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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 the previous exercise we added a sky to my Stonehenge photograph. We converted the sky to a Smart Object, and we scaled it to about 150%, so it better fits its new environment. We went ahead and masked the sky, so that it appears to be behind the rocks, and we blurred the sky using a Smart Filter application of Gaussian Blur. In this exercise we are going to take care of some of the aberrant edges inside of the sky mask. I am working inside of an image called Stone meets sky.psd, it's found inside the 11 layer masks folder, and I am going to zoom in on this right side of the image here, and you can see one of the problems, even though the mask is in pretty good shape, we have got a very sharp edge above the tree line here, which doesn't make any sense whatsoever, because the trees are entirely out of focus so this should be a very, very soft edge. And you can see that the hard edge goes all the way across the photograph.
Notice that, and it looks just weird as heck. So we need to take care of that problem, eventually, we are going to address this issue using something called a Knockout Layer, but first let me show you the more traditional approach. I would go over here to the Sky layer mask and click on it to make it active, then I would probably grab something like the Gradient tool, it's probably the simplest solution. So I will go ahead and click on the Gradient tool to make it active. I still have my radial gradient from the magazine cover exercise selected here, so I am going to right click on this down pointing arrow head and chose reset tool in order to restore the default settings. And my foreground color is black currently, make sure that yours is as well, and then I am going to switch the style of my Gradient to this guy right here, which is Foreground to Transparent. And then I will press the Enter key or the Return key on the Mac in order to accept that modification, and I will drag upwards. So I am actually shift dragging to constraint the angle of my drag to exactly vertical, and then I will release.
And it looks pretty good; it's nice that now the sky is being peeled away from the trees, and we have a soft transition there, but let's say that's not exactly what I want, I want the Gradient to be a little higher. So I would have to press Ctrl+Z or Command+Z on the Mac in order to Undo that gradient, and try again, and then if I think that's too much, I have to undo it again, and try a smaller gradient. Basically the thing is, even though, this is a point I am trying to make, even though, we are basically creating what most folks would term non-destructive modifications, because we haven't harmed a single pixel in the Stonehenge image.
And thanks to the fact that we applied a Smart Filter and a layer mask to a Smart Object, we are not harming a single pixel inside of the sky layer either. However, we are now making permanent modifications to the layer masks. So if I were to Alt+Click or Option+Click on this layer mask thumbnail, you can see that I am indeed editing the layer mask on a pixel basis, meaning that I can't really modify the gradient independently of the rest of the layer mask, the way that I would like to, What I would really like to have happen, is I would really like to have layers inside of my layer masks, so that I could put the Gradient on an independent layer and be able to modify it independently, but that would be a lot of nesting of functions inside of Photoshop. Can you imagine if you had a layer that had its own mask, and the mask has its own layers, and those layers had their own masks, and so on, and so on. We would have a layers palette that was about as wide as the screen with all the indents that would be going on.
So rather than go that route which would be pretty weird, we are going to go another weird route. We are going to apply a knockout layer. And that knockout layer will serve as an independent layer of transparency. So I am going to undo the gradient that I just added there. I am going to Alt+Click or Option+Click on a layer mask thumbnail in order to return to the RGB composite. I am going to press Ctrl+Shift+N or Command+Shift+N to make a new layer, and I am going to call this layer knockout. And you don't have to select anything else, you don't have to change to blend mode, just go ahead and click OK in order to make that new layer. I still have my gradient tool active. Now I could draw a Black to Transparent Gradient, but I am going to work with a different color just to make a point here. I am going to go ahead and expand my color palette for a moment, and I am going to change the saturation and brightness values to 100 a piece like so. So that we have 0, 100, 100 for Red.
It doesn't matter which color you choose, it could be any color you want, it could even be white, just some color needs to be a work there. Then you definitely want to be creating a Foreground to Transparent gradient, very important that it turns transparent, and now I am going to draw a gradient like so. Notice that I still have the shift key down so that I am constraining the angle of my gradient to exactly vertical, and then I will release, and at this point you might question what in the world I am doing? I mean, why am I covering the entire image with this bright blood red gradient, other than of course to destroy the image entirely? The reason is that once we convert this layer to a knockout layer, anywhere where the layer is Opaque will cut a hole, anywhere where the layer is transparent will cut no hole at all, and translucent pixels will basically soften the transition.
All right, so check it out. We have got this knockout layer. Right now it's just called knockout. I am going to double-click on it, either on the layer or on its thumbnail in order to bring up the Blending Options panel of the layers style dialog box. Let's move things over here a little bit. Notice we have got this option that's called knockout. Now by default it's set to None, you have two other options, Shallow or Deep. Shallow is designed to cut to the bottom of a group. If we had a group going, it would cut to the bottom of the group in no further. Deep will cut all the way through to the background layer.
Well, because we have no group at work here inside of the layers palette, meaning one of those little folders that's what a group is, because we don't have one, Shallow and Deep are going to do exactly the same thing. So I am just going to go ahead and choose Shallow. But you will notice exactly the same thing, but right now it means nothing. I am not doing anything to the image whatsoever. And that's because knockout alone doesn't do anything, you need to combine it with a function that's going to make the pixels transparent, and the best way to work is with this fill opacity value. It's not the only way to go, but it's by far the most predictable way to go.
Don't use Opacity, it doesn't work, you need Fill Opacity instead, and I am going to reduce the Fill Opacity value all the way down to 0%, thereby making those red pixels entirely transparent, and forcing them to cut through to the background layer. So once again that's a combination of knockout set to Shallow or Deep, either one would work for our purposes, and Fill Opacity is set to 0%, that's very important, all the way down to 0%. Then click OK, and we now have a knockout layer.
And check it out; let's see what it does here. You can see if I zoom in, that it is cutting down to the contents of the background layer, so this is what things look like without the knockout layer. This is what they look like with the knockout layer. And now, here is what I consider to be the coolest thing, I can press the Ctrl key in order to temporarily get the Move tool, that's the Command key on the Mac, and with Ctrl+Shift+Down, so that I am performing a vertical drag, I can move that gradient up and down. Every once in a while you may see it peel away from the edge, but that's just because for a second it's getting mixed up and performing a horizontal movement. So this would be command+Shift+Dragging on the Mac, and I can move that gradient up and down independently of the layer mask. So it really is an independent masking layer, which is exactly what I want. This is a truly non-destructive modification.
In the next and final exercise of this chapter we are going to take care of the remaining problems with the edges as you can see. See how they are a little far out right there, a little bit of haloing going on, and we are going to take care of those edges. We are going to choke them in, you are going to see a combination of Gaussian Blur and the Levels command, stay tuned.
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