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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."
In this exercise, I am going to introduce you to one of the oldest commands in all of Photoshop as well as one of the most powerful. That's this command right here, it's under the Image menu, and it's called Calculations. Now what the Calculations command does is it allows you to blend any two channels with each other. So they can be a color bearing channels such as Red, Green, and Blue or they can be alpha channels, and you can blend those two channels together in order to create a new alpha channel. The reason, you would want to do such a thing, is for one thing, to generate a better mask.
So as opposed to working directly from the Red, Green, or Blue channel, and then enhancing the levels, and overlay painting, and all that good stuff, you can blend two channels together in order to create the base alpha channel and get some additional work done. So you save yourself time plus you can create more details, more articulate, more accurate masks. Now that is, of course, assuming that you come to terms with the way the Calculations dialog box works. It is a little bit complicated but I assure you if you stick with me, you will get a sense of what's going on there and you will become a master masker. This is really an advanced command.
So switch back to the RGB version of the image or you to have the image open and I am not sure that you do because I haven't told you where it is. It's called Model on blue.jpg and it's found inside the 14_Calculations folder. This image comes to us from Klaas Lingbeek- van Kranen of the Netherlands and he works with iStockphoto.com. It features this woman with auburn hair set against a fairly, uniformly blue background, a little darker toward the top than it is at the bottom. She has got a little bit of fly-away hair going on, some softly focused skin. So she is a challenge. Also, she is alternately lighter and darker than her background. So we just can't just say, gosh, she is uniformly lighter or she is uniformly darker than her background as we have with previous sample files. This one, we are going to have to take a different approach, essentially.
So notice here is the Red channel, she is very bright, her skin is very bright, her hair isn't all that contrasty with the background; it's sometimes lighter, sometimes darker. Then dress is also sometimes lighter and sometimes darker than the background. More often than not, it's darker. So in other words, sometimes the foreground image is lighter, sometimes the foreground image is darker than the background. You understand, right? All right, so here is green image; not a lot of contrast going on in the skin tones, dark dress compared with the background, the hair is not a lot of contrast either. The Blue channel, we have the best contrast where the hair is concerned because all of a sudden she is turning into a Brunett, essentially.
Her dress is darker than the background, her skin is a little bit, the edges are a little bit darker than the background but we do have some highlights going on every once in while and, of course, there is not a heck of a lot of contrast. So there is no one channel that's leaping out and saying, "Hey! Choose me, choose me." Instead, we are left with three channels that really aren't any good for establishing a base alpha channel. When you encounter such an image, that's when you think about using the Calculations command. Sometimes, you can use the Color Range command. I have showed you that in the past but other times, when going gets really tough as it does in this image, we have got to resort to Calculations.
Now we are going to mix two channels together, bear that in mind. Of course, then we come to which two channels do you want to mix? More often than not, you want to mix the two channels that are most different from each other. So in other words, you look at these channels and you go, okay Red, she is light, background dark green. Pretty much the same, she and the background are both, sort of, in the mid-tone range. Then Blue, she has got dark hair against the light background. So she is the darkest in this channel and the background is the lightest. So in other words, the channels that are the most different from each other are Red and Blue. This is going to be true anytime you have a model, or a human being, or a portrait shot of any kind, set against the blue background; you are going to talking about merging the Red and Blue channels. If you have a model set against a green background, then you would be wanting to merge the Red and the Green channels with each other, and so on.
Also, if you had a portrait shot of a person against the blue sky, you would also be mixing using the Calculations command, you would be mixing the Red and Blue channels with each other. So once again, you look for the channels that have the highest degree of difference. Then you go up to the Image menu and you choose the Calculations command. Now before I choose this command, I want you to know, in this exercise I am introducing you to the way that the dialog box works. In the next exercise, we will actually use it to build the base alpha channel. So I am going to choose the Calculations command, it brings up a fairly murderous looking dialog box. Not so much because there is all that many options, there are quite a few options here but nothing compared to like Layer Properties where you have more than a 100 options organized in, like, more than ten different panels; that kind of thing. This time you just have a lot of superfluous options and a lot of options that don't make a, heck of, lot of sense.
For starters, Calculations is talking to you in terms of Source 1 and Source 2. What in world does it mean by that? Well, Source 1 is the first channel and Source 2 is the second channel but actually, the way that it works is the Source 2 channel is at the bottom of the stack. So imagine we are putting one channel on top of another channel inside of a layered stack. Source 2 would be at the bottom of the stack and Source 1 would be on top of it. So remember that because that becomes important when we start applying the blend modes down here. It's asking, all right, what image are we working in for starters? Where is the first channel coming from, image and layer wise? Where is the second channel coming from, image and layer wise? Well ostensibly, we are working inside of the same image but you could work inside of a different image. If you have two images open, two or more images open, that are exactly of the same physical size, the same number of pixels wide and tall, then you can select any of those images from this list.
In my case, we want to stick with Model on blue.jpg and for you Model on blue, may be the only image that's open. There is only one layer inside this image, Background. So Layer, in both cases, has to be set to Background. So then, we just ruled out four of the options here. These first two options in Source 1 and the first two options in Source 2, just leave them alone. Next comes Channel, this one is important. Now it's telling you, we are going to merge the two different channels, Red and Blue. So let's go ahead and set Source 1 to Red and we will set Source 2 to Blue. So in other words, Red is going to be sitting on top of Blue. Now, of course, we want to get to a state where she is very bright, including her hair, and her background is very dark.
So far we don't have that. Now inside the Red channel, we do. She is, if I move this out of the way, we can see this little thumbnail back here, she is fairly light in the Red channel and her background is fairly dark, overall, but in the Blue channel, she is, more often than not, darker than her background. So we need to invert the Blue channel and we are going to do that by turning on the Invert check box. Now notice, we get this ghostly image here, against a dark background. That's a good thing. So, more often than not, she is lighter than her background; which is a good thing, by virtue of the fact that we are blending these images together.
Now if at anytime, you want to see one of the channels by itself because right now, we are multiplying them together as per the Calculations dialog box default setting; but if you want to see one or the other by itself, go ahead and switch over to Normal. Now when you choose Normal, you are going to see the Source 1 channel, which is red in this case because it's on top. If you want to see the inverted Blue channel, then change the Opacity value to 0, like so. Then you can see that background channel, essentially here. So always remember, Source 1 on top, Source 2 underneath it.
All right, I am going to change that Opacity value back to 100%. The next option here is Mask, we are going to give that a slip for now, we will come back to it later. Then finally, we have got this guy, Result. What do you want to do with, whatever, you are making here? Which you can preview, by the way, we have got a Preview check box, so we can see what we are doing. Where do you want to put this amalgam of these two channels? The answer is, more often than not, put it inside of a new channel inside this document. You could also generate a selection right after that, if you wanted to or send it to the new document, instead, but we want New Channel which is going to deliver a new alpha channel here inside the Channels palette.
That's it. That's as far as we are going to go inside this exercise. In the next exercise, we are going to investigate, what we can achieve here using the Blending Options in order to generate a base alpha channel, in order, of course, to generate a mask.
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