# Adjust Levels

Gilles
Caulier

    caulier dot gilles at free.fr

2004-11-07
0.7.0

The Digikam image plugin Adjust Levels is a tool for to adjust manually the histogram channels of an image.

KDE
Digikam

# Introduction

A more sophisticated than Brightness/Contrast/Gamma Image Editor tool, and only slightly more difficult, way of correcting exposure problems is to use this plugin. The dialog for this tool looks very complicated, but for the basic usage we have in mind here, the only part you need to deal with is the Input Levels area, specifically the sliders that appear below the histogram.

This plugin contains a visual graph of the intensity values of the active layer or selection (histogram). Below the graph are five sliders that can be clicked and dragged to constrain and change the intensity level for the image. The left sliders position represents the dark areas and similarly, the right position represents the light areas.

## Using the plugin

Actually the easiest way to learn how to use it is to experiment by moving the three sliders around, and watching how the image is affected.

On the right, a both original and target preview image is available. The target preview is re-actualized dynamically, in according with the plugin settings. On the left, the following options are available:

- Modify levels for Channel : this combo box allows the selection of the specific channel that will be modified by the tool:

  - Luminosity: this option makes intensity changes against all pixels in the image.

  - Red: this option makes Red saturation changes against all pixels in the image.

  - Green: this option makes Green saturation changes against all pixels in the image.

  - Blue: this option makes Blue saturation changes against all pixels in the image.

  - Alpha: this option makes transparency changes against all pixels in the image.

- Modify Scale for channel : this combo change whether the histogram will be displayed using a linear or logarithmic Y axis. For images taken with a digital camera, the linear mode is usually the most useful. However, for images that contain substantial areas of constant color a linear histogram will often be dominated by a single bar. In this case a logarithmic histogram will often be more useful.

- Input Levels: the input levels allow manual levels to be selected for each of the ranges. The main area is a graphic representation of image dark, mid and light tones content. 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 (histogram). 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 two ways:

  - Three sliders: the first on the top for dark tones, the second one on the middle for light tones, and the last one on the bottom for midtones (often called Gamma value).

  - Three input boxes to enter values directly.

- Output Levels: the output levels allows manual selection of a constrained output level range. There are also two sliders located here that can be used to interactively change the output levels like Input Levels.

- Auto: this button performs an automatic setting of the levels based on the pixel intensities of the image.

- Save and Load: these buttons are used to do just that. Any Levels that you have set can be saved to the filesystem and loaded later. The used file format is The Gimp Levels format.

- Reset All Values: this button reset all Input Levels an Output Levels values for all channels.

## The plugin in action

Below, you can see an Input levels adjustment example apply on a photograph for the Red/Green/Blue channels. Intensity/Alpha channels and Output Levels are unchanged.

The Adjust Levels Image Editor Plugin in Action