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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."
We are going to start things off by creating a shadow mask that will allow us to convey the shadow as it was actually captured by the camera. So this is a natural shadow photographed against a white background, and you can see that it's a fairly natural white background that's going on here, it's not a fakey white background, the image hasn't been messed with. So we have a lot of gradations of luminosity going on here. We want to capture and convey all of those gradations into a different image. This image right here, this will be the final version of the composition. And you can see that I have made the shadow darker, so I have enhanced that shadow but it is the original shadow. I haven't gone in there and painted in some additional details.
And I have also managed to capture the varying degrees of focus that are associated with the hands. So the hand in the foreground is out of focus, whereas the pen nib which is relatively in the background here is in sharp focus. And I have managed to capture all of that information with this masking technique that I am about to share with you. Now, a word of advice upfront. When capturing images whose shadows you want to transfer, I recommend that you capture the shadows against a white background as was the case with this image right here. And the reason I suggest you to do that, not only in an absolutely smooth background like this, but a white background as well, don't capture against a blue screen or a green screen.
And the reason, I recommend that approach is because that way, you are going to have the highest degree of contrast between your shadow and the background, and it give you a sense of what I mean. I am going to go ahead and grab my Brush tool here for a second, and I am going to bring backup my palette and let's just pretend we are working with a blue screen photograph. I'll just go ahead and dial-up 255 blue, leave red and green down at 0, and I increase the size of my brush a little bit. Notice that I have the Mode set to Multiply, and so I can just go ahead and paint over the shadow a little. And then, we'll take a look at what it would like if we had shot the shadow against a green screen as well. I will go ahead and paint in some green over this shadow. So this should be a shadow against a green screen, let's pretend, and this would be a shadow against a blue screen.
Now let's check out the individual color channels. Inside of the Red channel, we have nothing to work with. We've got black on black essentially, and this is an exaggeration. If you are working with a real blue screen or a real green screen, you will have a little bit of distinction going on inside the Red channel, but not nearly enough. And then, inside of the Green channel, the shadow against the green screen looks okay, but the shadow against the blue screen looks terrible, and then, in the Blue channel, the shadow against the green screen looks terrible, and the shadow against the blue screen looks all right. But what you've done is you have waddled down your shadows to a single channel, if you are shooting against a green screen or a blue screen, which is why it is best in a perfect world here, if you are going to convey those shadows, you need to capture them against white.
Now here is the worst case scenario. I'll show you another image here just by way of contrast. This shadow right there, the shadow that's being cast by this couple, its a great shadow, and if the kind of the shadow that you might want to go ahead and try to capture and convey against a different background, if you are creating a special composition for this couple, for example, this shadow though, is not going to do you much good. It's not going to work out for you well, because not only it is captured against a colored background, we could probably work with this light sand color, the problem is, all these little micro shadows in the sand that are appearing outside the big shadow and inside the big shadow as well, and we are not going to be able to effectively deal with that shadow information. We are not going to be able to peel those out with any degree of success.
So this shadow is of no use to us whatsoever, you would have to rebuild the shadow, which would be very difficult. You have to take this couple and skew them, I suppose, but even then, it wouldn't look right. You wouldn't get the right dynamics inside of his leg. For example, this little area here doesn't really translate to this area here. With a simple skew, you would have to do some additional work. All right, so long story short here. Just go ahead, and make sure that you capture your shadow against a white background. Go get yourself an image, or of course, you can work with mine, the sample images that I am providing for you, and then, join me in the next exercise.
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