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In this exercise, I'm going to explain how the Add and Subtract Blend modes work, since they're so useful where masking is concerned. And how you can modify their behavior using the Offset and Scale values. I'm still working away inside Hair in flight.tif. I've gone ahead and chosen the Calculations command, set the first channel to Blue, the second channel to Red, and Inverted both of them, and then set the Blend mode to Add. The Add blend mode does what it says it does. It adds the luminance levels of one channel to the luminance levels of the other. And as you might imagine, that means the luminance levels get quite a bit brighter.
Now because we've inverted both the channels at this point, they're quite bright where the hair is concerned, for example. So the hair is gone from very dark to very bright now. And when you add the hair, this bright hair from one channel to the bright hair from the other channel, it goes beyond white. It's clipping like crazy. So the only colors that aren't clipping inside of this image are the ones that were bright before we inverted the channels and are now quite dark indeed. Well this is pretty typical behavior for the Add mode. It goes ahead and blows highlights across the board, which is why you can drop things down using the Offset value or the Scale value.
Now Offset just goes ahead and subtracts luminance levels from the result of the operation whereas the Scale value allows you to divide away the results. And we'll see what that looks like in a moment, but first I'm just going to say, you know what, let's go ahead and subtract, so I'll enter a negative value, -150 luminance levels and see what we come up with, and as a result, notice the hair remains white. That's how far beyond 255 it was. Remember that 0 is black and 255 is white, so it must have been out there in the stratosphere in a 400s or 500s or what have you.
By entering -150 though, we go ahead and sync the background. And then at this point you might want to tweak the value. Now, of course, you wouldn't just know that -150 was a good place to start, you would experiment and see what you come up with, but if I press Shift+Down Arrow I'm going to incrementally darken the image. And at some point I'll start to see a bunch of details show up in the hair which happens for me at about -200. The problem with this so far is that we don't have a lot of darkness to work with in the background. The hair is bright, it's great, but we're running out of darkness at the ends of the hairs.
And so that's going to create troubles for us. And I've got a workaround, don't worry about it, but for now let's get a sense of how the Scale value works. You can divide a number by anything from 1 to 2. So you don't have a lot of range here and what's funny is if I enter 4 or something like that and press the Tab key it's going to get mad at me and say nope, the only numbers are between 1.000 and 2.000 and the closest value is inserted which is 2. So you have a high degree of accuracy where this value is concerned, but you don't have much range.
Now what ends up happening when you scale the results of your modification is that you're going to darken things. So if I'm dividing all the luminance levels by 2, that's going to darken the heck out of them. So I'll go ahead and brighten things up again, let's take the Offset value to -100 to go brighter. And you'll notice that you have a lot less contrast as a result of that scale value. Now you only want to use the scale value when you want to reduce the contrast and that's typically if you want to smooth out the results. If you're getting super sharp transitions inside of your new alpha channel, then dividing the results by something between 1 and 2 can prove helpful.
However, if that's not the problem, if you're in need of more contrast not less, then you want to leave that value set to 1. Anyway, the closest I'm going to come to something halfway decent right now is something like Add with -200 or I could go ahead and try out the Subtract mode instead and see what I come up with. Now, Subtract is going to do exactly what it says it's going to do,e it's going to subtract the luminance levels of one channel from those of another channel. And you still have the option of adding an Offset and a Scale value. Now, of course, that's going to end up with something very dark for us because if we're subtracting one inverted channel from the other inverted channel, why then we're going to get a lot of zeros as a result, and zero is black.
So not surprisingly we're getting a very dark result. And then you could then add luminance levels using that Offset value or if you wanted to further darken things and smooth out the transitions, you could increase the Scale value. We're not going to do that, but notice that we're getting a terrible result. You might think well subtract, what a horrible mode. It's not delivering what we need. Well that's because we're subtracting two very similar images from each other. We need to exploit the differences once again. So we need to turn off one of these Invert checkboxes. So I'll start by turning off the Invert checkbox associated with Red and we end up getting dark hair against a fairly bright background.
Interesting, but not what I'm looking for. So I'll turn Invert back on and now I'll turn Invert off for Blue and I end up coming up with something that's much more in keeping with what I want, and then I could adjust this Offset value. So if the result of the Blend mode is too dark, I could raise that Offset value like so. And if it's getting to be too light, then I could go ahead and back off the Offset value. And in my case, for this image, I might take that Offset value down to 50 or something along those lines, but really I got to tell you, this just isn't doing it for me.
I don't have nearly enough contrast in this background over here and I'm running out of contrast over on the left -hand side. So what do I do? Well, tell you what, I'm just going to go ahead and accept this new base alpha channel for now. Subtract, Offset of 50, notice that, I'll click OK. And I'll go ahead and name this guy subtract and offset 50 just so I remember how I created it. And then I'm going to go back to the drawing board because you know what I need to do, I need to do something I've never really tried this before, and it's working out brilliantly, I have to tell you.
I'm going to increase the contrast of the image before I even start creating the mask. And I'll show you how to do that in the next exercise.
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