Levels Tool

Gimp version: 2.6.1
The Levels tool can be activated through Colors -> Levels or Tools -> Color Tools -> Levels.

The Levels tool shows you the distribution of color values in the active layer or selection. This information is often useful when you are trying to color balance an image. This tool is used to make an image lighter or darker, to change contrast or to correct a predominant color cast.

Fist of all you have to select the channel on which do you want to make changes. On a color image you can work with red, green, blue and value channels. On gray-scale (black and white) images, only value is available. On a color image, the value channel will contain the maximum value from the other channels on each pixel. Otherwise said, each pixel is represented in the value channel as MAX(red,green,blue).

At any time, if you want to cancel changes made on a channel and preserve the others you can click on the "Reset Channel" button.

At the right of the reset channel button are two buttons which allow you to switch the histogram scale: linear or logarithmic. Most of the time you will prefer the linear type. Only switch to the logarithmic scale if linear does not provide valuable representation, like in this example:



The most important part of this tool is the histogram. On horizontal, from left (darkest - 0) to right (brightest - 255) are the luminance values for the selected channel. On vertical is represented the number of pixels in the active layer for the corresponding luminance value on the selected channel. A well balanced image is an image with levels (tones) distributed all over the whole range. Too much levels on left means a darken image, too much on right the opposite.

Level ranges can be modified using those tree three slider triangles below the histogram: black for dark tones (Shadows), gray for mid-tones (Gamma), white for light (Highlights) tones. The black slider determines the black point : all pixels with this value or less will be black (no color with a color channel selected / transparent with the Alpha channel selected). The white slider determines the white point : all pixels with this value or higher, will be white (fully colored with a color channel selected / fully opaque with the Alpha channel selected). The gray slider determines the mid point. Going to the left, to the black, makes the image lighter (more colored / more opaque) . Going to the right, to the white, makes the image darker (less colored / more transparent). Alternatively for these sliders the eye-droppers or the numeric text boxes can be used.

Output levels allows manual selection of a constrained output level range. Output levels force the tone range to fit the new limits you have set.

At the bottom of the tool dialogue panel there are two more helpful options, buy these ones will affect all the channels. The first one is the auto button, which you will find often really helpful. Basically it tries to determinate the proper adjustments for the levels on all the channels.

A nice feature is represented by the three eye-droppers at the right of the auto button. This buttons allows you to select a black point, a neutral one (gray point) and a white point. Using this into the levels tool will try to fix the colors balance. This feature is often useful for color cast corrections.

For more detains on usage and features of the the Levels tool please consult the Gimp manual.