...a useful adjustment that increases the gain in deep shadow portions,while at the same time keeps *untouched* the already bright areas(above a certain predefined level)

Experts: is there a way to select just Shadows or just Midtones, or just Highlights?

Then one could just use the Levels or Curves tool on the selection.

I know that Color Balance can confine the Range To Adjust to one of these three samed subsets of the image, but then one would have to change Cyan-->Red, Magenta-->Green, and Yellow-->Blue separately (and even if one could "chain" them together, I am not sure that raising/lowering them together would achieve the same effect as raising or lowering Levels on the selection...) [By the way, how are Shadows/Midtowns/Highlights defined? Is there some standard, or does GIMP have its own definitiion, which might not equal some other one...?]

I google'd "GIMP select just Shadows", and found one pertinent result (from David Gowers on 2009-04-07): http://www.gimpusers.com/forums/gimp-user/11373-select-shadows which says:
"I typically use QuickMask mode.
1. Copy the image
2. Enter QuickMask mode
3. Paste the image and Anchor it
4. Colors->Auto->Normalize
5. Exit QuickMask mode
The above will select pixels based on their intensity -- a pixel of 0intensity being 0% selected, 127 intensity == 50% selected, 255 ==100% selected. You could modify this with various tools in the Colors menu to get thedesired effect. For instance, your original 'select the shadows' idea could be done by simply1. Colors->Invert 2. Adjust with Colors->Curves to taste, or threshold usingColors->Threshold if you wanted a hard-edged selection The above is the advanced method, with more control andsophistication. However, simply setting the 'select by' option to'Value' for the 'select by color' tool might do what you want. Hope that helps!
David
"