Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 67

The curve in the Corrections settings affects the brightness of the image.
You can edit this curve the same way you edit the base curve to increase image
contrast. The base curve is intended to apply readily available curves to the image.
When simply touching up the contrast of an image, use the Corrections control.
The settings left of the curve window control the black point. If your
image seems foggy or washed out, you can try to correct the image by clicking
the Auto adjust black point icon.
The Auto adjust curve (Flatten histogram) icon to the right creates a color
histogram that flattens an image. The picture can gain immensely in contrast,
but sometimes the result can appear artificial.
With the help of the curve Corrections , I correct and fine-tune the settings
from the Base curve . I slightly reduced the shadows as well as the highlights
by dragging the curve down in these sections. However, I lifted the midtones
to make the image appear brighter and fresher.
An essential setting in Corrections is Saturation . I raised the saturation a little
in this example image from 1.00 to 1.15 to intensify the colors. However, I raised
the saturation after correcting the curve, as this actually improves the colors.
Figure 2.9
The Corrections settings increased the saturation, the shadows and highlights were darkened with the help of the curve function, and the
midtones were brightened. The image now has fresh colors, without any over- or underexposed areas.