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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]](images/note.png)
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Note |
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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.
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5.5.1. Activating the Tool
There are different possibilities to activate the tool:
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You can access this tool from the image menu through
→ → ,
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or through
→ ,
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or by clicking on the
icon in Toolbox if this tool has been installed in it. You can do
that through the Tool dialog.
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Presets
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You can save the color settings of your image by clicking the
Add settings to favourites button
The
button opens a menu:
which lets you Import Settings from File or
Export Settings to File,
and gives you access to the Manage Save Settings dialog:
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Threshold range
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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.
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Preview
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The Preview toggle allows dynamic updating of the active layer or
selection while changes are made to the intensity level.
5.5.3. 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.
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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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Call the Threshold tool from the decompose image. By moving the
black cursor, fit threshold to isolate the best the element you want
to extract. This will probably not be perfect: we will enhance the
result with the selection mask we are going to create.
![[Warning]](images/warning.png)
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Warning |
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Make sure you have selected the right layer when you call the
Threshold tool: when it is opened, you can't change to another
layer.
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Make sure the image displaying the selected layer is active and
copy it to the clipboard with
Ctrl+C.
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Now, make the original image active. Click on 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.
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Voilà. Your selection mask is ready: you can improve the selection
as usually. When the selection is ready, disable the Quick mask by
clicking again on its button: you will see the marching ants around
the selection.
We used the Zoom to work at a pixel level, the Lasso 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.
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