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Grokking The Gimp
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4.1.2 Saving Selections to Channel Masks

One of the primary uses you will make of channel masks is to save selections. Why should you save a selection to a channel? There are many reasons. The first is that saving the selection protects your work, which is particularly important if you are making an involved and difficult selection. Second, it allows you to make your selections in parts. This helps divide and simplify the work. Third, it allows you to reuse a selection for several purposes. This is useful for all sorts of rendering operations, such as making drop shadows, highlights, and special textures (for example, see Section  8.5). Fourth, it allows you to refine your selections in ways that would otherwise be impossible. Finally, many special effects can be had using filter plug-ins on masks. As you can see, saving a selection to a channel mask really has a very broad range of benefits.

After making a selection with any of the tools described in Chapter  3, we can save the selection to a channel mask using the Save To Channel  function, found in the Image:Select menu. Figure  4.2(a)

  
Figure 4.2: Saving a Selection to a Channel Mask
Figure 4.2

illustrates an image of a selected falcon, and Figure  4.2(b) shows the Channels dialog after having used the Save to Channel  function. The thumbnail appears as a grayscale image, where white corresponds to fully selected, gray to partially  selected, and black to unselected pixels. As seen in Figure  4.2(b), the default channel mask title is Selection Mask copy.

After using Save to Channel, the active focus changes from the Layers dialog to the newly created channel in the Channels dialog. Nevertheless, the selection in the image window remains active. This has several important consequences. First, since the new channel is active, all ensuing filtering or painting operations are applied only to this channel. Furthermore, since the original selection is still active, the only parts of the new channel that can be affected are those inside the selection area. For example, trying to paint outside the selection will have absolutely no effect.

Thus, if the objective is to work on the newly created channel, the selection must be canceled by typing C-S-a in the image window, and the channel must be made visible by toggling on its Eye icon in the Channels dialog. If the objective is to save the selection as a channel and to move onto other image operations, the selection must be canceled, and the appropriate layer in the Layers dialog must be made active.

Grokking The Gimp
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