Thank you Ell for your quick answer.

When I read your explanation, I  have a feeling that I understand. But when I try to apply this, I get nothing. I tried with an alpha channel, a layer mask, a selection mask (where I can edit transparency) without any result in the image.

Please, give me an example applying this damned Alpha percentile. I am sure that I will understand with an example.


Le 31/10/2018 à 02:34, Ell via gimp-docs-list a écrit :
On 10/30/18 11:01 AM, Julien Hardelin wrote:
Hi all,

I don't understand the "Alpha percentile" option in Median Blur filter.

Is this "alpha" a term related to statistics or to transparency ? I
can't built an example of this option.
By default, the Median Blur filter finds the median value at the
neighborhood of each pixel.  In spite of its name, the filter can
actually find *any* arbitrary percentile, not just the median (i.e., the
50th percentile).  The "Percentile" parameter controls the percentile
used for the color components, and the "Alpha percentile" parameter
separately controls the percentile used for the alpha component.  In
other words, "alpha" is related to transparency in this context.

To see the effect of this parameter, you need an image with transparency
-- if the alpha channel is opaque everywhere, the result will also be
fully opaque, regardless of the percentile.  Much like the color
"Percentile" parameter, for which lower values shift the image toward
darker tones and higher values toward brighter ones, lower values for
the "Alpha percentile" parameter shift the image toward more
transparency, and higher values shift the image toward more opacity,
where a value of 50% is balanced.  Roughly speaking, values less than
50% make the opaque regions of the image smaller, while values greater
than 50% make the opaque regions of the image larger.

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