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The elusive alpha channel remains one of the most misunderstood yet powerful tools in Photoshop. Alpha channels are collections of luminance data that control the transparency of an image, and they inform just about every aspect of Photoshop. As he builds transitional blended layers, fashions a depth map, makes edge adjustments, and takes on extreme channel mixing, Omni Award-winning expert Deke McClelland teaches Photoshop users that where there's a will, there's a way. Photoshop CS3 Channels and Masks: Advanced Techniques covers mapping texture on an image, turning flesh into stone, using vector masks, working with all different channels, creating a rustic edge effect, and much more. Exercise files accompany the tutorials.
Download Deke's customized keyboard layouts for Channels and Masks from the Exercise Files tab."
So in the last exercise we used the Add mode combined with the Calculations command in order to generate an Alpha Channel, this one right here, for our superhero hair dude from photographer Chris Schmidt. And this is a very serviceable Alpha Channel, at least a base channel on which to build a mask. However, maybe we'd be able to do better and certainly we would like to gain some experience with the Subtract mode. So let's give it a shot, let's see if subtract might give us something better than Add. I want you to return to the RGB composite view of the image. I am going to press the F key -- oh, by the way before I press F however I am working with a catch up document, in case you want to open it up, it's called Hero with Add.tif, meaning it's our Hero guy with of course an Alpha channel created using the Add Blend mode.
All right, now I am going to switch back to the Full Screen mode here, so that I can keep an eye on as much of the vertical aspect of the image as I can inside of this fairly small screen view here. Then I am going to go out of the Image menu and I am going to choose the Calculations command. Now Calculation should come up with a last calculation we applied, and it certainly does for me, the Source 1 channel is set to Green, Invert; and the Source 2 channel is set to Blue, Invert. Fine! Blend is set to Add because that's the last thing we applied. This time let's try Subtract, so I am going to ahead and change the Blending from Add to Subtract.
And just to get a sense of what's going on, notice that this doesn't deliver anything like what we had a moment ago. We are not seeing any of the hairs. This mask would be pretty useless, I think, unless we were trying to select some sort of ghoulish version of the face. That's definitely not what we are going for. We are trying to separate the hairs from the background. So I am going to suggest to you that our channels are messed up at this point. We are not going to just go in and start adjusting the Offset and Scale value because we are near where we need to be. We should at least be in the range before we start fooling around with Offset and Scale. So our channels are indeed messed up, and let me show you what's going on. I am going to go ahead and change the Opacity value to 0 for a moment, so we can focus on the Source 2 by itself. And remember from a couple of exercises ago that the subtract equation is Source 2 minus Source 1.
All right, so we are going to start with whatever the Source 2 channel is and we are going to subtract Source 1 from it. So let's take a look at the Source 2 channel. It's Blue Invert so it looks like this, light hair against a darkish background. That's a good thing. Let's just see what our green channel looks like when Inverted as well. We'll just go and change Source 2 to Green since we can see the Source 2 channel right now thanks to that Opacity value of 0%. And it's also white against dark. Ding, ding, ding! That's the warning signal going off. It's what I am suggesting here, because if we are trying to make the hair turn white which we are against the black background then we can't be subtracting white from white.
Because white has a Luminance Level of 255, so if we take 255 minus 255 so the white herein the Blue Invert channel and we subtract the white here in the Green Invert what are we going to get? We are going to get 0 which is black. So we are going to send the hair black, which is exactly what's happening. So if I change this back to blue and I set the Opacity value to 100% you can see that we are sending the hair to black, we are sending the background to black, we are sending everything but a little bit of his face to black. That's why we need to subtract from the Blue Invert channel we need to subtract Green with Invert turned off, and we are going to get this effect right here instead.
