8.10. Niveles

The Level tool provides features similar to the Histogram dialog but can also change the intensity range of the active layer or selection in every channel. This tool is used to make an image lighter or darker, to change contrast or to correct a predominant color cast.

8.10.1. Activar la herramienta

You can get to this tools in several ways:

  • En el menú de imagen a través de HerramientasHerramientas de colorNiveles….

  • En el menú de imagen a través de ColoresNiveles….

  • By clicking on the tool icon in the toolbox if this tool has been installed there. For this, please refer to Sección 1.11, “Caja de herramientas”.

8.10.2. Opciones

Figura 16.152. Opciones de la herramienta «Niveles»

Opciones de la herramienta «Niveles»

Presets

You can save the color settings of your image by clicking the Add settings to favourites button

The button opens a menu:

Figura 16.153. Preset Menu

Preset Menu

which lets you Import Settings from File or Export Settings to File, and gives you access to the Manage Save Settings dialog:

Figura 16.154. Manage saved Settings Dialog

Manage saved Settings Dialog

Channel

You can select the specific channel which will be modified by the tool:

  • Value makes changes to the value of all RGB channels in the image: the image becomes darker or lighter.

  • Red, Green and Blue work on a particular color channel: the image gets more or less color. Remember that adding or removing a color result in removing or adding the complementary color

  • Alpha works on semi-transparent layers or selections: here, dark means more transparency, and white is fully opaque. Your image must have an Alpha Channel, otherwise this option is disabled.

  • Initialize channel cancels changes to the selected channel.

Input Levels

The main area is a graphic representation of the active layer or selection dark (Shadows), mid and light (Highlight)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 (Shadows), one grey for midtones (Gamma), one 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).

  • Two eye-droppers: when you click them, the mouse pointer becomes an eye-dropper. Then clicking on the image determines the black or the white point according to the chosen eye-dropper. Use the left, dark one to determine the black-point; use the right, white one to determine the white point.

  • Three numeric text boxes to enter values directly.

Input Levels are used to lighten highlights (bright tones), darken shadows (dark tones), change the balance of bright and dark tones. Move sliders to the left to increase lightness (increase the chosen color / increase opacity). Move the sliders to the right to lessen lightness (lessen the chosen color / lessen opacity).

Examples for Input Levels

The original image is a gray-scaled image with three stripes: Shadows (64), Mid Tones (127), Highlights (192). The histogram shows three peaks, one for each of the three tones.

Imagen original

  1. The Value channel is selected. The black slider (Shadows) has been moved up to the Shadows peak. The 64 value became 0 and the Shadows stripe became black (0). The Gamma (mid tones) slider is automatically moved to the middle of the tone range. Mid tones are made darker to 84 and Highlights to 171.

    Black slider has been moved

  2. The white slider (highlights) has been moved up to the highlight peak. The 192 value became 255 and the highlight stripe became white. The Gamma (mid tones) slider is automatically moved to the middle of the tone range. Mid tones are made lighter to 169 and Shadows to 84.

    White slider has been moved

Output Levels

Output levels allows manual selection of a constrained output level range. There are also numeric text boxes with arrow-heads located here that can be used to interactively change the Output Levels.

Output levels force the tone range to fit the new limits you have set.

  • Working with Value: values are compressed and look more alike; so contrast is reduced. Shadows are made lighter: new details can show up but contrast is less; a compromise is necessary. Highlights are made darker.

  • Trabajar con canales de color: si usa el canal verde por ejemplo y configura los niveles de salida entre 100 y 140, todos los píxeles con algo verde, incluso con un valor bajo, tendrán su valor del canal verde desplazado entre 100 y 140.

  • Trabajar con el canal alfa: todos los valores alfa se desplazarán al rango que haya establecido.

Ejemplo de «Niveles de salida»

  1. La imagen original es un degradado RGB desde negro (0;0;0) a blanco (255;255;255). Los niveles de salida no tienen histograma; aquí se usa VentanasDiálogos empotrablesHistograma.

    Imagen original (un degradado)

  2. Canal valor seleccionado. El deslizador negro se ha movido a 63 y el blanco a 189. El histograma muestra la compresión de los píxeles. Ningún píxel es menor que 63 y ninguno es mayor que 189. En la imagen, las sombras son más claras y las luces más oscuras: se reduce el contraste.

    Black slider has been moved

Todos los canales

Auto: realiza una configuración automática de los niveles.

Three eyedroppers , , . .  These three buttons respectively represent a white, a gray and a black eye-dropper. When you click one of these buttons, the mouse pointer takes the form of the eye-dropper it represents. Then, when clicking the image, the clicked pixel determines the white point , the black point or the mid point according to the eye-dropper you chose. Works on all channels, even if a particular channel is selected.

Figura 16.155. Example for Levels eye-droppers

Example for Levels eye-droppers

Above is original gradient from black to white. Below is the result after clicking with the white eye-dropper: all pixels with a value higher than that of the clicked pixel turned to white.


Edit these settings as Curves

To make your work easier, this button lets you turn to the Curves tool with the same settings.

Vista previa

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

8.10.3. Tool Options dialog

Figura 16.156. Opciones de la herramienta Niveles

Opciones de la herramienta Niveles


Although this tool is not present in the Toolbox by default (please refer to Sección 1.11, “Caja de herramientas” if you want to add it), 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.

8.10.4. Práctica real

Figura 16.157. Una imagen muy subexpuesta

Una imagen muy subexpuesta

Imagen original

Una imagen muy subexpuesta

El histograma muestra un predominio de las sombras y ausencia de las claras.


Figura 16.158. Ajustar el punto blanco

Ajustar el punto blanco

The white slider has been moved to the start of well marked Highlights. The image lightens up.

Ajustar el punto blanco

The resulting histogram (down) shows Highlights now, but Shadows are still predominant.


Figura 16.159. Setting the balance between Shadows and Highlights

Setting the balance between Shadows and Highlights

The mid slider has been moved to the left. This results in reducing the proportion of Shadows and increasing the proportion of Highlights.

Setting the balance between Shadows and Highlights

The resulting histogram (down) confirms the reduction of Shadows.