If you’re one of the 99 people out of 100 I’m thinking of right now, you’ll likely never need to know what an alpha channel is when it comes to Adobe Photoshop, but if you’re that one out of 100, you most likely do. Alpha channels (or channels overall) are those things you sort of hear about, but never quite understand. They’re hidden somewhere near the Layers panel, but not often looked into, which is a shame, because they’re actually quite handy to have around. And once you get a handle on what they are, you realize that they aren’t all that mysterious or difficult to comprehend after all. They’re very straightforward, but not talked about nearly enough.
In today’s post, I’d like to do two things that have to do with Adobe Photoshop. First, I’d like to give you the simplest description of an alpha channel as I possibly can and second, I’d like to show you what they’re good for. I’ll quickly run through that description and then leave it up to you to do what you will with it. Once you understand that alpha channels are tools that can be used for a variety of things, I think you’ll return to them time and time again.
It makes absolutely no difference which image I use today. All I need is for something to be opened up in Photoshop to assist me with what I’d like to cover. I pretty much closed my eyes and pulled this picture of an aging hallway out of a hat. Here you are.
What is an Alpha Channel?
I’d like to let you know that I’ve already written a few posts that discuss channels in general on this website. If this is your first experience with these things, you most likely won’t click on the following links because you don’t yet know how they’ll benefit you. But, if by the end of this post, I’ve piqued your interest, please feel free to come back up here to learn a bit more. Channels really are interesting creatures.
Okay, let’s get going. To kick things off, I’d like to define an alpha channel as best I can. Here you are:
Basically, an alpha channel is an additional “layer” in the Channels panel that controls the transparency for specific colors or selections. You already know that the Channels panel contains the RGB, Red, Green and Blue elements by default, but what you may not know is that we can also create new elements that can determine the opacity for an object in an image. If what I just wrote isn’t entirely understood, please continue on below and by the end of this post, everything should be clear.
How to Create an Alpha Channel
To create a new alpha channel, I’ll head into the Channels panel by clicking on the tab to the right of the Layers tab. If you’re following along and don’t see this tab, you can go up to the Window > Channels menu item and click. That’s just as easy.
Once I’m in the correct panel, I’ll click on the small menu icon that’s located in the upper right and then, from there, I’ll select the New Channel menu item.
After I select that item, the New Channel dialog will appear. Inside this dialog, I can change a few settings. For now, I’ll simply keep things the way they are and click the OK button to move forward.
After I click the OK button, the dialog box will disappear and I’ll be left with a new element in the Channels panel. The thumbnail will be black and it’ll be located directly below the other channels. Also, the visibility will be turned off by default.
If I were to turn the visibility for this channel on and off, I’d see those settings that were in the dialog box come alive and then disappear again. In my case, since the overlay value was red and 50%, I’ll see a semi-transparent red overlay.
I’ve Just Created a Mask
If you complete the steps in the above section, you should be able to create an alpha channel. That’s not so difficult, is it? That’s actually half the battle. From here on, all that’s left are a few things to understand. And they aren’t even that challenging of ideas.
Let’s first talk about what just happened. The only reason this might throw you for a loop is because you’re making too much of it. In the simplest sense, by following the steps above, I created a mask. That’s it. If you take a look inside the Channels panel located in one of the above screenshots, you’ll see that the new alpha channel’s thumbnail is black. That means that the mask that is the alpha channel is blocking the other channels from being visible, just like any other more traditional mask would do. Call it a “channel mask” if you want. And the only reason this mask is red and semi-transparent is because those are the settings I chose inside of the New Channel dialog box. The mask could be black or blue or any other color I choose. It just happens to be red and at 50% opacity today.
Here’s something else for you to chew on. If I were to save this image out as a JPEG file or something else, there would be no evidence of this mask at all. That’s why I like to think of these things as tools for use inside of Photoshop. In the sections below, I’ll show you what they can do and why they are so cool. First though, let me really hammer this mask point home for a moment. I’m going to use my Brush Tool to draw a squiggly line in white right on the image. Right on top of this alpha channel. Check this out.
That sort of looks like what happens when you draw on a mask in white, right? That’s because it is. To prove it, I’ll show you the alpha channel thumbnail. It’s black with a white line drawn on it.
Inverting a Channel Mask
In this section, I’m going to quickly show you how to invert a channel, meaning, change the mask from being black to being white. When the thumbnail is black, that means the red overlay will be visible (the mask will be intact, hiding the other channels). When it’s white, that means it won’t be. I’ll show you why this is important in a moment. Just stick with me here.
Okay, after creating the alpha channel, I can either use the keyboard shortcut of Ctrl+I to invert it or head up to the Image > Adjustments > Invert menu item and click. Both will change the black to white.
