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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 last two exercises we used both the Add and the Subtract mode combined along with the Calculations command in order to create a couple of base alpha channels. Now if you are just joining me, I am working inside of a catch up document called ANS in action.tif. ANS standing of course for the Add and Subtract modes, and this image is found inside the 14 Calculations folder. If you go down to the bottom of the Channels palette, you'll see that we have one alpha channel that's called Add, -90, 1.4. -90 being the Offset value, 1.4 being the Scale value. And we have another one called Subtract, 90, 1.4. 90 being the Offset value, 1. 4 being the scale value.
Bear in mind that these two variations on the Alpha channel were created using slightly different settings for the base channels, for the Source 1 and the Source 2 channels. So in the case of Add we were taking an Inverted version of the Blue channel and adding an Inverted version of the Green channel to it. In the case of Subtract we were taking an Inverted version of the Blue channel and subtracting a standard version of the Green channel from it. Now I was telling you that we can get similar though, not quite identical effects out of Add and Subtract to which you might have responded, really? Because these look totally identical to me. Well, we can test that, can't we? You may recall way back when we used the Blend mode in order to compare a JPEG file to the non JPEG, the uncompressed original. And we did that using the Difference mode. We can also use the Difference mode to find out the differences between different alpha channels such as the Add and Subtract variations. Why don't we go ahead and do that.
Go ahead and click on Add, the Add channel here, inside the Channels palette in order to make it active. Then let's go up to the Image menu and choose the Calculations command. And right now it's saying that it wants to take Source 1 set to Channel Add and Source 2 set to the exact same channel and subtracting from each other. Well let's go ahead and change Source 2 to subtract. It doesn't really matter who's Source 1 and who's Source 2. But one of them needs to be subtract and the other needs to be add. And then we'll change the blending mode to Difference. Now before I choose Difference let's review what it's going to do. Where we light pixels that means we have significant differences between the two channels. Where we see black pixels, it means we have no differences between the two channels, and other variations, Gray of course either means slight differences in the case of dark grays and big differences in the case of light grays.
All right, so if we see Black there's not much difference, if we see white there's a lot of difference. Let's choose difference, it turns Black meaning that there's not a heck of a lot of difference between these two channels. However, even though it looks like the entire darn thing is black now which would mean that the two channels are utterly and completely identical to each other, they are not. They just happen to be very similar, but there are some differences. All right, so we have gone ahead and created this new Alpha Channel using the settings that you see here. Once you have done that click OK and there's your new Alpha channel. Let's go ahead and call it, What's the diff? Because it's going to tell us the differences between these two channels.
Now we need to elaborate those differences using the Levels commands. So press Ctrl+L or Command+L on the Mac to bring up Levels. Notice that the histogram is shoved all the over to left hand side. So we don't have much histogram to work with but we do have a few bars meaning that the two channels are not identical, they are just very, very similar. Let's go ahead and take this White value down to 2 or 3, actually 2, I think, works out better for it. We could take it two 3, or 4, or one of these higher values because we will see that there are shades of difference going on but I say 2, is going to give us the most significant reading of the image. And then click OK.
So we have yanked this histogram all the way apart so that we only have really black, white and most -- couple of shades of are gray. I think really we just have one shade of gray. So now seeing the differences between the two channels you may still wonder, okay, I understand the effect at this point but I am still left wondering what's the diff. Deke, you know the big question still goes unanswered. Well, there's three reasons we perform this check here. One, is I want you to see that it's possible. I want you to see how you can use the difference mode in order to compare any two Alpha channels inside of your images, or any two channels in general inside of your image. Two, it's a darn cool effect, don't you think. There's something you might be able to use for whatever reason. Not for masking necessarily but for black and white posters maybe. And then finally three, it does demonstrate to us that there is one channel that's going to be even slightly better than the other.
So let's go back and check them out. Look at them very closely. Here's Add, here's Subtract, what did you see happen? Did you see a change occur on screen? Subtract got darker, the background darkened up a bit. I would say from my reading of the image and my experience with working with this image that subtract is going to be closer to our final goal because the background is slightly darker and there's a higher degree of contrast between the hair and background. So I am going to suggest we start working from that subtract channel in order to generate our final mask, and that is what we are going to be doing of course, starting in the next exercise.
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