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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 we are going to take the Difference mask that we created in the previous exercise. Notice that I am still working inside that exact same Applied images.psd composition and I created the Difference mask using, of course, the Calculations command. We are going to take that Difference mask and we are going to enhance its contrast so that it becomes a usable mask. Normally I would use the Levels command to enhance the contrast, but we have so little contrast going on inside of this Difference mask that I am going to have to bring up the big guns by which I mean the Curves command.
So press Ctrl+M or Command+M on the Mac to bring up the Curves command. You should be working in the difference channel, by the way. I am going to go ahead and drag inside of the image here. I want you to watch the bouncing ball in the graph. I am going to drag inside of the lightest portions of the hand and notice that we are still in a very dark region of the graph. We are down into the left of where the bouncing ball is and these are the highlights. This is as light as it gets folks. If we go into some of these darker areas of the hand, you can see that the ball is descending precipitously until we go, of course, down here into the blacks.
So this area of the histogram is the area that we want to send to white and then this region over here is the region that we would want to send to black. Now we could try to pull that off using the Point tool right there, but we are going to have better luck if we use the Pencil tool. The Pencil tool allows us to create an arbitrary map which allows us to create big, huge distinctions in contrast, which is what we need. So go ahead and get that Pencil tool and I am going to drag from about here. Notice the Input value, I want you to position your cursor at the top of the graph so the Output value is 255 and the Input value should be something like 15 for now, let's say.
I am just going to drag over to the right from that point, like so, and notice that I am dragging above the graph so that I get a strictly horizontal line that is all the way white, so that I am sending all those colors to bright white as you can see there, all those luminance levels. Then I am going to come down here to an Input of 14 and an Output of zero and I am going to drag over to the left and I will make all those colors black. Now this looks pretty good. We have made the hand nice and bright. That's nice. But we have also picked up some these sort of whiskers or iron filings, these white iron filings down here in the shadow detail and we don't want that. So let's go ahead and make our colors a little darker, make our image set just a little darker by dragging over one pixel at a time to the right with our Pencil tool here, with the Pencil tool.
At about an Input level of 19, things are looking pretty good. The way I am working here is I am sending everything that has a luminance level of 19 or brighter to white and everything that has a luminance level of approximately 18 and darker to black and I get this effect. Now if you decide to apply your changes slightly differently, that's okay, you could pick 20 as your threshold or something like 17 as your threshold, that kind of thing. Now so far we could have used the Threshold command to achieve this effect and had an easier time of it, frankly. But the Threshold command does not have this function right there, the Smooth button. I want you to click on Smooth once and that goes and creates a smooth transition, the smoothish transition between those two radical shifts in the arbitrary map.
As a result, we preserve a little bit, a modicum of natural edge right there. That's what we want. So just click on Smooth once and only once, then click on OK in order to accept that modification. So, so far so good, everything is looking great. This mask is coming along quite nicely. In the next exercise, we will use the Dodge and Burn tools along with Gaussian Blur and Levels in order to complete the hand mask.
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