Join Deke McClelland for an in-depth discussion in this video Making an alpha channel, part of Photoshop CS5 One-on-One: Mastery.
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In this exercise, I am going to show you how to make a base Alpha Channel here inside Photoshop. I'm working aside Hair in flight.tif found inside the 26_masking folder. When you're masking inside of Photoshop most of your activity happens inside the Channels panel. So, I'd like you to go ahead and switch over to that panel. The reason is because that's where you create your Alpha Channel in the first place. That's where you go ahead and develop the Alpha channel, exaggerate the contrast, finesse the details and ultimately store your mask for later use. Now, the other two panels that become helpful to you are the Layers panel and that's useful when you're applying and modifying layer masks.
Then, you might go ahead and finesse your layer mask from the Masks panel as well, but 90% of the activity happens right here. Now, we're working inside of an RGB image, notice that, which means that we are going to have to build our mask from the existing information in the Red, Green and Blue channels, because as I was saying masking is the art of using the image to select itself. Now, a question that I frequently get is, Hey! What if I am working with a CMYK image? Can you mask as effectively from a CMYK image as you can from RGB? The answer is no, you can't, and there are two reasons for that.
First of all, the transitions inside of a CMYK image are a lot choppier, and then secondly you have problems with your shadow details. Because when Photoshop converts from RGB to CMYK it has to offload a bunch of the shadow detail inside the Cyan, Magenta and Yellow channels off to the Black channel. The Black channel is virtually useless to us for masking purposes and the Cyan, Magenta and Yellow channels are effectively ruined because they have been robbed of the shadow information.
So, what do you do if you're working with a CMYK image? Well, you go up to the Image menu and you choose the Duplicate command to create a copy of that image. Then, you turn around and take that copy, choose Mode and then choose RGB Color to convert it to an RGB image. You then create your mask inside the RGB version of the image and transfer it over to the final CMYK image. All right, but we are working inside RGB, so we don't have to take those steps. What we need to do is evaluate the information that already exists inside the image. Now bear in mind, this is the effect we are going for.
So, this is one of our two Alpha channels inside the image. When you think Alpha channels just bear in mind these are not color-bearing channels, they are extra channels inside the image. So you can modify them without harming the image itself. All right, but the image doesn't magically contain one of these things. we have to make it. What the image contains is the Red, Green, and Blue channels. So, let's checkout what we got. We have this Red channel right there and notice we have the keyboard shortcuts as well, which I am going to be using just so that I can keep my cursor out here in the Image window, and they are Ctrl+3 or Cmd+3 on the Mac for the Red channel, Ctrl+4 or Cmd+4 for the Green channel and Ctrl+5 or Cmd+5 for the Blue channel.
So, here we are looking at the Red channel. We've got some dark hair against a fairly light background. We have some very light skin going on, on the left arm but not so much on the right arm. Sort of murky shadows back there in the right side and then of course the dress is alternately lighter or darker than its background, big mess. Anyway, Ctrl+4 or Cmd+ 4 for the Green channel. The hair brightens up slightly so does the background. The arm gets a little bit darker. Then, finally Ctrl+5 or Cmd+5, the hair lightens up even more so does the background, the arm grows darker.
So, we are losing contrast as we go through these channels. We have the most contrast there in the Red channel, a little bit less inside the Green channel and less still inside of the Blue channel. Now, when you're working with portrait shots, I'll just say the Red channel is going to be where the skin tones reside. That's where you see the brightest skin tones. Then things are going to grow darker inside the Green channel. But, often times you are going to have better detail inside the Green channel. You never know. Then finally once you get to the Blue channel, that's where you are going to start losing the brightness in the skin tones, that's where you are going to start losing the detail as well.
All right, so in our case let's say we look at one of these channels and we like one of them. We think one of then might work for us pretty well as a base for a mask. So, I might look at the Red channel and say okay, that where I have the most contrast. It's not an awful lot of contrast but it's better than the other channels. So, I'll start by modifying the contents of this channel. Well, obviously you don't start changing this channel right out of the gate. For example, I'd say you know her hair is dark and the background is light. I want it to be the other way around. I want to select that hair and have the background be dark.
So, I'll press Ctrl+I or Cmd+I on the Mac in order to invert this channel. But of course, if I do that, why then I have pretty thoroughly messed up that full-color image. All right, so I don't want to do that, I'll go back to Red. The nice thing about inverting an image is that you can undo it just by pressing Ctrl+I or Cmd+I on the Mac once again, because every single luminance level has an exact opposite, inverting is a nondestructive operation. Anyway, I am going to grab that Red channel and I'm going to drag it down onto this little Page icon at the bottom of the Channels panel in order to make a Duplicate of it.
Now, I have an Alpha channel, that's all it takes to make one. I'll go ahead and name this guy, based on red, just so that we know where it came from. All right, and I still need to perform that first up of inverting because after all once again, bear in mind, I'll go back to tougher mask for a moment. That white represents the selected area, black represents the deselected area. You might also think of this channel in terms of a layer mask where white would represent opacity and black would represent transparency. And of course, there's that old adage that white reveals and black conceals.
So, we need her to be light against the dark background. I would indeed press Ctrl+I or Cmd+I on the Mac to invert the image. Right away, you see that we've got some problems here. It's even more obvious how little contrast we have to work with. There is just a little bit of a dark corona around the hair on the left-hand side of the image and it fades. Notice, that toward the far left side we are starting to lose that hair detail and then over on the right -hand side, oh, my gosh! We've got so little contrast to work with. Now, we can punch up that contrast using something like the Levels command.
But, were we to do that, I'll go ahead and press Ctrl+L or Cmd+L on the Mac to bring up Levels. I'll drag this black point quite a good distance over here to the right-hand side so that we have a moderate amount of contrast. I'll go ahead and set it to 190, between the dark background and the somewhat light hairs. But, notice what a number that does on the left-hand side of the image. We are losing hairs like crazy. So, this probably isn't the best way to work, I am going to go ahead and Cancel out.
Rather than creating an Alpha channel based on the contents of a single color-bearing channel, we are better off merging a couple of channels in order to exploit what differences we can inside the image and you do that using the Calculations command under the Image menu. I'll show you how that guy works in the next exercise.
Recommended prerequisites: Photoshop CS5 One-on-One: Fundamentals and Photoshop CS5 One-on-One: Advanced.
- Using masks and blend modes in radically new ways
- Mastering the Pen tool and Paths panel
- Transforming and maximizing Smart Objects
- Employing Smart Filters to create complex effects
- Exploring the capabilities of Bristle brushes and the Mixer Brush
- Merging multiple images into seamless panoramas
- Exploring the full range of luminance with HDR Pro
- Recording actions and batching-processing images