If you're not entirely clear about what I mean by the term mask, I'll tell you: A mask is a selection outline expressed as a grayscale image.
* Selected areas appear white.
* Deselected areas appear black.
* Partially selected parts of the image appear in gray. Feathered edges are also expressed in shades of gray, from light gray near the selected area to dark gray near the deselected area.
Figure 9-1 shows two selection outlines and their equivalent masks. The top-left example shows a rectangular selection that I inverted by choosing Image ^ Adjust ^ Invert (Ctrl+I). Below this example is the same selection expressed as a mask. Because the selection is hard-edged with no antialiasing or feathering, the mask appears hard-edged, as well. The selected area is white and is said to be unmasked; the deselected area is black, or masked.
The top-right example in Figure 9-1 shows a feathered selection outline. Again, I've inverted the selection so that you can better see the extent of the selection outline. (Marching ants can't accurately express softened edges, so the inversion helps show things off more.) The bottom-right image is the equivalent mask. Here, the feathering effect is completely visible.
When you look at the masks along the bottom of Figure 9-1, you may wonder where the heck the image went. One of the wonderful things about masks is that you can view them independently of an image, as in Figure 9-1, or with an image, as in Figure 9-2. In the second figure, the mask is expressed as a color overlay. By default, the color of the overlay is a translucent red, like a conventional rubylith. (To see the overlay in its full, natural color, see Color Plate 9-1.) Areas covered with the rubylith are masked (deselected); areas that appear normal — without any red tint — are unmasked (selected). When you return to the standard marching ants mode, any changes you make to your image affect only the unmasked areas.
Figure 9-1: Two selection outlines with inverted interiors (top) and their equivalent masks (bottom).
Now that you know roughly what masks are (the definition becomes progressively clearer throughout this chapter), the question remains what good are they? Because a mask is essentially an independent grayscale image, you can edit the mask using paint and edit tools, filters, color correction options, and almost every other Photoshop function. You can even use the selection tools, as discussed in the previous chapter. With all these features at your disposal, you can't help but create a more accurate selection outline in a shorter amount of time.
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