8.54. Seuil

The Threshold filter transforms the current layer or the selection into a black and white image, where white pixels represent the pixels of the image where the value for the chosen Channel is in the threshold range, and black pixels represent pixels with a value outside the threshold range.

You can use it to enhance a black and white image (scanned text for example) or to create selection masks.

[Note] Note

As this filter creates a black and white image, the anti-aliasing of the original image disappears. If this poses a problem, use the Levels tool instead.

8.54.1. Activating the Filter

There are different possibilities to activate this:

  • This filter is found in the main menu under ColorsThreshold…,

  • or by clicking the icon in the Toolbox if Threshold has been enabled as a tool (which is disabled by default). For this, please refer to Section 6.13, « Boîte à Outils ».

8.54.2. Options

Figure 16.230. Threshold filter dialog

Threshold filter dialog


« Presets » are a common feature for several Colors commands. You can find its description in Section 8.1.1, « Fonctions communes aux couleurs ».


This allows you to select which channel(s) will be used to apply the Threshold. The default is Value, which will use all color channels, but you can also choose to use a single channel: Red, Green, Blue and Alpha (only available when the layer has an alpha channel), Luminance, or RGB.

Linear Histogram, Logarithmic Histogram

These two buttons to the right of the Channel selection let you choose the type of histogram shown.

Intervalle de seuils

The Threshold tool provides a visual graph, a histogram, of the intensity value of the active layer or selection. You can set the threshold range either using the minimum and maximum input boxes, or by clicking and dragging on the graph. It allows you to select a part of the image with some intensity from a background with another intensity. Pixels inside the range are white, and the others are black. Adjust the range to get the selection you want in white on black background. The Auto button can be used to automatically adjust the threshold.

Blending Options, Preview and Split view

These are common features described in Section 8.1.1, « Fonctions communes aux couleurs ».

8.54.3. Utilisation de Seuil et de Masque rapide pour créer un masque de sélection

That's not always the case, but an element you want to extract from an image can stand out well against the background. In this case, you can use the Threshold tool to select this element as a whole. Grokking the GIMP described a method based on a channel mask, but now, using the Quick mask is easier.

  1. First start decomposing your image into its RGB and HSV components by using the Decompose filter. A new grayscale image is created and the components are displayed as layers in the Layer dialog. These layers come with a thumbnail but it is too small for an easy study. You can, of course, increase the size of this preview with the dialog menu (the small triangular button), but playing with the « eyes » is more simple to display the wanted layer in the decompose image. Select the layer that isolates the element the best.

    Figure 16.231. Image d’origine, image décomposée et sa Boîte de dialogue des calques

    Image d’origine, image décomposée et sa Boîte de dialogue des calques
    Image d’origine, image décomposée et sa Boîte de dialogue des calques
    Image d’origine, image décomposée et sa Boîte de dialogue des calques

  2. Appelez l’outil Seuil à partir de l’image décomposée. En déplaçant le curseur noir, adaptez le seuil pour isoler au mieux l’élément à extraire. Ce ne sera probablement pas parfait : nous améliorerons le résultat grâce au masque de sélection que nous allons créer.

    [Avertissement] Avertissement

    Assurez-vous d’avoir sélectionné le bon calque quand vous appelez l’outil Seuil : quand il est ouvert, vous ne pouvez plus changer de calque.

    Figure 16.232. Le calque sélectionné après réglage du seuil

    Le calque sélectionné après réglage du seuil

    Nous obtenons le meilleur contour possible ici pour la fleur. Il existe plusieurs éléments rouges que nous devons enlever.

  3. Assurez-vous que l’image affichant les calque actif est active et copiez-la dans le presse-papier avec Ctrl+C.

  4. Now, make the original image active. Click the Quick Mask button at the bottom-left corner of the image window: the image gets covered with a red (default) translucent mask. This red color does not suit well to our image with much red: go to the Channel Dialog, activate the « Quick mask » channel and change this color with the Edit Channel Attributes. Come back to the original image. Press Ctrl+V to paste the previously copied layer.

    Figure 16.233. Le masque

    Le masque

  5. Voilà. Your selection mask is ready: you can improve the selection as usual. When the selection is ready, disable the Quick mask by clicking its button again: you will see the marching ants around the selection.

    Figure 16.234. Le résultat

    Le résultat
    Le résultat

    We used the Zoom to work at a pixel level, Free Select to remove large unwanted areas, the pencil (to get hard limits), black paint to remove selected areas, white paint to add selected areas, especially for stem.