GIMP Version: 2.8.10
Operating System: Windows
GIMP Experience: Beginner Level

List any relevant plug-ins or scripts:
selective gaussian blur

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maybe the request looks a bit crazy:
is there a way to apply a "reversed selected gaussian blur"?
the standard gaussian blur is such that the threshold is used to limit the blur action below the delta
for instance, using the defaults:
- it blurs with a radius of 5 wherever the delta of the values between two pixels is less than 50
could it be possible (or could be created a filter such that..) the opposite is performed? that is:
- it blurs with a radius of 5 wherever the delta of the values between two pixels is GREATER than 50
Sometimes could be useful...

This article on -blur at G'MIC by Garry Osgood might not solve your problem but it's an interesting read on the subject.

Thanks, Odin
Yes, it's interesting, unfortunately not in the "reversed" use I'm looking for.
I consider G'MIC coders good candidates for creating a "reversed" use of the delta in performing a selective blur.
Maybe someone is willing to try it...
Spes ultima dea!

Yes, it is possible with a G'MIC script, definitively.
Anyway, I'm not really sure it would be interesting, let me explain why.
A selective gaussian blur uses different amount of blur on different regions of the image, and the way it selects this amount of blur locally is basically by looking at the so-called gradient-norm of the image. This gradient norm tells locally if a pixel is on a region of high contrast/variation (probably an edge), or on a region with very few variations (mostly backgrounds or flat regions). And of course, the selective blur tries to blur less on the edges than on the flat regions.
Now, if you reverse this behavior :

1. You will blur more on the edges -> you will destroy them, like a regular blur does (with a constant amount of blur over the image).
2. You will blur less on flat regions -> this will be probably barely visible, because if the region is already perfectly flat, it won't change anything. On the contrary, if there is some 'noise' on these regions, they will be preserved.

In my opinion, this will look very close to what a "standard" gaussian blur (non-adaptive) does, except that you will keep some noise texture on flat region.
Even from an artistic point of view, I don't think this is of much interest

Thanks David for your reply.
You are right in 90% of the cases or even more.
But I have in mind situations in which:
1-the edges are too "hard" (lines having a width of 1-3 pixels too heavily contrasting with the rest of the image)
2-the non-edges areas have a nice (not "noise") structure to be preserved
In those cases, the standard non-selective gaussian blur will destroy the non-edges fine structure.
So, if the filter is writable easily, I will appreciate if implemented.
But only if it doesn't take too much of your precious time, always devoted to many interesting topics.
Thanks