Curves

From the Curves dialog box, you can change the image effect by adjusting the curves 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. To change the level, first select the channel. If the Master box is checked, the adjustment will apply to all the three color channels (red, green and blue). If another box is checked, the adjustment will only apply to that color channel. After choosing the channel, click on the tonal map or drag the diagonal line with your mouse and the line will become curved, while the input/output ratio is changed. When you get the desired effect, press the OK button to confirm your settings. Otherwise, you can press the Reset button to reset them, or you can press the Cancel button to cancel your settings.