The Channels dialog is the main interface to edit, modify and manage
your channels. Channels have a double usage. This is why the dialog is
divided into two parts: the first part for color channels and the
second part for selection masks.
Color channels: Color channels apply to the image and not to a specific
layer. Basically, three primary colors are necessary to render all the
wide range of natural colors. As other digital software, the
uses Red, Green, and Blue as primary colors. The first and primary
channels display the Red,
Green, and Blue
values of each pixel in your image. In front of each channel is a
thumbnail displaying a grayscale representation of each channel, where
white is 100% and black is 0% of the primary color. Alternatively, if your
image is not a colored but a Grayscale image, there is only one primary
channel called Gray.
For an Indexed image with a fixed number of known colors there is also
only one primary channel called Indexed.
Then there is a
optional channel called
This channel displays transparency values of each pixel in your image.
In front of this channel is a thumbnail displaying a grayscale
representation of the transparency where white is opaque and visible, and
black is transparent and invisible. If you create your image without
transparency then the Alpha channel is not present, but you can add it
Layers dialog menu.
Also, if you have more than one layer in your image, GIMP automatically
creates an Alpha channel.
GIMP doesn't support CMYK or YUV color models.
Representation of an image with channels
The right image is decomposed in three color channels (red, green, and
blue) and the Alpha channel for transparency. On the right image the
transparency is displayed as a gray checkerboard. In the color channel
white is always white because all the colors are present and black is
black. The red hat is visible in the red channel but quite invisible in
the other channels. This is the same for plain green and blue which are
visible only in their own channels and invisible in others.
Calling the Dialog
The Channel dialog can be activated in many ways :
from the toolbox-menu:
from the image-menu:
from another dialog-menu:
Using the Channel dialog
The top channels are the color channels and the optional Alpha
channel. They are always organized in the same order and they cannot
be erased. Selection masks are described below and displayed as a list
in the dialog. Every channel appears in the list in form of a
thumbnail. A right-click in a channel thumbnail opens the
Every channel is shown in the list with its own attributes. The main
attribute is the name of the channel itself. You can edit selection
masks by double-clicking on their name. A double_click on the
thumbnail opens a full dialog where you can also set the visual aspect
of the channel in the image window. In front of the thumbnail there is
an eye icon:
by clicking on it you define whether the channel is visible or not. As
a result of this visibility, the view of the image changes in the
image window and a white image becomes yellow if you remove the view
of the blue because yellow is the complementary color for blue. If you
remove the view of the Alpha channel, everything becomes transparent
and nothing else than a grey checkerboard is visible. The aspect of
this virtual background can be changed in the
Preferences. The chain icon
enables grouping of channels for operations on multiple channels.
Activated channels appear highlighted in blue in the dialog. If you
click on a channel in the list you toggle activation of the
corresponding channel. Disabling a color channel red, blue, or green
has severe consequencies. For instance if you disable the blue
channel, all pixels from now on added to the image will not have
blue component, and so a white pixel will have the yellow
Under the channel list is a set of buttons allowing you to perform
some basic operations on channel list.
Edit channel attributes,
only available for selection masks. Here you can change the
Channel name. The other two parameters
affect channel is visibility in the image window; they control
Opacity and color used for the mask in the
image window. A click on the color button displays the Gimp
color selector and then you can change the mask color.
you can create here a new channel available to save a selection.
Displayed dialog lets you change Opacity
and mask color used in the image to represent the selection.
only available for selection masks : you can here put the
channel up a level in the list. Press Shift
key to move channel to top of the list.
You can here put the channel down a level in the list. Press the
Shift key to move the channel to bottom of the
You can create here a copy of the active channel. Name of new
channel is suffixed with a number.
You can also duplicate a color channel or the Alpha channel.
It's an easy way to keep a copy of them and to use them later
as a selection in an image.
Channel to selection
here you can transform the channel to become a selection. By
default the selection derived from a channel replaces any
previous active selection. It's possible to change this by
clicking on control keys.
the selection derived from a channel is added to the
previous active selection. The final selection is merged
the final selection is the substraction of selection derived
from a channel from the previously active one.
the final selection is the intersection of selection derived
from a channel with the previously active one. Only common
parts are kept.
only available for selection masks: you can here delete the
Revision $Revision: 1.22 $
You can get the channel context menu by right clicking on a channel
thumbnail. This menu gives the same operations on channels as those
available from dialog buttons. The only difference concerns
transformation to selection operations, each of them having its own
entry in the menu.
Edit channel attributes,
Delete channel: see
Channel to selection:
Selection derived from channel replaces any previous active
Add to selection:
Selection derived from channel is added to previous active
selection. Final selection is merging of both.
Substract from selection:
Final selection is substraction of selection derived from a
channel from previous active selection.
