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
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.
5.5.1. Activate Tool
The Threshold Tool can be called in the following
order, from the image-menu:
Tools->Color Tools->Threshold,
or by clicking on the
icon in Toolbox if this tool has been installed in it. You can do
that through the Tool dialog.
5.5.2. Options
Figure 8.63.
Threshold tool options
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.
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.
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.
Figure 8.64.
The original image, the decompose image
and its Layer Dialog
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
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.
Figure 8.65.
The selected layer after threshold fit
We got the best outline for our flower. There are several
red objects which we must remove.
Make sure the image displaying the selected layer is active and
copy it to the clipboard with
Ctrl-C.
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 Attributs. Come back to the
original image. Press
Ctrl-V to
paste the previously copied layer.
Figure 8.66.
The mask
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.
Figure 8.67.
The result
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.
Published under the terms of the GNU General Public License
![[Note]](../images/note.png)
![[Warning]](../images/warning.png)
