Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 65

The drop-down menu offers you two choices to begin with: Linear curve
or Manual curve . Linear curve corresponds to the image's existing settings.
Manual curve lets you correct the brightness and contrast settings separately,
using various brightness ranges (see section 2.5.9 on gradation curves).
If you click the lower area of the linear curve and drag it downward while
pressing the left mouse button, you'll notice that the dark shades in the image
become darker. Repeat the process for the upper area of the curve, dragging
upward this time, and the bright colors become brighter. By experimenting,
you can correct any brightness problems or just creatively play with the image.
You can produce a color inversion of a true photonegative by dragging both
end points of the curve vertically. You can also create curves of your own and
save them to reuse with your images by clicking the Save base curve icon (it
looks like a disk).
The base curve is applied directly to every color channel. You should
apply it after you apply the exposure and white balance settings so that it
affects each color channel equally. However, you should do this before the
gamma correction so it affects only the linear data.
I use the base curve function to bring light into dark areas of backlit photos
and darken overexposed areas in photos to achieve more contrast. For best
results, I place three dots on the linear, diagonal curve. First, I place the point
at the bottom left. This section represents the shadows, the dark areas of the
image. By holding the left mouse button and dragging the dot upward, I lighten
up the dark regions in the photo, thereby altering the entire course of the curve.
It shifts upward so that all colors become brighter. To counteract the brightening
of the colors, I place the second dot in the middle of the curve and drag it back
to the center. The middle tones now correspond to the initial setting again. Then
I place one dot at the top right of the curve and drag it downward. This darkens
the highlights and the bleached-out sky gains in contrast. One additional, last
dot to correct the curve for the mid shadows and I'm done.
Color Management
Color management, or color profiling, involves the color space of the different
devices being used. The use of color profiles secures a constant rendition of colors
between the various devices and various programs when you're editing a picture.
For our purposes, sRGB is the standard color profile—it is essential for
cameras and monitors. This is the only color profile UFRaw offers. Other color
profiles, such as Adobe RGB (for printing purposes), can be found online. As
soon as you save a color profile on your hard disk, you can load it into the
program over the folder icon (next to the drop-down menu).
A click on one of the folder icons opens the Load color profile window.
You can use it to search your computer for installed color profiles (ICC and ICM
files). Under Windows Vista, you will find it in the Windows installation folder.
On my computer, I found it under the following path: C:\Windows\System32\