Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 120

Figures 2.47
Comparing the image, before and after
At the top of the Levels dialog is a Preset drop-down menu. Since GIMP
2.6, you can find this in almost all program windows. You can use this menu to
name and save your settings.
Click the Channel: Value drop-down menu (top left in Figure 2.46) to edit
each of the red, green, and blue color channels individually. This is important
when working with color cast images (more about this later).
To the right of the Channel: Value menu, you'll see two additional buttons.
If you hover your cursor over these buttons, you'll see that the left button
is called Linear and the right button is called Logarithmic . Depending on the
button you select, the representation of the histogram curve will change.
The logarithmic method is more data intensive, thus also more exact. The
histogram you see when you select the linear method is more significant,
so this is the method that was chosen for this exercise. For most images the
logarithmic histogram is smooth and quite flat.
You also find buttons represented by black, gray, and white eyedropper
icons. If you select the black eyedropper and click on an area of the image
that should be set in pure black, the program will recalculate the lightness
values. The same holds true for the white eyedropper. Using the black and
white eyedroppers may be sufficient to obtain a good tonality correction. The
additional gray eyedropper can be used to set the midtones of the image. This
can be helpful when working with colorcast images because it tells the program
what hue you want to assign to a gray shade (e.g., a shadow on a white surface).
If you wish, you can open the image with the settings that you just made
for further editing in the Curve function (gradation curves) by selecting the
Edit these Settings as Curves button.
The Reset button allows you to discard your settings without closing the
window.