As we promised we are back to continue the discussion about the
channels, and today we’ll enter in the world of a mighty channel, the
alpha channel. Photoshop has one other type of channel: alpha channels.
Their job is to store selections so you can use or edit them later.
These channels get their name from a process called alpha
compositing, which combines a partially transparent image with another
image. (Filmmakers use this process to create special effects and fake
backdrops.) Information about the shape of the transparent area and the
pixels’ level of transparency has to be stored somewhere, and that
somewhere is an alpha channel.
This is powerful stuff because the same technology lets you save
selections. And, as you’ve know, making selections can take a ton of
time. And since clients change their minds occasionally—“Put the model
in front of this bush, and change her hair color while you’re at it”—the
ability to save selections so you can mess with them later is a
lifesaver. As long as you save your document as a Photoshop file (PSD),
that alpha channel will always be there for you to use. That ought to
make you sleep better at night!
You can drag alpha channels between documents as long as both documents have the same pixel dimensions.
Folks sometimes refer to alpha channels as channel masks because,
once you’ve made an alpha channel (as explained in the next section),
you can use it to help you adjust certain portions of your image—kind of
like when you use a layer mask.
In fact, creating a layer mask by loading an alpha channel as a
selection is the most common use for alpha channels. That’s because, as
you’ll learn on next steps, you can use channels to make incredibly
detailed selections that are tough to get any other way. When you’re in
Quick Mask mode, you’re actually working on a temporary alpha channel.
Creating an Alpha Channel
It can be helpful to think of an alpha channel as a grayscale
representation of your selection. Unless you change Photoshop’s
settings, the black parts of the channel are the unselected portion of
your image—also referred to as the protected or masked part—and the
white parts are the selection. And, just like in a layer mask,shades of
gray represent areas that are only partially selected, which means
they’re partially transparent.
Photoshop gives you several different ways to create an alpha channel:
Create a selection and then choose
Select>Save Selection. Create a selection and then click the “Save selection as channel” button at the bottom of the
Channels panel. It looks like a circle within a square. Click the “Create new channel” button at the bottom of the
Channels panel. When you do that, Photoshop creates an alpha channel named
Alpha 1 and sticks it at the bottom of the
Channels
panel. The new channel is solid black because it’s empty. To create a
selection, turn on the composite channel’s visibility to summon the red
overlay of
Quick Mask mode so you can see your image. Then grab the
Brush tool
(B) and paint the area you want to select white (think of this process
as painting a hole through the mask so you can see—and therefore
select—what’s below it). Though you can certainly start with an empty
alpha channel, it’s usually easier to create your selection (or at least
a rough version of it) on the full-color image before adding the alpha
channel. In most cases, you’ll find it easier to create a selection
first (even if it’s rough) and then add your alpha channel, as shown
blow. (That way, you see the full-color image instead of a screen full
of black or red.) To do that, select something in your image and then,
once you’ve got marching ants, click the “Save selection as channel”
button (circled below). Photoshop adds an alpha channel—which includes
your selection—to the bottom of the
Channels panel.
Choose
New Channel from the
Channels panel’s
menu. When you choose this command, a dialog box opens that lets you
name the new channel and tell Photoshop how to display the channel’s
info. Straight from the factory, Photoshop shows selected areas (the
parts of your image inside the marching ants) in white and unselected
areas in black. Partially selected areas, which have soft edges, appear
in shades of gray. If you’d rather see your selections in black and
everything else in white, turn on the dialog box’s
Selected Areas radio button. If you want to edit your alpha channel using
Quick Mask mode (as described later in this section), you can change the
Quick Mask’s color and opacity here. When you’ve got everything the way you want it, click OK to make Photoshop create your alpha channel.
Editing Alpha Channels
Once you’ve got yourself an alpha channel, you can fine-tune it just
like a layer mask by painting with the Brush tool or using any
selection tool. If you use a selection tool, you can choose
Edit>Fill
and then pick black or white from the Use pop-up menu, depending on
whether you want to add to or subtract from your selection (selected
areas are white, and everything else is black). If you want to reverse
the way Photoshop displays the channel’s info—so that your selection
appears in black instead of white—just double-click the alpha channel’s
thumbnail in the
Channels panel and, in the resulting
Channel Options dialog box, turn on the
Selected Areas option. When you do, Photoshop flip-flops your mask’s colors, as shown below.
Double-clicking an alpha channel’s thumbnail in the
Channels
panel summons the dialog box shown below, which lets you tell Photoshop
to reverse what the mask color (black) represents. You can also use this
dialog box to turn the alpha channel into a spot color or to change the
color and opacity of the
Quick Mask overlay. To do the latter, click the red color swatch, pick a new color from the resulting
Color Picker, and then click
OK.
You can also edit your alpha channel using
Quick Mask mode. To do that, in the
Channels panel,
activate the alpha channel and then click the composite channel’s
visibility eye, as shown below. When you do, Photoshop puts
Quick Mask
mode’s signature red overlay atop your image. (If you’re editing an
alpha channel in an image with a lot of red in it, you won’t be able to
see clear through the mask, so change the overlay color as described
earlier.)
If you activate an alpha channel and then turn on the composite
channel’s visibility eye (circled in the image below), you can edit or
create a selection from scratch in
Quick Mask mode. In this example, the
Brush tool
(the white circle beneath the dress brim) set to paint with black is
being used to fine-tune the masked area around the girls dress. If you
mess up and mask too much—by painting with black across part of the
dress, thereby subtracting it from your selection, say—press X to
flip-flop your color chips and paint across that area with white to add
it back to your selection (just like you would with a layer mask).
You can also run filters on an alpha channel, just like you can with a layer mask. Among the most useful are
Gaussian Blur for softening the selection’s edge (helpful if you’re trying to select a slightly blurred area) and the
Minimum filter for tightening your selection .
Loading an Alpha Channel as a Selection
Once you’re finished editing your alpha channel, you can transform it
into a selection so you can actually do something with it. You can
summon the marching ants in several ways:
Ctrl-click the alpha channels thumbnail in the
Channels panel. Click the “
Load channel as selection” button at the bottom of the
Channels panel (it looks like a tiny dotted circle) while you’ve got an alpha channel active. Drag the alpha channel onto the “
Load channel as selection”
button (let go of your mouse as soon as Photoshop highlights the
button). Now you can perform all the amazing color and lighting
adjustments, and they’ll affect only the area you’ve selected.
Deleting Alpha Channels
When you’re finished using an alpha channel (or if you want to start
over with a new one), you can get rid of it by dragging it onto the
Delete button (the little trash can) at the bottom of the
Channels panel. Or just click the trash can while the alpha channel is active and then click
Yes when Photoshop asks if you’re sure you want to throw it away.
I hope you’ve learned something new and you enjoyed it. Go alpha!
Girl stock: 
