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Grokking The Gimp
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4.1.6 Combining Channels

Section  3.2 describes how selections can be combined; selections can be added, subtracted, and intersected. These operations are useful because they allow the results from different selection tools to be combined and because they also allow the work to be performed incrementally. This can be particularly important when selecting from a large image or when making a complicated selection.

Channel masks can also be combined. This is done by converting the masks to selections and then saving the combined result back to a mask. Figure  4.7

Figure 4.7: Two Channel Masks
Figure 4.7

illustrates the procedure. Figure  4.7(a) shows an underwater scene for which two selections have been previously made and saved to channel masks. The channels strips for the two masks, labeled Fish and Rock, can be seen in the Channels dialog, shown in Figure  4.7(b).

After making the channel labeled Rock the active channel, it is converted to a selection by clicking on the Channel to Selection button (represented by the dashed circle icon) in the button bar. The active channel can be seen in Figure  4.8(b).

Figure 4.8: Converting the First Mask to a Selection
Figure 4.8

Figure  4.8(a) shows the resulting selection in the image window. Note that it was not necessary nor useful to toggle on the visibility of the channel mask to do this.

A similar operation is performed with the channel mask labeled Fish. The mask is made active and then converted to a selection. Normally the conversion of a mask to a selection replaces any selection already active in the image window. However, the selection represented by the mask can also be added to the active selection. This is accomplished by choosing the Add to Selection  function from the Channels menu, shown in Figure  4.6(b). The Add to Selection function can also be invoked by Shift-clicking the Channel to Selection button in the button bar. The result of adding the selection is shown in Figure  4.9(a),

Figure 4.9: Converting the Second Mask to a Selection and Adding it to the First
Figure 4.9

which shows the combined selection in the image window.

Two other combining modes are also available in the Channels menu. These are the Subtract From Selection and the Intersect With Selection functions. These can both be used from the Channel to Selection button in the button bar. Control-clicking the button produces a subtraction and Control-Shift-clicking produces an intersection.

The final step to obtain a channel mask of the combined selection is performed by using Save to Channel from the Image:Select menu. The procedure is illustrated in Figure  4.10.

Figure 4.10: Creating the Combined Channel Mask
Figure 4.10

The resulting channel mask, which has been labeled Rock+Fish, is shown in Figure  4.10(b).

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