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Grokking The Gimp
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4.5 Masks and Selections

Chapter  3 described in detail the use of the GIMP's selection tools; however, that discussion is incomplete. A full understanding of how to effectively work with selections requires a discussion on how to integrate masks. This section shows how masks are complementary to the selection tools and illustrates why the combination of selections and masks is so powerful.

4.5.1 Using Masks to Refine Selections

Masks are terrific tools for refining selections. A careful examination of a mask can often reveal several problems. Figure  4.36

Figure 4.36: Image Illustrating a Selection
Figure 4.36

illustrates a selection made with the Bezier Path tool. As will be seen in a moment, this selection exhibits the three basic types of selection problems. To better examine these problems, the selection is converted to a channel mask, and the selection itself is canceled.

The resulting channel mask is shown in Figure  4.37(a),

Figure 4.37: A Mask Converted from the Selection
Figure 4.37

and Figure  4.37(b) shows the associated Channels dialog. Because it is difficult to make out the light blue water background through a 50% transparent, black channel mask, the color of the mask has been changed to yellow, as shown in Figure  4.37(c).

To see the problems associated with the selection, the Zoom  tool is used to magnify the image window. This produces the result shown in Figure  4.38(a).

Figure 4.38: Illustrating the Three Basic Selection Problems
Figure 4.38

This figure shows that in several regions the light blue color of the background is showing through from around the edges of the yellow mask. This means that these pixels have been erroneously included as part of the selection.

Figure  4.38(b) shows the same image as in Figure  4.38(a), but with the colors of the mask inverted. The color inversion is done by making the channel mask active and then using the Invert    function found in the Image:Image/Colors menu. Inverting the colors inverts the regions of the mask that correspond to selected and unselected pixels in the image. Now it can be seen that in some places, the dark pixels from the subject are showing through around the mask edges. This means that they are mistakenly not included in the set of selected pixels.

Finally, in both Figures  4.38(a) and (b) a rough-edge, aliasing effect can be seen.

Each of these three problems can be solved by refining the mask. This can be accomplished using several different methods, but for this type of fine work near a mask edge, the best choice is the Airbrush  tool from the Toolbox. The Airbrush can apply a very light coat of paint, so it is a great touch-up tool. Working near the edge requires some blending of the background with the subject to avoid aliasing. When used with a light pressure the Airbrush is perfect for this.

Figure  4.39(a)

Figure 4.39: Introducing the Airbrush Tool
Figure 4.39

shows the Tool Options dialog  for the Airbrush. It is the Pressure option that interests us here. The Pressure slider is in units of percent, and the default value of 10% is shown in Figure  4.39(a). The effect of using 10% pressure in conjunction with the soft brush chosen in Figure  4.39(b) produces the top line painted in Figure  4.39(c). Each of the other lines is painted with the Pressure value labeled to the right of the line. This figure shows that, for low pressures, the Airbrush tool produces a very light layer of paint, great for touching up imperfect and aliased edges like the ones seen in Figure  4.38.

Using the Airbrush tool on the problem pixels shown in Figure  4.38 produces the results shown in Figure  4.40.

Figure 4.40: Solving the Three Basic Selection Problems with the Airbrush Tool
Figure 4.40

The technique used in applying the Airbrush tool is as follows:
Make the channel mask active.
Use the Zoom tool to magnify the image to a sufficient resolution so that the paint can be applied to the problem edge areas with precision.
Set the Active Foreground Color to black by typing d in the image window. 
Lightly apply black paint to the problem areas with the Airbrush tool. The black paint is useful for removing pixels which should not be part of the selection.
Invert the mask colors using Invert from the Image:Image/Colors menu, and work the new problem areas. Because of the inversion of color, now the black paint is useful for including pixels that should be part of the selection.
Evaluate the precision of the applied paint, and correct for mistakes by making liberal use of the Undo (C-z) and Redo (C-r) functions.

Grokking The Gimp
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