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Grokking The Gimp
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4.1 Channel Masks

A channel mask is a special grayscale layer only 8 bits deep and is used to store and edit selections. This section describes the Channels dialog, how to save selections to channel masks, how to convert channel masks to selections, and the other GIMP functions used for organizing masks and operating on them.

4.1.1 The Channels Dialog

As an image can have any number of layers, it can also have any number of channel masks. Thus, analogous to the Layers dialog, the Channels dialog is the the tool for organizing and working with channels. The Channels dialog is obtained by typing C-l in the image window and clicking on the Channels tab in the resulting Layers & Channels window. Figure  4.1

Figure 4.1: The Channels Dialog
Figure 4.1

illustrates the various elements of the Channels dialog.

The most important feature of the Channels dialog is the Channels Palette.   This region of the dialog consists of horizontal strips where the top three entries are always the Red, Green, and Blue channels of the image, already described in Section  2.2.1. The rest of the strips are channel masks. Figure  4.1 shows a single channel mask. The channel mask strip displays a thumbnail image of the mask's contents, and the area to the left of the thumbnail contains an Eye icon. Analogous to layer strips in the Layers dialog, the visibility   of the Eye icon for channel strips can be toggled on and off, making the channel mask either visible or invisible in the image window.

To the right of the thumbnail is the Channel Title Area. In Figure  4.1 the channel is named A Channel Mask. The title can be changed by double-clicking in the Channel Title Area. This brings up a dialog where the new title can be entered.

The channel strip in Figure  4.1 is highlighted in blue, indicating that it is the active channel.  A channel mask is made active by clicking on its thumbnail or Title Area. Only a single channel mask can be active at a time, and, when a channel mask is active, none of the image layers can be active. Because GIMP tools applied to the image window are applied to the active layer  or the active channel,  this can sometimes be a source of confusion. After working on a channel mask, to work on a layer it is necessary to explicitly switch to the Layers dialog and to make the layer active.

The remaining elements of the Channels dialog are the Channels menu and the button bar. These are described in more detail shortly.

Grokking The Gimp
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  Published under the terms of the Open Publication License Design by Interspire