Curves Tool

Gimp version: 2.6.1
The Curves tool can be activated through Colors -> Curves or Tools -> Color Tools -> Curves.
The Curves tool is the most sophisticated tool for changing the color, brightness, contrast or transparency of the active layer or a selection. While the Levels tool allows you to work on Shadows and Highlights, the Curves tool allows you to work on any tonal range. It works on RGB images.

You have to select the channel on which do you want to make changes. At any time, if you want to cancel changes made on a channel and preserve the others you can click on the "Reset Channel" button.

You can change the histogram scale like for Levels tool.

The horizontal gradient represents the input tonal scale (like the horizontal gradient just below the histogram in the Levels tool). It, too, ranges from 0 (black) to 255 (white), from Shadows to Highlights. When you adjust the curve, it splits up into two parts; the upper part then represents the tonal balance of the layer or selection.

The vertical gradient represents the destination, the output tonal scale. It ranges from 0 (black) to 255 (white), from Shadows to Highlights (like output gradient in Levels tool).

The curve is drawn on a grid and goes from the bottom left corner to the top right corner. The pointer x/y position is permanently displayed in the top left part of the grid. By default, this curve is straight, because every input level corresponds to the same ouput tone. GIMP automatically places an anchor at both ends of the curve, for black (0) and white (255).

If you click on the curve, a new anchor is created. When the mouse pointer goes over an anchor, it takes the form of a small hand. You can click-and-drag the anchor to bend the curve. If you click outside of the curve, an anchor is also created, and the curve includes it automatically.

Meanwhile, on the canvas, the mouse pointer has the form of an eye-dropper. If you click on a pixel, a vertical line appears on the chart, positioned to the source value of this pixel in the selected channel. If you Shift-click, you create an anchor in the selected channel. If you Ctrl-click, you create an anchor in all channels, possibly including the Alpha channel. You can also Shift-drag and Ctrl-drag: this will move the vertical line and the anchor will show up when releasing the mouse left button.

For more detains on usage and features of the the Curves tool please consult the Gimp manual.