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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."
Wow, outright out to my throbbing head. Well, the pain is just is going to increase inside of this exercise. But it's good pain, all right. So you are going to wander away from it, going wow, that's something but that's interesting. Now if I can just see it working I might understand what's going on and you will see it working over the course of the next few exercises. So in the previous exercise I gave you a sense of how the Add blend mode works. What the mechanics of that Mode are. In this exercise, I am going to give you a sense of how the Subtract Mode works. Under the hood we are going to take a look at the math. Now bear in mind that both Add and Subtract only appear inside the Apply Image in Calculations dialog boxes. They are not available as blend modes to any of the Painting, or Editing tools, they are not available inside the Layers palette and so on. And that's because they have those special additional numerical values that you can modify in order to bring the extreme results back into the visible plain basically.
All right, so here I am looking at the Add Mode. You may recall just for the sake of sort of laying down the basics here, this is the Source #1 image, this is our first channel, it's an Alpha channel. And it's basically a soft ellipse a fuzzy ellipse, and I applied Gaussian Blur with a Radius value of 24. And this is Source #2, again Gaussian Blur, Radius value of 24 but this time we have a bigger circle. This is how the two overlap each other so that the Shadow is smaller than the Sun. And the Shadow with subtract we are actually going to use the shadow to cast a shadow on the Sun to create an Eclipse effect as you'll see. This is how things look when we added these two channels together.
This is how -- we are going to skip down here, this is how things look if we subtract one channel from the other. And in this case we are taking Source #2 and we are subtracting Source #1 from it. And in case you are unclear on what I am talking about, you may recall if we go to the Image menu and choose the Calculations Command, that both Source # 2 and Source #1 are channels. They can be color bearing channels, they can be Alpha Channels. We are working with Alpha Channels to keep things more or less simple here. And Source #2 is in back of Source #1. that's why we start with Source #2. So we start with Source #2 in the background and then we subtract Source #1 in the foreground away from it. So let's go ahead and cancel out of that, because we don't want to ruin the image.
So Source #2 - Source #1, I think that will do it. In this case we are taking the Sun channel which is on top, and we are subtracting the lower channel from it. And we are going to divide it by the Scale value, the Scale value can vary, you may recall between 1 and 2, and it can be a decimal value as well but it can't be any lower than one and I can't be any higher than two. And then you add the Offset value. You can add or subtract really the Offset value, because the Offset value can vary from -255 to +255.
When you are adding you are more likely to subtract with the Offset value. When you are subtracting that is applying the Subtract blend mode you are more likely to want to turn around and add to the Offset value. In our case though for this slide we are seeing Sun - Shadow, so we are applying the Subtract Mode to these two channels here. And we are dividing by 1.0 so the Scale value is set to its default setting 1, and the Offset value is set to its default 0 and we get a standard Eclipse. Now, something to bare in mind here is that Subtract allows us to take two areas of white and subtract them from each other so that they turn black. So if we have white overlapping white that turns black, which is great. Think about this for a second if you've got clouds in your skies so imagine that you have a foreground image, a foreground element like a person for example, that you shot against the sky background and the sky has clouds inside of it. Well, you can neutralize those clouds by subtracting them away from each other, because those clouds are going to be white in the Red, Green, and Blue channels. So if you apply the Subtract Mode the clouds are going to go black and presumably the foreground image, the person is going to stay nice and bright so that you can mask them out.
So here we've gone ahead and subtracted white from white and the area of difference where the Shadow channel was black and the Sun channel was white remains white, so we get an eclipse. So again this, this is the Sun minus this, this is the Shadow ends up producing this effect right here. Now let's take a look at what happens if we add a little bit of Offset. This is the result right here and I am working inside the Layer Comps palette by the way. This is the result of taking Sun - Shadow divided by 1.0, so I left the Scale value at 1.0 and I applied an Offset value of 128, so I am adding medium gray.
Notice that it makes the white area ultra hot, ultra white. It goes ahead and takes what was formerly black that is to say anything that went black, that ended up with a result value of 0. It adds 128, and that makes this area a medium gray. Then there is area that went ultra-black that is it sunk below a brightness value of 0, because we were taking in our case black from the Source #2 channel and subtracting white, this little edge over here of white and gray from the Source #1 channel. That means we ended up getting -128 as our result in many cases. And by adding an Offset of 128 we brought it up to black. So end up getting this effect here. So we are raising all of the colors together in order to produce this hotter and sharp effect on a set.
Now if we want to soften it, what do we turn around and do? Softening is always a function where Add and Subtract are concerned. Softening is a function of the Scale value, so I would go ahead in this final slide right here. I went ahead and kept my Offset value at +128, but I applied a Scale value of 2.0, the highest setting I can apply, and that ends up softening the effect quiet dramatically. So what I want you to remember is this, Add if you want to brighten colors, when you are adding channels together, Subtract if you want to darken colors or cancel out Luminance levels inside of your channels.
Use Offset in order to bring the extreme pixels back into the visible range. So you can either subtract Luminance levels from the Add Mode or add Luminance levels to the Subtract Mode. And then finally, the key item here is that you can use the Scale value, you can raise the Scale value in order to soften the effect. All right, that's a heck of a lot of information, that's more information than a lot of people want to hear about Add and Subtract but it's very important to understand. Now that you have that under your belt, even if you just have a rough understanding of what I just passed by you here, the following is going to make a lot more sense when we actually put these modes into play for the purpose of creating a very accurate mask, starting in the next exercise.
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