How to use threshold tool in GIMP software
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The Threshold tool transforms the current layer or the selection into a black and white image, where white pixels represent the pixels of the image whose Value is in the threshold range, and black pixels represent pixels with Value out of the threshold range.
You can use it to enhance a black and white image (a scanned text for example) or to create selection masks.
[Note] Note
As this tool creates a black and white image, the anti-aliasing of the original image disappears. If this poses a problem, rather use the Levels tool.
8.53.1. Activating the Tool
There are different possibilities to activate the tool:
You can access this tool from the image menu through Tools → Color Tools → Threshold…,
or through Colors → Threshold…,
or by clicking on the icon in Toolbox if this tool has been installed in it. For this, please refer to Section 1.11, “Toolbox”.
8.53.2. Options
Figure 16.233. Threshold tool options
Threshold tool options
Presets
You can save the color settings of your image by clicking the Add settings to favourites button
The button opens a menu:
Figure 16.234. 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:
Figure 16.235. Manage saved Settings Dialog
Manage saved Settings Dialog
Threshold range
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 input boxes or clicking button 1 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 will be white, and the others will be black. Adjust the range to get the selection you want in white on black background.
Preview
The Preview toggle allows dynamic updating of the active layer or selection while changes are made to the intensity level.
8.53.3. Using Threshold and Quick Mask to create a selection mask
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.
First start decomposing you image into its RGB and HSV components by using the Decompose filter. A new grey-scaled 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.
You can use it to enhance a black and white image (a scanned text for example) or to create selection masks.
[Note] Note
As this tool creates a black and white image, the anti-aliasing of the original image disappears. If this poses a problem, rather use the Levels tool.
8.53.1. Activating the Tool
There are different possibilities to activate the tool:
You can access this tool from the image menu through Tools → Color Tools → Threshold…,
or through Colors → Threshold…,
or by clicking on the icon in Toolbox if this tool has been installed in it. For this, please refer to Section 1.11, “Toolbox”.
8.53.2. Options
Figure 16.233. Threshold tool options
Threshold tool options
Presets
You can save the color settings of your image by clicking the Add settings to favourites button
The button opens a menu:
Figure 16.234. 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:
Figure 16.235. Manage saved Settings Dialog
Manage saved Settings Dialog
Threshold range
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 input boxes or clicking button 1 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 will be white, and the others will be black. Adjust the range to get the selection you want in white on black background.
Preview
The Preview toggle allows dynamic updating of the active layer or selection while changes are made to the intensity level.
8.53.3. Using Threshold and Quick Mask to create a selection mask
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.
First start decomposing you image into its RGB and HSV components by using the Decompose filter. A new grey-scaled 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.
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