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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, if you have been working along with me here then you know that our goal in life or at least our goal in this small portion of our lives is to see the Add and Subtract modes in action combined of course with the Calculation command, and that's what we are going to do, and when I say 'Action' I mean with this guy here Mr. Superhero hair.psd found inside the 14 Calculations folder. An image, that is a by-product of the work of photographer Chris Schmidt of the U.K. All right, so we can generate a mask for this guy, an Alpha channel version of this guy that is going to work for us using either the Add or Subtract mode.
Turns out that the Subtract mode is probably going to work a little better for us, but we might as well try both, I figure, just so that we have some hands-on experience. All right, so let's go ahead and switch in my case because I have so little screen real estate to work with. I am going to switch to the absolute Full Screen mode so that I don't have those scrollbars showing up at the bottom window and I can see more of the vertical image at a time including all the way to the top of the head. I really need to see that part because that's the part that's in the worst shape where masking is concerned.
Now let's go up to the Image menu and choose Calculations command, and let's wander through the channels here, and we can wander through the channels using Source 1 by changing Source 1, because Source 1 is on top of Source 2 as long as Blending is set to Normal and Opacity is set to 100%. So go ahead and make that occur first. And now we can see the Red Channel. So this is another way just to go through the channels as opposed to having to click on each one of them out here outside of the Calculations dialog box. All right, so we are doing everything inside the dialog box in other words.
This is the Red Channel, it looks pretty darn good. I should move the dialog box over just a little, and actually, you know what, I will move the image over a little bit too because we really need to keep an eye on the contrast between the hair and the background. And it's pretty darn and good at this point, obviously, it's hopeless at the top of the image, but that's going to be the case throughout the channels. Now let's try the Green Channel just to get a sense of what's going on, and now it looks pretty darn good. It looks better, we've got a high degree of contrast going between hair and background that probably if I was generating a mask from a single channel, Green is what I would select for this image. But let's move our way down to the Blue Channel because really we haven't seen it yet, and it looks like this.
Now the Contrast is weaker between the hairs and the background, much weaker in fact, but one is the Blue Channel have going for it empirically if you sort of take a look at it here. What it has going forward is it very accurately imparts the degree of difference and focus between the different hairs. So for example the foreground hairs are very, very dark; the background hairs are very, very light, almost indistinguishable from the background blue, and we need that in order to be able to accurately generate the various levels of focus associated with his hair. So we need that Blue Channel. We also like the Green Channel, so let's work with both of them.
Now in each case the foreground image is showing up darker than the background image. So we need to invert the channel. So let's go ahead and set the top channel to Green, and we will set it to Invert of course, and that's what it looks like when its inverted and then we will set the bottom channel, the channel underneath the Blue and we will invert it. Now we didn't make any change to the Composite View here, and that's because Blending is set to Normal. So we are only seeing the Source 1 channel. Of course we are not changing the Source 1 image. It's going to be Superhero.psd, and then actually happens to be the image that I can choose at any rate, and then layer is going to be background because there is only one layer inside this image, the Background layer.
All right, now let's go down to Blending; let's just give everybody else a slip. If you wanted to fool around with the channels, we would start with Hard Light just as I was showing you way back at the outside of this chapter and then we would try out the other modes by arrowing through them here on the PC or selecting them manually on the Mac. However, I know that I am going to be working with Add or Subtract. Now Add and Subtract are simpler blend modes that most of the other blend modes. We know that Add is the same as Linear Dodge at face value because they both add luminance levels. However, where they get more complicated because adding and subtracting are pretty simple formulas, is with the Scale and the Offset values that gives us control over the blend mode process. And actually between you and me it'd be great if we had those kind of controls with all of the blend modes. Although, it really would up the complication, quite considerably I think, but still my goodness, the stuff we would be able to get done.
Anyway, let's go with Add and it goes ahead and adds the Luminance levels. This is the same effect we would get with the Linear Dodge mode. Now then I could say, well, okay, that's too high. So let's go ahead and take the Offset value down, and what I typically do is I select the Offset value and then I start pressing Shift+Down Arrow. When I am working with Add, when I am working with Add, I take the Offset value down, when I am working with Subtract I take the Offset value up because you are trying to balance the equation. So you Subtract from Add you Add to Subtract. All right, so I am going to Shift+Down Arrow with the Offset value until I get something that looks a little more reasonable. So we have a higher degree of contrast between the background. We are really trying to darken up this background while keeping the hairs nice and light. And we can take this value pretty darn low because the hairs are very, very light and the background is pretty darn light as well.
Now if we start finding that we have too much choppiness between the hairs and the background, in other words; we have got too much noise; we have got too many jaggies; we have got too much contrast in general, then we can soften things using the Scale value. Now the Scale value is a little more difficult to work with, because notice, if I just start pressing the Up Arrow I really can't go down, the Scale value varies between 1 and 2. But if I start pressing the Up Arrow I am changing the value in 1 -- what is that 1000th of an increment here, that's craziness. It is just so small, and even if I press Shift along with the Up Arrow, I am only adding 100th of a divider here.
And so that's not changing things very dramatically or very quickly for me, but what I can do is I can press Shift and the Up Arrow key and hold, and then you are going to start making some big changes very quickly. All right, so Shift and Down Arrow, hold as well and you will see that value scale along right there, and then at this point you might say, well gosh, something like I think a value of something around the neighborhood of 1.4 does a good job of softening the effect, but now I have over-darkened it because it's Offset down too far. So let's Shift+Up Arrow this value. You can also just press the up or down arrow key if you prefer to vary the value and increments of 1, but I prefer to work in increments of 10 because that gets the job done quicker essentially.
All right, so let's go with these values right here, Offset of -90, so Add of course; we've got the Add Blend mode; we've Offset -90; we've got Scale of 1.4. So we are darkening the results of the Add equation here, and we are also scaling it so that we have softer transitions. Not way soft transitions, just softer transitions. So we don't have too many jaggies going on. And then note of course Channel just to make sure Channel is set to Green Invert. This channel is set to Blue Invert, and we are done. Go ahead and click OK and that becomes our Add Channel. Let's go ahead and name it, so I will call it Add and then I will call it -90, 1.4, and I know that that means an Offset value of -90 because it couldn't mean a scale value of -90 and a Scale value of 1.4.
All right, so there the deed is done and this is the Alpha Channel that we come up with at any rate right here. In the next exercise we will try something similar with a Subtract mode and we will decide which of the two Alpha channels we like better.
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