Modify Levels for Channel

You can select the specific channel which will be modified by the tool: Value makes changes to the luminosity of all pixels in the image. Color channels allow to change saturation. Alpha channel works on transparency. Initialize channel cancels changes to the selected channel.

Input Levels

The main area is a graphic representation of image dark, mid and light tones content (the Histogram). They are on abscissa from level 0 (black) to level 255 (white). Pixel number for a level is on ordinate axis. The curve surface represents all the pixels of the image for the selected channel. A well balanced image is an image with levels (tones) distributed all over the whole range. An image with a blue predominant color, for example, will produce a histogram shifted to the left in Green and Red channels, signified by green and red lacking on highlights.

Level ranges can be modified in three ways:

• Three triangles as sliders: one black for dark tones, one grey for midtones (often called Gamma value), one white for light tones.

• Two eyedroppers: use the left one to pick the darkest color and the right one to pick the lightest color on the image. You can use View/Info Window (Cursor tab) to find these colors.

• Three input boxes to enter values directly.

Output Levels

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.

All Channels

Open: This button allows you to select a file holding level settings.

Save: Allows you to save any levels you have set to a file that can be loaded later.

Auto: Performs an automatic setting of the levels.

Three eyedroppers: These three controls determine three points on the grayscale. Any level below the "Black Point" is black; any point above the "White Point" is white. The "Gray Point", which must be between the other two, determines the middle level of Gray. All shades of gray are calculated from these three levels.

Preview

The Preview button makes all changes to the levels dynamically so that the new level settings can be viewed straight away.

Tool Options dialog

Although this tool is not present in Tool box, nevertheless it has a Tool Option Dialog under the Toolbox. These options are described here:

Histogram Scale

These two options have the same action as the logarithmic and Linear buttons in the Levels dialog.

Sample Average

This slider sets the “radius” of the color-picking area. This area appears as a more or less enlarged square when you maintain the click on a pixel.