1. Press at the toolbar and such a dialog box will
pop up:
In this dialog box, you can change the image effect by adjusting the curve of the image. In the tonal map, the horizontal axis represents the original input brightness and the vertical axis represents the enhanced brightness. When you open this dialog box, the diagonal line in the tonal map is straight which means the brightness level ratios between the input image and the output image is equal at any point.
2. Choose the channel.
3. Move the cursor to the tonal map, and its shape will turn to a cross. Click on any point or drag the diagonal line to the point in the map, and the input/output brightness ratio is changed.
4. Press the "OK" button to save your setting and close this dialog box.
*To cancel your adjustment, press the "Cancel" button or "Reset" button to restore the default setting.