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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 use the Depth Mask combined with the Lens Blur Filter in order to fashion a depth of field effect, in which we maintain the focus of the foreground image, and we blur away the focus of the background. If you would like to join me, I am working inside of a catchup document called Grass masks.tif that features the original photograph from iStockphoto.com, Bobby Osborn, as well as three variations on our depth mask. The rough version of the depth mask here inside of the rough alpha channel. We also have the field alpha channel. It's cleaned up version of the rough alpha channel.
Then we have the depth mask, the actual, final depth mask in which I added a Black to Transparent gradient over top. Let's go back to the RGB image. Let's switch over to the layers palette here. I want you to go ahead and press Ctrl+Alt+J or Command+Option+J on the Mac in order to jump this image to a new layer. We also have the option of renaming this new layer of course, and I am going to call it Lens Blur, because we are going to be applying the Lens Blur filter. Now I will click OK in order to create that new layer. Then I want you to go up to the Filter menu, choose Blur, and then choose this guy right there, Lens Blur. Now I am choosing to approach the image in this way. That is to say, I am applying Lens Blur to an independent version of the image, for two reasons.
First of all, I want to effect the image while leaving the original intact, just in case I want to come back to the original image at some point in time. You might ask why am I not applying Lens Blur as a smart filter, however. The reason is that Lens Blur is the only true filter in Photoshop's arsenal that is not available to us as a smart filter. So we have to go the static route instead. Go ahead and choose the Lens Blur filter, and you will bring up this big dialog box that takes over your entire screen. Make sure that the preview function is set to Faster, otherwise it's going to take forever to preview the effect.
Then I want you to increase the Radius value from 15 to 45, which is going to blur the heck out of the image. You also have the option of changing the shape of the iris of the lens. So the Lens Blur filter is trying to simulate an actual defocused effect. So softly focused effect as per produced by an actual film or digital camera. So you have got a lot of options available to you, but I am going to suggest that we just go ahead and ignore these options down here, leave them set to their defaults, that is to say.
Just go ahead and increase the Radius value in order to increase the degree of blur. Here are the options up toward the top, these are the options that I am more concerned about, depth mask. Notice that you have the option of setting the masked area to one of your masks that are in here inside the image. So you can work with the Transparency Mask, which is of course the transparency that's associated with the selected layer. You can work from the layer mask, if you were to have one, we don't have one actually in this case, but we do have a series of alpha channels rough field and depth.
Let's go ahead and for the mean time and choose Depth in order to apply that final depth filter, and you can see what a terrific job it does of maintaining the focus of the foreground image, the model herself, as well as the grass toward the bottom of the image. And it also blurs away the grass toward the top of the image, and we have a gradual transition between the high focus and low focus grass as well. Thanks to the gradient that's inherent inside of that depth mask. Now a few other options that are going on here, notice that there is this Blur Focal Distance value that's set by default to 0, which is a luminance level. So what its saying is that where the luminance level is 0, i.e., black the image is highly focused. So if we were to change this, we are going to change the Blur Focal Distance to 255, then White would indicate high focus.
So toward the top of the image you may recall that's where we have white inside of the alpha channel, toward the top of the image it would be highly focused, and towards the bottom of the image down here where we have black would be out of focus. So you can turn that equation on its head if you want to. You can also specify that any other luminance level is sharply focused. So if I were to tweak this down to 48, I would say that this region right here, this little bit of grass down here is highly focused, everything else is in low focus to some degree or other, depending on how close it's luminance levels are, to 48.
So I want to go ahead and switch this back to 0, so that the black portions of the mask are highly focused, and the white portions are out of focus. I could of course also invert that mask if I wanted to, in order to reverse which portion are focused or out of focus. You get the idea. Anyway, I am just going to leave the default settings intact. So Invert is turned off, Blur Focal Distance is set to 0, Source is set to depth. Now if I wanted to, that's all I'd have to do. I can click OK, and be done with it, or I could tell you, there is a better way to work, and that is to say we could be effecting the depth and the model independently of each other, using a Knockout Mask.
So here is what I want you to do instead. Rather than going with depth, rather than having the Source set to depth, which does go ahead and apply a depth to field effect, let's go ahead and give ourselves a little more control over the effect by switching the mask to field. So that she is sharply focused, the grass is out of focus, and will effect the grass independently using a Knockout layer. After you select field here, so set the Source to Field, set Radius to 45, and then go ahead and click OK in order to create this effect right here. Now we will take a moment to go ahead and apply the Lens Blur filter, but once it's finished, we should see this effect on screen. And notice it's not a particularly realistic effect. It is pretty nifty, and that it's done a good job of preserving the model protecting her from harm.
The background, the grass that is to say, is uniformly out of focus. So it's Lens Blur to throughout, but where things kind of breakdown is down here in the foreground grass, even though the grass is out of focus, the edge of the grass blades are in sharp focus. So they are masked very nicely here, which looks completely ridiculous, and of course, the dandelion are in focus in various locations as well. We are going to go ahead and fix that. So you can create a really great looking depth of focus effect that's also very flexible, in the next exercise using a Knockout Mask.
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