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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, in this exercise we are going to create an edge mask inside of Photoshop. Now it's always the same seven steps whenever you are creating an edge mask inside of Photoshop, you always follow these seven steps; you duplicate a channel in order to create an alpha channel, you apply the Levels command, you apply the Find Edges filter, you invert the image, you apply the Maximize filter, you apply the Median filter and then you apply Gaussian Blur. So those seven steps every single time duplicate Levels, Find Edges invert, Maximize, Median and Gaussian Blur, got it, okay, good.
Now of course we are going to walk through those steps here and you will see that the exact values that you apply inside the Levels dialog box and inside the Maximize filter will vary, but otherwise things are pretty darn predictable. So I want you to make sure that you've got Plains dwellers.psd open, it's found inside of the 12 Specialty folder and make sure that only the background layer is visible, but the other two layers inside the Layers palette are turned off, make sure also that the background layer is active. Then I want you to go to the Channels palette here, and let's check out our various color channels that are available to us. Now notice that went ahead and scanned this image from 90 years old print positive, I went ahead and scanned the image as an RGB image, it's a full color image. And I'll frequently -- you'd get this question, I mean basically all of these images -- sufficiently old images are going to be black-and-white images in the first place, it may have a little bit of sepia from the aging process or they may have some other coloring associated with them. But the biggest question I get is, should I scan these as grayscale images or should I scan them as color images? And the answer is, go ahead and scan them in color because you may get different results inside of your red, green and blue channels that way and the results that you get, you may find to be advantageous, especially if you have a lot of color variation going on inside of your photograph.
Now in my case, we don't have a lot of color variation going on. The red channel and the green channel and the blue channel are all pretty darn similar to each other. There is no channel that's in any way, shape or form like obviously better than the other. So what I recommend you do in the case of this image is just go ahead and grab the when in doubt channel which is a green channel and duplicate it. So I am going to grab that green channel, drag it down to the little page icon at the bottom of the of Channels palette in order to create a new alpha channel called Green copy. Then I am going to double-click on its name and I am going to change that name of course to Edge mask because that's what it's going to be. So that's step number one, duplicate the channel.
Step number two, let's go ahead and rule out some of the garbage inside of this image using the Levels command, and this isn't the first time we have used the Levels command in order to go ahead and enhance the contrast of the mask right at the outset, it won't be the last either. Generally speaking a good first step. So I am going to go up to the Image menu, I am going to choose Adjustments and I am going to choose Levels like we just go ahead and press Ctrl+L or Command+L on the Mac as well, and I am going to take the final input levels value down to 170, so I am saying anything with the Luminance level of 170 or lighter becomes white and that ends up getting rid of a lot of those pockmarks inside of the lighter details inside of the image. Now bear in mind we are not harming the original image, we are just creating an edge mask here. So we want to get rid of as much of that bad art effecting inside of the images as we can.
And then I am going to go ahead and raise the Black value, the black point value here to 60. So I am saying anything with a Luminance level of 60 or darker becomes black. And these aren't by no means magic values of course, these just happened to work very well for this particular image. Now I am going to go ahead and click OK, I have enhanced the contrast of the image, that's step number two. Step number three is to find the edges inside of the image, and Photoshop provides a special filter for this very purpose. Go up to the Filter menu, choose Stylize and choose this guy right there, Find Edges which will trace the edges inside of the image. And notice what a fantastic job it does; it doesn't bring up a dialog box, it just goes ahead and applies this effect automatically, and if I go ahead and zoom in on my great grandfather's face here, you can see that it has done a terrific job of tracing those edges, it's also given us some soft transitions, something that we don't get from the Threshold function inside the Unsharp Mask dialog box. We get these nice soft edges which is a really, really great thing.
Now we do get some trash edges, we still get a lot of sort of artifact tracing going on in the background and inside of the cheeks, inside of my great grandfather's face, for example, it makes him look a little bit unshaven, sort of give some Fred Flintstones/Homer Simpson look here, but in fact everybody has got it inside of this image, whether you are a man or a woman, you are still getting a little bit of bearding going on, a few whiskers going on. And that's because the Find Edges filter is tracing the artifacts, not the whiskers but the artifacts inside of the image.
Now we want exactly the opposite of this effect, meaning that we want to go ahead and adjust the edges so they ought to be selected and we want to ignore the non-edges so ought to be deselected so we need to invert this mask by pressing Ctrl+I or Command+I on the Mac, that's step number four. So immediately after you apply the Find Edges filter's step number three then you invert the image, step number four there. Next, we need to enhance these edges. I want to thicken these edges up so I want to expand the edges and I do that using the Maximize filter as you may recall, maximize expands the maximum luminance level which is white. So I will go up to the Filter menu, once again choose Other and then I will choose Maximum.
Now the last Radius value I applied was apparently 40 pixels, that's way too high. I want something in the neighborhood of 2 pixels for this image. Now this is again, by no means a magic value, you are going to want to adjust this setting, to taste, according to the size and resolution of your image. But a Radius of 2 works well for this image. Now here is the problem. Notice as I increase this Radius value, I am tracing square pixels so I am creating larger and larger squares inside of the image, and so I am going to want to round off these squares, hence I will be applying the Median filter in the next step. So, let's go ahead and take this Radius value as I said, I just want to ensure you that the square is going on there.
Let's go ahead and take this Radius value to 2 pixels and click on OK. Now the fact that I just apply 2 pixels of Radius that's not a firm, always do that step. You always will want to apply the Maximize filter but exactly what value you use is up to you. However, the next steps, you are going to want to apply these exact values. Let me show you what they are. Next, I am going to go because I have got all these square pixels going on, I want to round off the pixels and I will do that by going to Filter-Noise-Median which rounds off the edges as you may recall. So I will go ahead and apply Median, and this time, I want to exactly match the Maximize value. Whatever I applied for Maximize, I want to apply it again for Median. This is important, so I did 2 before, I do 2 again. If I had done 3 before, I do 3 again. You get the idea. So I will go ahead and click OK and that gives me exactly as much roundness as I need.
In the best cases, it's going to go ahead and smooth off the edges, worst case scenario, it's going to convert squares to circles. So circles, still better than squares. And finally we want to soften the edges and I am going to do that by applying the Gaussian Blur function. So I will go up to the Filter menu, I will choose Blur and I will choose Gaussian Blur, this guy right there, and this time I want 2x. So whatever values I applied for Maximum and Median, I will now multiply that value times 2, so this time I want a Radius value of 4 pixels for Gaussian Blur. If I had been using 3 I would go 6, if I had been using 4, I would go with 8 and so on, but in our case, we use 2, this time we use 4 of course, 2x and I will go and click OK in order to create the final edge mask.
Now I am going to zoom out so that you can see it, and it doesn't look like anything, any great thing here, any great shakes, it looks pretty gooey, pretty gummy, but this actually will serve as a splendid edge mask as you are about to see starting in the next exercise.
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