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!