Making Color Corrections with GIMP

The first step when editing most images is to correct the brightness, contrast, and color saturation. This helps overcome some of the deficiencies that are commonly found in digital photographs or scanned-in images. To do this, right-click the image and select Colors. You’ll find a variety of options to let you tweak the image, allowing you a lot of control over the process.

For simple brightness and contrast changes, selecting the Brightness-Contrast menu option will open a dialog box where clicking and dragging the sliders will alter the image.
The changes you make will be previewed on the image itself, so you should be able to get things just right.

Similarly, the Hue-Saturation option will let you alter the color balance and the strength of the colors (the saturation) by clicking and dragging sliders. By selecting the color bar options at the top of the window, you can choose individual colors to boost. Clicking the Master button will let you once again alter all colors at the same time.

The trouble with clicking and dragging sliders is that it relies on human intuition. This can easily be clouded by a badly calibrated monitor, which might be set too dark or too light. Because of this, GIMP offers another handy option: Levels.

To access the Levels feature, right-click the image and select Colors -> Levels. This presents a chart of the brightness levels in the photo and lets you set the dark, shadows, and highlight points. Three sliders beneath the chart represent, from left to right, the darkest point, the midtones (shadows), and the highlights within the picture. The first step is to set the dark and light sliders at the left and right of the edges of the chart. This will make sure that the range of brightness from the lightest point to the darkest point is set correctly. The next step is to adjust the middle slider so that it’s roughly in the middle of the highest peak within the chart. This will accurately set the midtone point, ensuring an even spread of brightness across the image.

A little artistic license is usually allowed at this stage, and depending on the effect you want in the photo, moving the midtone slider a little to the left and/or right of the highest peak might produce more acceptable results. However, be aware that the monitor might be showing incorrect brightness or color values.

Source of Information : Apress Beginning Ubuntu Linux 3rd Edition