.7.1 in Lucid, I use it
extensively since I co-own a photographic studio and have to do heavy
editing on the pictures, its a fad over here and everyone wants pics
with visual effects so ALL pictures must be edited. I noticed on some of
the finished pictures some sort of pattern like artifacts and pixels
with offending colors whenever there was transparent gray or black over
white o clear backgrounds, I also noticed that these were downscaled
using Sinc(Lanczos3) interpolation. I've been conducting some tests to
compare Cubic and Sinc interpolations on my pictures, I know Sinc is the
theoretical optimum but I needed to see in practice if the difference
was noticeable enough and if so always use it (Right now I'm biased
towards cubic) -- Some details on the Sinc VS Cubic on digital
photography resizing would be much appreciated. I already studied the
source code, but I'm far from understanding it completely although
that's where I caught the curiosity bug regarding the cubic and lanczos
To replicate the issue I'm talking about lets create a new image in
Width: 3000 px
Height: 2000 px
Color space: RGB color (Grayscale gives the same result)
Fill with: White
Once the image is created, go to Image->Scale Image... change the image
size to 99% (2970x1980 pixels), select Sinc(Lanczos3) as Interpolation
method and scale the image. (upscaling shows the same behaviour)
After the scaling is done create a new layer on top and fill it with
Black set its transparency to 90% and zoom to 800%, you will see the
Scaling to 10%, 20%, 25%, 40%, 50% or 80% of the original size the
patterns are not observed, any other size is affected (Yes I tried ALL
percent sizes, but only downscaling, upscaling is different)
This also occurs if the layer is filled with black, but it's harder to
see if it is the only layer, so try adding an alpha channel and scale
The alpha channel is affected if it is present on the layer, the pixels
affected are 1% more transparent than the others. The pixels affected by
transparency may or may not be the same affected by color.
Black and Gray(808080) colors are affected only by transparency if the
alpha channel is present, otherwise the color is affected.
In any other case the affected color is 1 unit smaller per channel, for
example with White 'ffffff' the affected pixel color is 'fefefe', Gray
'808080' turns to '7f7f7f'
Black instead turns to '010101' (I'm having a hard time replicating this
on a layer without alpha channel)
With various layers it affects all of them, in some cases not but it's
very hard to try finding some sense to it since if somehow the patterns
are not created in some layers, trying again may affect others, the same
or all of them.
It affects all colors, but it is easily seen in neutral colors.
Use the Color Picker using the info window and sweep the canvas, you
will run into the affected pixels...
I don't know if this has been reported already, if so can someone
please give an explanation about why it happens, it would be much
Right now I can't use the Sinc(Lanczos3) method because the patterns
shows in common pictures, not only the ones I've been testing on.
 Accessing Internet around here is very difficult hence the reason I
am using the mailing list for this instead of other channels.
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