The Levels tools provides similar features to the Histogram tool but can also change the intensity range of the active layer or selection.
The Levels tool contains a visual graph of the intensity values of the active layer or selection. Below the graph are three Arrow-heads that can be clicked and dragged to constrain and change the intensity level for the active layer or selection. The black Arrow-head represents the dark areas and similarly, the white Arrow-head represents the light areas.
Levels Tool Settings
The Modify Levels for Channel button allows the selection of the specific channel that will be modified by the tool.
Value: This option makes intensity changes against all pixels in the image.
Red: This option makes changes to he Red channel only.
Green: This option makes changes to the Green channel only.
Blue: This option makes changes to the Blue channel only.
The Input Levels input boxes allow manual levels to be selected for each of the ranges.
Output Levels allows manual selection of a constrained output level range. There are also Arrow-heads located here that can be used to interactively change the Output Levels.
This button performs an automatic setting of the levels based on the pixel intensities of the image.
The Save and Load buttons are used to do just that. Any Levels that you have set can be saved to the filesystem and loaded later.
The Preview button makes all changes to the levels dynamically so that new Levels settings can be viewed straight away.