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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."
Now that we have a basic sense of how a Calculations command works, we are going to put it to use to create a base Alpha channel for masking this particular image, and specifically we are going to see some blue screening techniques. So in this case, we've got this image called Model on blue.jpg, it's found inside of the 14 Calculations folder, and you'll notice that she is very warm, and rosy, and light, and her background is very cool, and blue, and so on, so that she has got a high degree of contrast; complimentary color contrast between foreground and background.
You can set the setup in front of a traditional blue screen, or even a green screen if you like, or you can set your model against this sky, a cloudless sky frequently works well, and you can even get away with a clouded sky when you are using the Calculations command. The only thing that you want to make sure is that if you are working with a blue background, you don't have your model set in blue, that she is not wearing blue, for example, wearing blue jeans . If he or she was wearing blue jeans, probably I want to set them against a green background, if masking was your intention. In our case, she does have some blue threads working through the fabric inside of her dress but it's far enough toward the interior, just enough that we have an edge to work with here, so that we should be able to get rid of that stuff fairly easily.
Now she had some blue jewelry, we would just want to make sure that wasn't dangling directly over the blue background that was are set against her flesh, her hair, or something along those lines. So she is going to work out really nicely, where blue screening is concerned. I've got the RGB composite selected here inside the Channels palette. I am going to go up to the Image menu, and we are going to choose the Calculations command, and just as in the previous exercise, I want you to make sure that the first channel, Source 1 is set to Red and the second channel is set to Blue, and Invert is turned-on.
And you should see this by default because by default, the blend mode is set to Multiply so that we are darkening up the background fairly fiercely, and we are leaving some little lightest sort of darkish grays in the foreground. So, when you are working with a blue screen or a green screen, you'll probably want to start off with your blending set to Hard Light, that's just the good place to start. I am not suggesting that it's going to necessarily work for you, it's just a starting point, and in fact, you are going to have to experiment with the blend modes in order to figure out what blend mode produces the best results.
So I am going to go ahead and choose Hard Light. Now most of the time, you leave the Opacity value set to a 100%. I'll show you an exception shortly, but for now, we are going to leave it set to a 100. Now, notice here on the PC, that my blend mode is sticky, meaning than I can arrow through it, so if I press the Down arrow key, I'll advance to the Vivid Light mode that we see here. On the Mac, you can't do that. You have the actually manually select the modes which means this is one of those rare occasions, one of the very, very rare occasions where sticky options here under Windows is actually helpful, instead of just extraordinarily irritating, the way it is normally.
Now when you are working inside of a dialog box, it can actually be helpful sometimes, particularly, when you are cycling through blend modes. So anyway, I've got Vivid Light, you can see that it works pretty nicely. We've got a very light foreground image against a very dark background, and there are those little blue threads showing up as dark as well. We could work with this mode if we wanted to, but I would like to checkout some other ones, here is Linear Light, not nearly as good, here is Pin Light, forget about it, here is Hard Mix, now this is pretty interesting, in that we get either white or black pixels with nothing in between, and it does a pretty good job of differentiating the foreground from the background here, but we've got jagged edges.
Now if I want to reduce the jaggedness of those edges, if I want to soften them up a little bit, I would switch over here to the Opacity value, and I would reduce it. So I press Shift+Down arrow to reduce it to 90%. You can see, now we are introducing some soft edges, and if I go further, we will introduce some more, and more soft edges, and more gray values into the image as well. So our Opacity value is especially useful when combined with Hard Mix, not really useful at all when combined with the other options. All right, I am going to set that back to a 100%. Let's keep cycling down, we've got two modes that are unique to the Calculations and Apply Image dialog boxes. They don't appear inside the Layers palette and those are Add and Subtract, and they do just what they say, you are going to either add brightness value in the case of the Add option right here, which means that you are going to elevate the brightness of the image, or you can subtract brightness values, in this case, I've got my Offset value set to 140, it would really look like this by default, so very, very dark.
We'll come back to Add and Subtract, because they are actually very useful functions here inside the Calculations dialog box. We'll come back to those in a future exercise. Here is Difference, that's going to go ahead, and trace around the edges. It's going to use one channel in order to invert the contents of the other channel of course, and sometimes, it's very good in terms of tracing edges inside the image, other times, not so great like now. And then finally, we've got Exclusion set here, almost never useful inside of this dialog box because it delivers such tepid results.
Let's switch back to Hard Light, and let's up arrow now through the blend modes here. This is hard light, we saw that before. This is Soft Light, actually better. If you check it out here, we've got a fairly light image in the foreground against the fairly dark background, not a heck of a lot of contrast but we do have uniformity, that is the foreground image is more or less uniformly lighter than the background which is a good thing. Up arrow to Overlay a little stronger, then Soft Light but not much. Now let's go up to Lighter Color. Now this is interesting. This is a mistake, actually Lighter and Darker color don't produce any effects, they only produce effects on a full color composite image. They don't work on grayscale images.
So they shouldn't be here inside the Calculations dialog box at all, they got stuck here inadvertently. It's basically what happen in Photoshop CS3. So go ahead and skip them, they never do you any good, that's Lighter and Darker color are always the same as Normal. Here is Linear Dodge, Add in parenthesis, that's because it does the same thing as the Add mode. The Add mode just gives you a few other options to work with. So you can modify the results of the Add mode. You've just got to accept what you get with the Linear Dodge, and this is what Linear Dodge gets you, does the same thing we saw when we choose Add a moment ago.
This is Color Dodge, actually better because the background are little bit darker there. This is green; not so good, this is lighten, bad, this is darker color, non-functioning, this is Linear Burn, gosh! Interesting, that's not going to work for masking. This is Color Burn, a little lighter, a little better than what we had before, but still the dress isn't separating from the background, which means this is Multiply, we already saw that, she just looks freaky, and this is darkened, she just looks freakier. She does look freaky and bad because she is set against a light background. Here is what we want, of all of those, the one that I think looks the best is Color Dodge.
Now Color Dodge isn't necessarily always going to deliver the best results but it often does, so it's definitely one of the ones to check out. So you would basically want to work with your strong modes, meaning your Color Burn and Linear Burn, your Color Dodge and your Linear Dodge, and your Hard Light, your Vivid Light. Sometimes Linear Light is going to come through for you, and then Add and Subtract are extremely powerful. We'll come back to them later, but for now, we'll go with Color Dodge. This looks great to me, this is a nice starting point for our New Channel. So just go ahead and make sure all of your settings are set as you see them on screen right now. It's very important that the first channel is set to Red and the second channel to Blue Invert, then click OK, and you'll generate a new Alpha channel.
I am going to go ahead and name it, mask and I am ready now to go ahead and craft the mask using the Levels command, and an Overlay Brush, and so on, and that's what we will set about doing in the next exercise.
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