And that's because we are subtracting black hair because Green Channel has black hair in it. Let's go ahead and look at the Green Channel thumbnail here inside the Layers palette. It has got black here. So we are taking the White here inside the Blue Invert channel we are subtracting Black from it, 255 minus zero gives you 255 so the hair remains white. Now it's going to take you sometime to get used to this kind of stuff and come to terms with what's going on with subtract but that gives you an early sense of what to look for. You should also just bear in mind that you can have a little bit of fun, irritating fun but still you can have some fun with turning the Invert checkboxes On and Off and see what you come up with.
For example, how did I know that I wanted to subtract Green normally from Blue Invert as opposed to Green Invert from Blue Normal? Well, just try it out. Turn one on and turn the other off, see what you come up with. Gosh! That's exactly the opposite of what I want, I don't have another Invert checkbox that I can select. So I need to turn these guys on and then back off like so. All right, so experiment, that's going to give you a clue as to what's going on as well. I am now going to take the offset value up, because we are roughly in the right neighborhood here. Because I have applied subtract I need to raise the offset value, so I am going to press Shift+Up Arrow, and I held on that Up Arrow key there and I raise that offset value to 160. Now another thing I should be able to do with Scale is telling you how slowly the Scale value reacts. I should be able to Shift+Drag on the word Scale in order to scrub it. Notice what happens when I Shift+Drag on the scale, I start scrubbing the offset value instead.
That's nice. Thank you for that, Calculations dialog box. Here's another thing you can thank the Calculations dialog box for. I was telling you that you can change the Scale value from 1 to 2. So this is the Scale value of 1, and this is the Scale Value of 2. Fine! What if I decide to change the Scale value to 4, which is out of range, by the way you can't use a Scale value of 4. If I were to press the Tab key, I'll get a warning. That's wrong buddy it's got to be between 1.000 and 2.000. But I can change it to four. It's not going to nag me immediately; it's just going to ignore me. But here's what I love. 5, I get ignored, 6, I get ignored, 7 I get ignored. And notice that I am getting ignored to the tune of it; it's just treating it like I have applied the maximum value of 2.
If I take it up to 9, I am still just getting ignored. If I take it up to 10, I get ignored in a different way. It now treats it as if I have a Scale value 1, because it's paying attention to the initial 1, is what it seems to be doing, and then ignoring the second value. So like if I change it to 16, same thing. You got to remember because otherwise you know you got to press Tab and it complaints at you that kind of thing. You just got to remember, you are going between 1 and 2. I am going to change this value to 1.4 again, that's the same scale value we used a moment ago for the Add mode and that is going to be consistent. Whatever you come up with for Add is going to work for subtract as well, is what I am trying to say. And then I'll go back to Offset, now that the image is too bright of course because I ended up brightening it additionally with a Scale value in addition to softening the edges. So I'll take that Offset value down to 90, and last time Blending was set to Add and the Offset was -90 and Scale was 1.4. This time it's Subtract, Offset is 90 as opposed to -90, and scale is 1.4. So scale is the same. And the only difference we have applied up here is to turn the Invert checkbox off for Green. It gives us the similar result.
All right, so once you have established these values, go ahead and click Okay for me, would you and you will create a second Alpha channel, actually it's a fourth alpha channel, and we'll call it subtract of course, and we'll call it 90 and then 1.4 like so. And then press the Return key or the Enter key in order to accept that modification. So this is the Add version of the mask and this is the subtract version of the mask. They are almost identical to each other, thanks to the modifications that we applied. So you can do the job using Add or Subtract, it's entirely up to you which approach you take. Sometimes you are getting slight differences here and there. Other times you are going to get almost identical result. And the approach you take as I say, is entirely up to you. You may find that one mode makes a lot more sense to you than the other for example.
All right, so we now have practical experience with both Add and Subtract, very useful modes for creating image calculations to create a base Alpha channel for a complex mask. In the next exercise we are going to set about modifying this mask using some traditional techniques in order to create an accurate selection outline that conveys all the different degrees of focus associated with this fellow's hair.
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