[Gimp-user] how to obtain the same adjustment in GIMP like this one in Photoshop Express

[scott092707 aol com]

> ...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
"