Documentation

Curves

The Curves adjustment can be used to adjust the colors and/or luminosity of an image with a very high degree of control. Curves allows you to fine tune the intensity of every shade in an image. In particular, it can be used to highlight or dim only certain ranges of intensity present in an image in a way that Levels and Brightness / Contrast cannot.

Unlike Levels and Brightness / Contrast, Curves can be used to freely adjust the exactly how bright each shade of intensity in an image can be. In fact, the colors of an image can be completely inverted using curves.

For our curves examples, we will be using the following image:

Original Image

Original Image

Interface

Curves lets you remap the colors of an image using control points on a graph, as in the examples below. When the Curves dialog first appears it will have the graph shown below. This graph is simply a function in the mathematical sense. The horizontal values are the intensity input, and the vertical values are the intensity output.

Curves

Curves

Put more simply, this means that reshaping the left half of the curve affects the darker half of an image, and reshaping the right half of the curve affects the brighter half of the image. Moving a curve above its default position on the diagonal guide line will cause the affected areas to brighten, moving a curve below its default position on the diagonal guide line will cause affected areas to darken.

Example

Lighter Curves

The left side of the curve (darker areas in the picture) has been moved up vertically (lightened)


Lighter Apple

The resulting image

Example

Lighter Curves

The right side of the curve (lighter areas in the picture) has been moved down vertically (darkened)


Lighter Apple

The resulting image

Control points are used to affect the curve. Control points are added by clicking anywhere that there is not currently a control point. Control points may be removed by right-clicking on the desired control point. To move a control point, simple click and drag the control point to the desired new location. Note that it is not possible to have two control points vertically in line with each other, as this would mean that a given intensity of color would be both brightened and darkened (which isn't possible).

Multichannel Adjustments

By default, Curves uses the luminosity of an image to adjust the intensities. Alternatively, this can be performed using the Red, Green, and Blue channels of an image by selecting "RGB" from the drop down list near the top of the dialog box.

When this mode is selected, new control points will apply to the color channels selected with the checkboxes near the bottom of the dialog. To work with an individual channel or pair of channels, simple change which channels are selected using the checkboxes.

Example

Lighter Curves

The picture has been adjusted with RGB Curves to increase the saturation of the image. Notice how the Blue and Green channels have been reduced and the red increased, resulting in a much more vibrant apple


Lighter Apple

The resulting image

Other Possibilities

As stated above, you can even use Curves to invert an image without reversing colors. In the example below, our apple was adjusted where all dark areas are lightened and all light areas are darkened. Using Curves can produce a result that is not possible with the other adjustments, such as Levels or Hue / Saturation.

Example

Lighter Curves

The curves of the image have been inverted, dark areas are completely lightened and light areas are completely darkened


Lighter Apple

The resulting image