Join Deke McClelland for an in-depth discussion in this video Establishing a base alpha channel, part of Photoshop Smart Objects.
In this exercise, we are going to establish a base Alpha channel that will ultimately enable us to mask this woman against her new background. Now I am going to make fairly short work of this. We are going to be employing a command called Calculations, and I discussed that command at length, in case you are curious, inside of a series that I've done as I am recording this. That series is called Photoshop CS3 Channels and Masks: Advanced Techniques. And then you go to Chapter 14 Calculations, aka Channel Operations, and that will give you all the information you ever wanted to know about the command we are about to employ.
As I say, we are going to make pretty short work of it now. Now I have gone ahead saved my progress as Ready to mask.psd. It's found inside at 01_how_they_work folder. And I have switched over to my Channels palette, which is what you'll need to do to, if you're working along with me. Let's check out the channels we have to work with. Here is the Red channel. Notice how light she is against this white background. The background is going to be white in all three channels because white is a combination of red, green, and blue light working together. But she's also very light inside of this red channel, even though, if we switch back to RGB, she is a person of some color, another words she is fairly dark skin in the grand scheme of things.
Now, regardless of skin tones, whether you are as pale as I am or as dark as many other people are, you are going to resonate most lightly inside of that Red channel because we are all just filled with blood, and there's various other reasons for it as well. But we resonate in the red-to-orange- to-yellow zone and that's covered mostly by that Red channel. So she is lightest in the Red channel. She darkens up in the Green channel. This is going to be true of all your portrait shots, by the way, and her skin gets darker still inside the Blue channel.
Some of the Highlights inside of her face get lighter because of the make-up that she's wearing. We are looking for contrast, the highest degree of contrast between the foreground and background, and we are looking for two channels that have the most contrast and those would be, in this case, Blue and Green because she starts to lighten up so much in Red channel and we start losing some of that contrast there. All right, so now we've identified Green and Blue is being our source channels. Let's go back to RGB because, again, they contain the most contrast.
Now we will go up to the Image menu, and choose the Calculations command. The Calculations command allows you to take two channels inside your image, in our case Green and Blue, and merge them together to create an Alpha channel that will serve as the basis for a mask. So I will go ahead and choose the Calculations command. Now the first thing you need to do is identify your Source 1 and Source 2. These are those two channels I was telling you about. They are going to be Green and Blue. However, if I go ahead and change the first channel to Green and the second channel to Blue, let's say. Notice that nothing happened when I switched the second channel to Blue. We saw it change when I switched the first channel to Green, but nothing for Blue and that's because our layers are all mixed up.
So let's take a look at our other source options here. Source 1 and Source 2 are both set to the same document, Ready to mask.psd. That's what we want. That's the only image I have open right now. However, our layers are goofed up. Right now, we were mixing a merged version of the entire composition with just the white layer. Well, the white layer is always white. So that is not going to do us any good. So we want to switch it to Merged as well. So let's go ahead and mix Merged with Merged like so, Green plus Blue, and then I am going to set the Blending mode to Subtract, which is the darkest mode we have available to us. I really want to darken things up here so that she gets extremely dark against an extremely light background.
So I will go ahead and choose Subtract. Now that's a little bit too dark and that's because we are subtracting all the luminance in the Green channel from all the luminance in the Blue channel, and that means we are subtracting white from white and we get black that way, we don't want that. So we are going of have to take one of these channels and invert them. That's why we have these Invert checkboxes here. I am going to turn on Invert for Green, and we end up getting the best result possible that I discovered. Now you might be able to fool around with these settings and come up with something different. You have got a bunch of different Blend modes you can experiment with, if you want. Add and Subtract tend to be your buddies for this kind of stuff, but you can experiment.
Opacity, I would leave that set to 100%. You don't need to worry about the Mask option. And then it's a matter of selecting Green and Blue and deciding who's going to be inverted and who's not going to be inverted, depending on the Blend mode you select. But as I am concerned, these are the best settings that I discovered. I will click OK in order to accept that new channel. And notice now we have Alpha 1. So the original channels were not harmed. We just now have an extra channel. And recall, that channel can be no larger than the canvas size, which is why I went ahead and scaled the canvas in the previous exercise to give myself more room to work.
And that way I can mask the entire model in one fell swoop instead of being presented with some weird edge details that I'd have to address differently later. Anyway, having done that, I will go ahead and rename this channel 'base mask' or something along those lines. Now there is one other thing we need to do at this point. When you're creating a mask, white represents the area you are going to keep, because we are working with a layer Mask, and black represents the area that's going to go transparent. You also hear people say white reveals and black conceals.
We want to reveal the model and we want to conceal her white background so that we are revealing the sky below. So we want the model to be white and we want the background to be black, exactly the opposite of what we almost have here. So I will go up to the Image menu, I will choose Adjustments, and I'll choose the Invert command, which is going to make what's currently white, black, and what's currently black, white. Now I think everybody knows how the Invert command works. Here's the thing though. You might wonder, "Well, why don't you apply an Adjustment layer? "Why are you applying a static color adjustment here?" Because when you're working inside of a Mask, static color adjustments are what you have.
We don't have layers inside of masks. We do, however, have masks inside of layers and we'll come to that shortly. But we have to work static right now. So I'm to go ahead and choose the Invert command and we get this effect here. That's 90% of the masking done in just those two steps, Calculations and then Invert. We will do the remaining 10% that's necessary to make her appear totally white against the totally black background in the next exercise.
Download Deke's customized keyboard layouts and color settings for Photoshop from the Exercise Files tab.
- Preparing a composition for masking
- Manually adjusting problematic edges in a composition
- Combining layer effects and adjustment layers
- Roughing in a polygonal mask
- Cloning a super-massive Smart Object
- Applying Variations as a Smart Filter