2.2.  Channels Dialog

Revision History
Revision $Revision: 2405 $ 2007-08-29 j.h

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 (See Alpha Channel in Glossary). 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.

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.

Every channel is shown in the list with its own attributes. These attributes are very similar to the ones described in Section 2.1, “ Layers Dialog ” for the layer dialog.

[Caution] Caution

Activated channels appear highlighted (generally) 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 consequences. 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.

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. The displayed dialog lets you set Opacity and mask color used in the image to represent the selection. (If you use the New Channel button in Channel Menu, you can create this new channel with the options previously used by pressing the Shift key when clicking). This new channel is a channel mask (a selection mask) applied over the image. See Selection Mask

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.

Delete channel

only available for selection masks: you can here delete the active channel.

Overview

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.

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 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.

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 be deleted from the channel 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.

  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.