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Grokking The Gimp
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4.3 Basic Tools for Working with Channel and Layer Masks

This section discusses techniques for editing channel and layer masks. Although, layer masks are used to edit a layer's alpha channel, and channel masks are used to store and edit selections, these two mask types behave similarly. Thus, the techniques for editing and filtering them can be treated in a somewhat unified manner.

4.3.1 Painting Tools

One of the most direct ways to modify masks is with the GIMP's painting tools. The Paintbrush,  Pencil,  Airbrush,  Eraser,  Ink Pen,  Dodge and Burn,  Smudge,  and Bucket Fill  tools can all be used. The versatility of these tools allows masks to be created that could not be duplicated using the selection tools alone. This is one of the reasons why masks are complementary to the selection tools.

Figure  4.17

Figure 4.17: Using Painting Tools on a Channel Mask
Figure 4.17

illustrates the use of the Paintbrush tool to edit a channel mask. As can be seen from Figure  4.17(a), the Paintbrush tool has been selected from the Toolbox, and white is the Active Foreground Color. Furthermore, as shown in Figure  4.17(b), a medium hard brush has been selected from the Brush Selection  dialog. Figure  4.17(c) illustrates the Channels dialog, which shows that an active channel mask labeled Butterfly has been created. The mask's color is set to yellow, its opacity is 50%, and its Eye icon is toggled on, which means the the channel mask can be seen in the image window. The effect of the mask in the image window is shown in Figure  4.17(d).

The partial transparency of the mask makes it easy to paint in the image window while following the butterfly's outline. As already noted, the mask acts like digital tracing paper. Some white strokes can be seen in the channel mask thumbnail shown in Figure  4.17(c). These were created by painting with the Paintbrush tool in the image window. Because the channel mask is active, the Paintbrush modifies the mask, not the image layer. However, the effect in the image window is to reveal parts of the image layer, as can be seen in Figure  4.17(d). Regions that have been painted white can be repainted black, which restores the mask. Note that although the mask appears yellow in the image window, it actually remains a grayscale image. For this reason white, black, and grays are the only colors that should be used when painting in channel masks.

Grokking The Gimp
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