4. Channels dialog

The Channel 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, theGimp 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 secondary optionnal 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.

[Warning] Warning
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.

Dialog call

The Channel dialog can be called in many ways :

  • from the toolbox-menu: File/ Dialogs/ Channels
  • from the image-menu: Dialogs/ Channels
  • from another dialog-menu: Add Tab/ Channels

Using the Channeldialog

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

  • 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.

4.1. 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.

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.

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 mask by 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 change the 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 mask will be removed from the channel list and converted to a selection. 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 a subject 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).