Check out the next screenshot. The alpha channel is now white and that means that it’s not blocking any of the other channels from being shown. It’s behaving just like any other mask would. Remember, black conceals and white reveals.
Really, all I’ve done so far in this post is create an alpha channel and invert it. I also showed you that you can draw on a mask with white. But now that the mask is white itself (no mask at all), let’s see what happens when we draw with black to hide some of the other channels. I’ll use the Brush Tool again to draw another squiggly line. This time though, I’ll draw with black instead of white.
Oh man, now we’re getting somewhere. By drawing with black, I just hid some of the other channels. I guess my question now is, who the heck cares? What can I do with this? Why is it important? I’ll answer those questions below. I needed to lay some groundwork before I covered this new material. Before I forget though, take a look at the alpha channel thumbnail now. It’s got the black squiggly line in it.
Creating a Selection
Okay, check this out. It’s going to knock your socks off. If I head down to the bottom of the Channels panel and click on the Load Channel as Selection button, the area other than the squiggly line will turn into a selection.
If I wanted to select the area that isn’t masked, I can simply deselect what I just selected by using the Select > Deselect menu item and then invert the mask. Then, I’d click that same button as I just did to create the selection. Actually, I think I’ll do that. Take a look. Now just the squiggly line is selected and not the other way around.
Using the Selection For an Adjustment
Okay, we are now rocking and rolling. We’ve gone over how to create an alpha channel and how to make a selection inside of it. Now, let’s talk about why in the world you’d want to do something like this.
Since I have an existing selection, all I have to do is click on one of the adjustments up in the Adjustments panel to work with that selection. For this example, I’ll click the Curves adjustment, which will immediately place a new Mask channel in the Channels panel. Take a look.
I’ll pull up on the curve, just to show you that it can edit whatever content is inside the selection. Here you go.
Do you see how the squiggle is now brighter? This works exactly the same way as if I had created a selection on the image the regular way and then applied an adjustment after that. All I did in this post was go through the back door and do things a bit differently.
What’s the Point of an Alpha Channel?
You’re probably asking yourself what the point of an alpha channel is. Here’s the point: creating an alpha channel and editing it as you would edit a mask, saves that mask for you to work with again in the future. It’s stored right there in the Channels panel. So in my case, if I wanted to create many very intricate masks that I’ll need to use over and over, I’ll have them all stored as alpha channels. As an example, after applying the Curves adjustment, I went ahead and made the same alpha channel a selection again. Then, I applied the Levels adjustment from it. Let’s take a look at the Layers panel now.
Now I’ve got two adjustment layers that have been created off of the same mask in the Channels panel. I could keep working from that same channel if I needed to. It’s quite versatile.
Creating a Channel From a Mask
To really bring my point home about how regular masks and alpha channels work together, I’ll move back into the Layers panel. Then, I’ll apply the Exposure adjustment. I’ll invert the mask in the adjustment so the thumbnail is black and then I’ll use the Brush Tool to paint some white on the image. Finally, I’ll reduce the exposure a bit. Let’s take a look at the Layers channel now.
Do you see the new Exposure adjustment layer? Good. There’s nothing new going on here. All I did was apply a regular old adjustment layer and then adjust the mask that comes with it. Now let’s check out the Channels panel again to see if anything new was added there.
Well lo and behold, a new channel has been formed. This one is called the Exposure 1 Mask. Do you see how these things are actually one and the same? Masks are alpha channels and vice-versa. Since we’re now in the Channels panel, let’s make sure this new mask channel is selected and then I’ll go down to the bottom of the panel again and click the Load Channel as Selection button, like I did up above. Let’s check out the selection that’s created from this mask.
Yup, just as it should be. Another selection was created. I could go on forever like this, flipping back and forth between the Layers and Channels panels.
Saving a Selection as an Alpha Channel
This will be the last section, I promise. I know this post is getting a bit long.
One of the most helpful features when it comes to working with alpha channels is having the ability to save regular selections as these types of channels. To show you what I mean, I’ll go ahead and use the Rectangular Marquee Tool to draw a square (or rectangle) selection on the image. I’m doing this the way I would normally do this, as if I had just opened the image in Photoshop and made the selection. Nothing fancy here.
Now, I’ll click into the Channels panel and head down to the bottom of it and click the Save Selection as Channel button. Let’s see what happens.
Notice how there has been another alpha channel saved. It’s the selection I just made. Now it’s a mask and I can use it over and over in my project. Very handy!
I know this post was long, but there was a lot to cover. You’ll likely have some questions, so please don’t hesitate to ask in the comment section below. Also, if you’d like to discuss how to save selections as alpha channels or how to create and work with alpha channels in Photoshop in general, please visit the Photoshop discussion board to do so. Thanks for reading!