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Grokking The Gimp
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4.3.4 Transparency as a Tool for Editing Masks

To effectively edit masks, the image layer must be partially visible behind the mask so that it can be used as a guide for editing. The semi-transparency of channel masks was discussed in Section  4.3.1, but how is it done for layer masks?  It is done by using the Opacity slider in the Layers dialog. The following example demonstrates its use.

Figure  4.27

  
Figure 4.27: Image with Two Layers and a Layer Mask
Figure 4.27

illustrates an image consisting of two layers. The Layers dialog shown in Figure  4.27(b) shows that the lower layer is a photo of a woods and the upper layer an image of a fish, on an otherwise transparent layer. The upper layer also has a layer mask, which is to be used to edit the fish image.

The goal is to make parts of the fish transparent to create the illusion that the fish is partially behind some of the trees in the woods. This can be done with the layer mask by painting with black on the desired areas of the mask (see Section  4.3.1) or by cutting away parts of the mask with a selection (see Section  4.3.2). However, to effectively edit the layer mask using either approach, you must be able to see the outline of the trees through the fish. This is achieved by making the Fish layer active and, then, lowering the value of the Opacity slider.

Figure  4.28(a)

  
Figure 4.28: Decreasing the Opacity of the Top Layer and the Selection Obtained Using the Bezier Path Tool
Figure 4.28

illustrates the effect of lowering the Fish layer's opacity. Because the trees can be perceived through the fish, it is easy now to edit the layer mask to achieve the desired effect. The result of using the Bezier Path tool to make a selection is shown in Figure  4.28(b) (note that the opacity of the Fish layer has been reset to 100%).

Making sure that the Active Background Color in the Toolbox is set to black and that the layer mask is active in the Layers dialog, the selection is cut by typing C-x in the image window. The result on the image is shown in Figure  4.29(a),

  
Figure 4.29: The Result of Editing the Layer Mask
Figure 4.29

and the thumbnail of the resulting mask can be seen in Figure  4.29(b).

Grokking The Gimp
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