# Adjust Curves

Gilles
Caulier

    caulier dot gilles at free.fr

Gerhard
Kulzer

    gerhard at kulzer.net

2005-02-26
0.7.2

The Digikam image plugin Adjust Curves is a tool that allows histogram channel curved adjustments of color corrections.

KDE
Digikam

# Introduction

The Curves tool is the most sophisticated tool used to adjust the images' tonality. Start it from the Fix Colors Curves Adjust Image Editor menu.

This plugin provides visual curves to modify the intensity values of the active layer (histogram) in a non-linear way. In Smooth curve edition mode, you change the curves shape by adding new curve points or by moving end point positions. Another mode is to draw all the curve manually in Free curve edition mode. In both cases the effect is immediately available in image preview area.

## Using the plugin

To the right, both an original and a target preview image is shown. The target preview is updated dynamically according to the plugin settings. To the left the following options are available:

- Modify Channel : with this combo box you can select the specific channel to be modified by the tool:

  - Luminosity: changes the intensity of all pixels.

  - Red: changes the Red saturation of all pixels.

  - Green: changes the Green saturation of all pixels .

  - Blue: changes the Blue saturation of all pixels.

  - Alpha: chages the transparency of all pixels.

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

- Curve Type for channel : this combo box determines whether the curve can be edited using a Smooth curve or a Free hand mode curve. Smooth mode constrains the curve type to a smooth line with tension and provides a realistic render. Free mode lets you draw your curve free-hand with the mouse. With curve segments scattered all over the grid, result will be surprising but hardly repeatable.

- Main Curves Editing Area: the horizontal bar (x-axis) represents input values (they are value levels from 0 to 255). The vertical bar (y-axis) is only a scale for output colors of the selected channel. The control curve is drawn on a grid and goes from bottom left corner to top right corner. Pointer x/y position is permanently displayed on the upper left part of the grid. If you click on the curve, a control point is created. You can move it to bend the curve. If you click outside the curve, a control point is also created, and the curve includes it automatically. So each point of the curve represents an 'x' level, corresponding to an 'y' color. If, for example, you move a curve segment to the right, i.e. to highlights, you can see that these highlights are corresponding to darker output tones and that image pixels corresponding to this curve segment will go darker. With color channels, moving right will decrease saturation up to reaching complementary color. To delete all control points (apart from both end points), click on the Reset Values button. To delete only one point, move it onto another point.

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

- Reset All Values: this button resets all curve values for all channels.

## The plugin in action

Below you can see a curve correction example appied to a photo in the Blue channel.

The Adjust Curves Image Editor Plugin in Action