Quoting norseman <norse...@hughes.net>:
> In Gimp, to add (SUM) the RGB values for a pixel and then
> change all pixels who's RGB SUM is greater than a user stated value to
> Second side of that is to change all RGB SUMs less than value given to
> Black. BUT not doing both at same time, thus allowing user to "step" in
> and see what got lost.
* Duplicate the layer.
* Desaturate the duplicate using the "Average" method.
* Add layermask to the duplicate using the
"Initialize to: Grayscale Copy of Layer" option.
* Bucket fill the duplicate layer with white.
* Run "Colors->Threshold" on the layermask, setting
the value appropriately.
* Bucket fill your original layer with black.
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