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Grokking The Gimp
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4.2 Layer Masks

Layer masks are special layers that are only 8 bits deep and that represent the alpha channel  of an image layer. The main difference between channel and layer masks is that the layer mask represents the alpha channel of the layer it is linked to, whereas channel masks represent selections and exist independently of any particular layer. Nevertheless, as will be seen later in this chapter, there is an intimate relationship between alpha channels and selections and, by extension, between layer masks and channel masks.

The interpretation of white, black, and gray pixels in a layer mask is slightly different than that for a channel mask. A layer mask represents an alpha channel, which, in turn, controls the transparency of a layer. In this context, for the layer mask, white represents 100% opacity, black 100% transparency, and gray an intermediate level of opacity/transparency. This section describes the basic operations that can be performed on layer masks.

4.2.1 Creating a Layer Mask

To create a layer mask, make the desired layer active in the Layers dialog and choose Add Layer Mask  from the Layers menu. This creates the layer mask, but only if the active layer already has an alpha channel. If it does not have an alpha channel  create one using Add Alpha Channel  from the Layers menu.

Figure  4.11(a)

Figure 4.11: The Layers Menu
Figure 4.11

illustrates the Layers dialog for an image consisting of an alpha-less background layer, and Figure  4.11(b) shows the associated Layers menu. As can be seen, all the functions in the two lowest groups of the Layers menu, including Add Layer Mask, are grayed out except for the function Add Alpha Channel. Selecting this function, adds the required alpha channel, and bringing up the Layers menu a second time now produces the Layers menu shown in Figure  4.11(c). As can be seen, the function Add Layer Mask is now available.

Selecting the Add Layer Mask function from the Layers menu brings up the Add Mask Options  dialog. This dialog allows a choice of three initial states for the new mask. The default is White (Full Opacity), which is what you will choose most often. This choice produces a layer mask that leaves the image layer fully opaque. It can sometimes be useful to choose the second option, Black (Full Transparency), which creates a layer mask that makes the image layer fully transparent. The final option is Layer's Alpha Channel,  which loads the active layer's alpha channel into the layer mask.

Figure  4.12

Figure 4.12: The Creation of a Layer Mask
Figure 4.12

illustrates a layer mask created using the White (Full Opacity) option. Figure  4.12(a) shows the image window, and Figure  4.12(b) shows its associated Layers dialog. The layer strip for the image shows the thumbnail of the image and, next to it, the thumbnail of the newly created white layer mask. That a layer mask is associated with a single image layer is underscored by the fact that the mask's thumbnail appears in the same layer strip as the image layer's thumbnail.

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