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2.2.  Channels dialog

Revision History
Revision $Revision: 1.41 $ 2006-03-09 lexa

Figure 9.7.  The Channel dialog

The Channel dialog

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 Gimp 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 Alpha. 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 from the Layers dialog menu. Also, if you have more than one layer in your image, GIMP automatically creates an Alpha channel.

[Note] Note

GIMP doesn't support CMYK or YUV color models.

Figure 9.8.  Representation of an image with channels

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.

2.2.1.  Calling the Dialog

The Channel dialog can be activated in many ways :

  • from the toolbox-menu: File->Dialogs->Channels

  • from the image-menu: Dialogs->Channels

  • from another dialog-menu: Add Tab->Channels

2.2.2.  Using the Channel dialog  Overview

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 channel menu.  Channel attributes

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.

[Caution] Caution

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 complementary color.  Managing channels

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.

New channel

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.

Raise channel,

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.

Lower channel

You can here put the channel down a level in the list. Press the Shift key to move the channel to bottom of the list.

Duplicate channel

You can create here a copy of the active channel. Name of new channel is suffixed with a number.

[Tip] Tip

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.

  • Shift: the selection derived from a channel is added to the previous active selection. The final selection is merged from both.

  • Ctrl: the final selection is the substraction of selection derived from a channel from the previously active one.

  • Shift-Ctrl: the final selection is the intersection of selection derived from a channel with the previously active one. Only common parts are kept.

Delete channel

only available for selection masks: you can here delete the active channel.  Channels Menu
Revision History
Revision $Revision: 1.22 $ 2006-02-27 j.h

Figure 9.9.  Channels Menu

Channels Menu

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, New channel, Raise channel, Lower channel, Duplicate channel, Delete channel: see Managing channels.

  • Channel to selection: Selection derived from channel replaces any previous active selection.

  • 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 are kept.

2.2.3.  Selection masks

Revision History
Revision $Revision: 1.29 $ 2006-02-27 j.h

Figure 9.10.  A selection composed out of channels.

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.  Using Selectionmasks

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 image.

[Caution] Caution

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.

2.2.4.  Quick Mask

Revision History
Revision $Revision: 1.41 $ 2006-03-09 j.h

Figure 9.11.  Dialog Quick Mask

Dialog Quick Mask

A Quick Mask is a Selection Mask 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 selection tools 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.  Activate dialog
  • The QuickMask can be activated in the following order, from the image-menu: Select/Toggle QuickMask.

  • 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 shortcut.  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 (right).


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 paint tool 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.

[Tip] Tip

To save in a channel the selection done with the Quickmask select in the image menu Select/Save to Channel  Usage
  1. Open an image or begin a new document.

  2. Activate the Quickmask using the left-bottom button in the image window. If a selection is present the mask is initialized with the content of the selection.

  3. Choose a drawing tool and use it with greyscale colors on the QuickMask.

  4. Deactivate the Quickmask using the left-bottom button in the image window.

  Published under the terms of the GNU General Public License Design by Interspire