Editing an alpha channel

Alpha channels, like color channels, are grayscale representations. As such, you can edit them like you would any grayscale image. Click the channel in the Channels palette to make it active and visible. You see it in the image window as a grayscale (or black and white) representation of the saved selection. If you want to see the image while you work on the channel, click in the left column (the eyeball column) to the left of the RGB (or CMYK) channel. The alpha channel then appears as a red overlay on top of the image. Figure 8-17 shows you what the screen looks like with just the alpha channel visible (left) and how it appears when the alpha channel is active and the RGB channel is also visible. (And take a look at the difference in the Channels palette. See the eyeball column on the left?)

Figure 8-17: You can see just the alpha channel itself (left) or as a red overlay (right).

Here are some of the things that you might want to do to an active alpha channel:

i Blur the alpha channel. Blurring an alpha channel (FilterOBlur commands) is much like feathering a selection — it softens the edges. One of the big differences is that you can see a preview of the blur, which is much better than guessing how much feathering you need.

ii Sharpen the alpha channel. Sharpening a saved selection (FilterOSharpen commands) makes the edges cleaner and more precise.

i Paint in the alpha channel. Painting with the Brush tool using black, white, and gray in the channel changes the selection. Paint with white to add areas to the saved selection; paint with black to remove areas from the selection; paint with shades of gray to partially select areas of the image. You can edit an alpha channel with the Brush tool very precisely, adding and eliminating stray pixels, as well as creating precise edges. See Figure 8-18, upper left, for an example of a channel that needs some touch-up that's best done by hand.

Figure 8-18: You have lots of ways — and reasons — to edit alpha channels.

i Use levels or curves on the alpha channel. If the saved selection has feathering or other areas of partial selection (grays in the alpha channel), you can manipulate them with a Levels or Curves adjustment (from the ImageOAdjustments menu). The Levels adjustment is particularly appropriate for controlling the feathering along an edge (as you can see in the upper right in Figure 8-18).

i Apply one or more filters to an alpha channel. Use artistic filters on a saved selection to create special effects, such as borders and frames. Take a look at the bottom of Figure 8-18. On the left is the original; to its right, you see a couple of samples. (This is one of those great places where you can let your imagination run free across the open plains of Photoshop!) Chapter 15 covers filters.

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