Intersect with selection:
Final selection is intersection of selection derived from a
channel with the previous active selection. Only common parts
Revision $Revision: 1.29 $
A selection composed out of channels.
Channels can be used to save and restore your selections. Clicking
on the Quick mask button on the Image window automatically
creates a new channel called Qmask and saves the
displayed active selection to a thumbnail in front of the
channel.There are many selection tools in the GIMP like rectangular
selection tool or fuzzy selection for continuous selections.
Selection Masks are a graphical way to build selections into a gray
level channel where white pixels are selected and black pixels are
not selected. Therefore gray pixels are partially selected. You can
think of them as feathering the selection, a smooth transition
between selected and not selected. This is important to avoid the
ugly pixelization effect when you fill the selection or when you
erase its content after isolating a subject from background.
Creating Selection Masks
There are several ways to initialize a selection mask. From the
image window menu
Select->Save to Channel
if there is an active selection. From the image window
the bottom-left button creates a Quick
Mask; the content will be initialized with the active
selection. From the channel dialog, when you click on the
New channel button or from the context menu.
When created, this Selection mask appears in the Channel dialog,
named “Selection mask copy” with a queuing number.
You can change this by using the context menu that you get by
right-clicking on the channel.
Once the channel is initialized, selected (highlighted in blue), visible
(eye-icon in the dialog), and displayed as you want (color and opacity
attributes), you can start to work with all the paint tools. The colors
used are important. If you paint with some color other than white, grey,
or black, the color Value (luminosity) will be used to define a gray
(medium, light, or dark). When your mask is painted, you can transform
it to a selection by clicking on the
Channel to selection
button or from the context menu.
You can work in selection masks not only with the paint tool but also
with other tools. For instance, you can use the selection tools to fill
areas uniformly with gradients or patterns. By adding many selection
masks in your list you can easily compose very complex selections. One
can say that a selection mask is to a selection as a layer is to an
As long as a selection mask is activated you are working in the mask
and not in the image. To work in the image you have to deactivate all
selection masks. Don't forget also to stop displaying masks in the
image by removing the eye icon. Check also that all RGB and Alpha
channels are activated and displayed in the image.
Revision $Revision: 1.41 $
Dialog Quick Mask
A Quick Mask is a
intended to be used temporarily to paint a selection. Temporarily means
that, unlike a normal selection mask, it will not be saved in the list
after its transformation to selection. The
sometimes show their limits when they have to be used for doing complex
drawing selection, as progressive. In this case, using the QuickMask is a
good idea which can give very good results.
The QuickMask can be activated in the following order, from the
The QuickMask can also be activated by clicking the left-bottom
button showed in red on the screenshot.
It can also be activated by using Shift+Q
Creating a Quick Mask
To initialize a Quick Mask,
click the bottom-left button in the image window. If a selection was
active in your image, then its content appears unchanged while the
border is covered by a tranlucent red color. If no selection was active
then all the image is covered by a tranlucent red color. At every moment
you can hide the maskby clicking on the eye icon
in front of the QMask.
From the channel dialog you can double click on the name or the
thumbnail to edit the QMask
attributes. Then you can changethe Opacity
and its filling color. Once a quick mask is initialized click on it to
be sure it is selected and blue highlighted in the list, and start to
paint on it with any GIMP paint tool. The mask is coded in gray tones,
so you must use white or gray to decrease the area limited by the mask
and black to increase it. The area painted in light or dark gray will be
transition areas for the selection like feathering. When your mask is
ready, click again on the bottom-left button in the image window and the
quick maskwill be removed from the channel list and converted to
aselection. Quick mask's purpose is to paint a selection and its
transitions with the paint tools without worrying about managing
selection masks. It's a good way to isolate asubject in a picture
because once the selection is made you only have to remove its content
(or inverse if the subject is in the selection).
Using Quick Mask's
Screenshot of the image window with activated QuickMask. The
QuickMask is filled with a gradient from black (left) to white
The QuickMask is now disabled and a selection is initialised from
the QuickMask, which was filled by a gradient before. You see the
selection borders in the middle of the image.
A stroke is now added during the enabled selection. The key is, that
the black color will have no opacity of the resulting stroke (right)
and white color will have a full opacity of the stroke (left).
After the QuickMask Button is pressed, the command generates a temporary
8-bit (0-255) channel, on which the progressive selection work is
stored. If a selection is already present the mask is initialized with
the content of the selection. Once QuickMask has been activated, the
image is covered by a red semi-transparent veil. This one representes
the non-selected pixels. Any
can be used to create the selection on the QuickMask. They should use
only greyscale color, conforming the channel properties, white enabling
to define the future selected place. The selection will be displayed as
soon as the QuickMask will be toggled but its temporary channel will not
be available anymore.
To save in a channel the selection done with the Quickmask select in
the image menu
Select/Save to Channel