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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."
All right gang, you may recall in the previous exercise we went ahead and applied our Depth mask using the Lens Blur Filter, but rather than working from the actual Depth mask, this guy right here the Alpha Channel that's called Depth, I went ahead and applied the Field mask instead which blurs all of the grass in a quite unrealistic way, so that we could independently adjust which portion of the grass is focused and which portion of the grass gets out of focus using a Knockout layer and that's what we are about to do.
So if you are just joining me by the way, I am working inside of an image called Depth mask.psd that's found inside the 12 Specialty folder and it features two layers, one is called the Background layer of course and the other is called Lens blur. I want you to go ahead and click on the Lens blur layer to make sure that it's active. Then I want you to press Ctrl+Alt+N or Command+Option+N on the Mac in order to create a new layer and let's call this one Knockout of course, because we are going to be creating a knockout effect here. Go ahead and click OK and then let's go ahead and grab the Gradient tool once again and just to make sure that we can see our gradient quite independently of the rest of the image, I want you to set the foreground color to something that compliments the image that is to say a color that we are not finding elsewhere inside the image. So blue comes to mind.
I will go ahead and change the Hue value to 210 here inside the Color palette and I will change the Saturation and Brightness values to 100 a piece, so we have got a little bit of a cobalt going and I will go ahead and make sure that my Gradient Style is set to Foreground to Transparent as it is. The Linear option is also turned on at this point, Opacity is set to 100%, Mode is Normal, Reverse is turned off and so on. Now I am going to draw a gradient from this dandy line just like we did a couple of exercises ago. We will draw the gradient from the top of this dandy line up like so and I have got the Shift key down this entire time and then I will release in order to draw a gradient that looks like this. So she looks like she is mired in some weird blue pudding or something along those lines.
Now we will take this Knockout layer and we will turn it into a Knockout, a true Knockout so that it's knocking out the effects of the Lens blur layer below it. Go ahead and double-click somewhere on the Knockout layer in order to bring up the Layer Style dialog box, then I am going to change the Knockout setting to Shallow, just because we want to burrow through one layer down to the background. I will set the Fill Opacity value of course to 0% in order to make not only the layer transparent so that we are not seeing any of that blue gradient, but also we are using the blue gradient to burrow through the layer below it. Then I will click OK in order to apply the effect.
Now you can see that we have indeed burrowed down to the background image. So we are burrowing through the foreground grass toward the bottom of the image. This is without -- I will go ahead and turn off the Knockout layer for a moment. This is without the Knockout layer, you can see the grass toward the bottom of the image is out of focus, this is with the Knockout layer and so it is doing its job, it's knocking out the layer below. But not only that, now we have the option of moving this Knockout layer and deciding exactly which portions of the grass are in focus or out of focus. I will go ahead and grab the Move tool and I will go ahead and drag the gradient layer down so that much of the grass is now out of focus. You can see that this entire big range toward the top of the image is out of focus at this point.
Now I will drag it back up and as I drag by the way I have got the Shift key down so that I am constraining my drag to exactly vertical and as I drag my gradient upward, you can see that less and less of the grass is out of focus. So this was before my move, lots of the grass out of focus toward the top of the image and this is after the move much more of the grass in focus. Now I have revealed this big area toward the bottom of the image that's now out of focus and that's because we have got a gap inside of the Knockout layer itself. I would go ahead and Shift+Tab the palettes away so that I can better see what I am doing. I would get my Marquee tool in order to fix this problem down here and I will drag around this region and I am trying to avoid an auto scroll, but I bet I won't be successful.
Actually I was successful, nice, and because I want this area -- I will go ahead and press Shift+Tab once again so that I can see my Knockout layer. I want this area to be filled with blue. My foreground color is currently set to that same shade of blue. So I will press Alt+Backspace or Option+Delete on the Mac in order to fill that region with color so that we are burrowing a hole through that region to the Background layer. So this gives us total flexible control over exactly which portion of the image is in focus and out of focus. It also gives us a higher degree of control if we want to apply say a blend mode to this effect that we have created here.
Let's say that I want this blur to exclusively lighten the background. Then I would bring up my Layers palette once again and I would go ahead and change the blend mode that's applied to this Lens blur layer, I will change it to Screen so that we are getting this nice lightning effect associated with the Lens blur as well. Now the problem is at this point even though it's just affecting the top grass, it's also affecting the top of her as you can see here, I will go ahead and drag her over a little bit so that you can see this is what the image looked like before I changed the blend mode, this is what it looks like after.
So I am lightning the top of her head and her shoulders and so on. So that means I just need to mask away that Lens blur layer inside of the model. I will do that by going back to the Channels palette. I will go ahead and load the Field layer in order to load that selection that traces around the model right there. I will switch back to the Layers palette. I will click on this little layer mask icon in order to convert the selection to as layer mask, she is once again protected inside of this layer and the grass is affected exclusively.
So you can see now, if I turn off that layer or turn it back on, she doesn't change, her luminance doesn't change, the luminance of the grass changes alone. So there we have it, the final version of the depth of field effect created using a combination of the Depth mask, the Lens Blur filter and a Knockout layer.
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