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Simplify alpha channels and unmask their potential

Added on Tuesday 16th of October 2007 06:26 am EST


Adobe Photoshop CS/CS2/CS3

Operating Systems:

Macintosh, Microsoft Windows



Alpha channels perplex many Photoshop users who then, unfortunately, avoid using them entirely. But if you don’t use alpha channels, you’re missing out on an indispensable, versatile, and timesaving tool. We’ll bring you up to speed on alpha channels so you can reap their rewards.


To maximize your alpha channel skills we’ll:

     Give you an understanding of an alpha channel’s fundamental purpose.

     Clear up the confusion surrounding alpha channels, color overlays, spot colors, and clipping paths. 

     Point out some of alpha channels advanced uses so you can harness their flexibility.



One of the greatest characteristics of alpha channels is precisely what confounds many Photoshop users—they are extremely versatile and multi-faceted. For example, say you’ve just read a tutorial on how to remove the background from a portrait, and in the example, the alpha channel displays as red on the image. Then you see another example and the alpha channel displays as black. You overhear your expert co-worker refer to an alpha channel as a clipping path, and all along you thought it was just a mask. You think to yourself, What’s going on, will somebody please explain!? Sure, we will!


Alpha channel fundamentals


On the simple side, an alpha channel is nothing more than a grayscale image. An alpha channel is comprised of black, white, and different shades of grey pixels. The black pixels are opaque, the white pixels are transparent, and the different shades of gray pixels have different transparency levels depending on their value. On the functional side, alpha channels have many uses, the top two being as selections or masks.


Alpha channels as selections

An alpha channel’s primary purpose is to save and store selections. Because Photoshop is pixel-based, you can’t easily select a segment of your image just by clicking on it, as you can in Adobe Illustrator, which is vector-based. Instead, you have to use one or more of Photoshop’s many selection tools and techniques to craft your selection. Since that usually isn’t a quick process, an alpha channel offers a convenient place to store that selection until you’re ready to use it again.



Tip: To store a selection as an alpha channel, make your selection and then click on the Save Selection As Channel button at the base of the Layers palette.



Alpha channels as masks

Alpha channels are also used to save and store masks. Selections and masks are often interchanged, since when you want to mask out an area of your image, you’ll need to start with a good selection. What’s sometimes confusing is that when you create a new layer mask (in the Layers palette), and have that layer selected, that mask shows up in the Channels palette.

This is a layer mask alpha channel and it’s treated slightly different from a regular alpha channel. It doesn’t offer all the same options as an alpha channel does as it’s temporary, meaning that if you apply or delete the layer mask, the channel disappears. Additionally, it’s only visible in the Channels palette if the layer with the mask is selected. Double click on the layer mask alpha channel and your only option is to change the opacity and color of the overlay, as shown in Figure A.



Unmasking alpha channels


Many Photoshop users find alpha channels confusing because of how versatile they are, or perhaps they’re stumped by all the alpha channel options. Some folks are simply puzzled because they hear terminology misused. We’ll point out some of the most common alpha channel characteristics responsible for this confusion.


Alpha channel options

One of the most confusing things about masks for some Photoshop users is differentiating between masked areas and selected areas. This is because there isn’t just one way that Photoshop displays masks. You’re controlling the helm here; you can set the options to view the image’s masked areas or selected areas.

Double-click on the alpha channel and you’ll see the Channel Options dialog box shown in Figure B with the following options:

        Masked Areas. If you select the Masked Areas check box, your alpha channel’s black pixels will represent the masked areas and the white pixels will represent the selected areas.

        Selected Areas. If you select the Selected Areas check box, your alpha channel’s black pixels will represent the selected areas and the white pixels will represent the masked areas.



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