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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 show you how to create a Difference mask. Now the great thing about a Difference mask is that it highlights everything that changes from one channel to another and it gets rid of everything that doesn't change, that's the same from one channel to the next. By the way, I am working inside of a catch-up document called Applied images.psd, found inside the 16_tough_stuff folder. I have got my hand layer selected. It's active and it's also fully opaque. I am going to go ahead and press Ctrl+1 in order to see the red channel, Ctrl+2 for the green channel and Ctrl+3 for the blue channel. That's Command+1, Command+2, Command+3 on the Mac and you can see how things are shifting from one channel to the other.
So let's start by looking at the shadow. Keep your eyes locked in this region down here. Here is the red channel. Notice that the shadow is a bit lighter there. There is the green channel, darkens up ever so slightly and the blue channel darkens up still more. So there is a little bit of variation in this shadow detail. The pen also changes a little bit. You will see the pen is brightest in the red channel, especially these gold areas of the pen right here. It gets a little darker in the green channel and goes pretty dark, pretty darn dark in the blue channel. Then we have got all kinds of change happening in the hand. So this is the red channel where the hand is concerned, very bright. This is the green channel, quite a bit darker and the blue channel darker still.
So the biggest amount of contrast overall is between the red channel and the blue channel, which is not surprising, given that there are flesh tones involved here. I am going to press Ctrl +Tilde or Command+Tilde on the Mac to return to the RGB composite image. Then let's go up to the Image menu and choose the Calculations command. I want you to make sure that Source 1 and Source 2 are both set to the same image, the image that you are working on which in my case, of course, is Applied images.psd. I would go ahead and set both of the Layer options to hand, just to be careful, Merged and hand will give you the same result, but might as well do it the right way, again in good habits here.
Then Channel should be Red, the first channel should be Red, the second channel should be Blue, because there is the most difference between those two channels and then change the Blending mode, not surprisingly to Difference, because it's capable of finding the differences inside the two channels. As soon as you do, notice what happens. You all but totally get rid of the background. It goes black. So basically, where the Difference mode is concerned, everything that doesn't change is going to show up as dark. So we have quite a bit of change going on inside the hand, not all that much though, because this is a pretty dark hand at this point, a little bit of change going on inside of the gold sections of the pen and very, very little change, next to no change going on in the shadow detail and the white background. So we can see a little bit of shadow, but that's it. We will be able to scrub that away pretty easily.
Now at this point with these settings, I want you to click OK in order to create your difference mask. Let's go over to the Channels palette, let's name it, difference, like so, and we are good to go. That is our base difference mask. We will increase its contrast so that we get something we can use in the next exercise.
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