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### 3.2. 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
  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
    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.

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