The color curves layer allows you to control the tonal range of an image. Add a curves layer by clicking the Color Curves icon on the Color tab of the toolbar.
The channel pulldown lets you pick a color channel to adjust.
- RGB will adjust all color channels at once.
- Red will only adjust the red values of your image.
- Green will only adjust the green values of your image.
- Blue will only adjust the blue values of your image.
The horizontal axis of the graph represents the input values of each color in your image, with dark values on the left and light values on the right.
The vertical axis of the graph represents the output of the effect, with dark values on the bottom and light values on the top.
The curve represents how the colors in your image are mapped from input values to output values.
Click anywhere on the graph to add a control point. The control point can be dragged to change the curve. The control point can also be reset/deleted either by selecting it, then press delete key or press the mouse's right click in front of the control point
Steep sections of the curve will result in high contrast. Flat sections of the curve will result in low contrast.