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This exercise is something of a sidebar. I'm inserting it for folks who are curious about a very specific question; if you fall into that camp, great, I'm going to answer that question. If you don't fall into that camp, by all means, feel free to skip this exercise and move on to the next one in which we'll talk about the contrast modes, but here is the question I'm anticipating, some of you might have any way. If Linear Dodge is the same as Add, is Linear Burn is same as Subtract? The answer is no by the way. One of the reasons I want to answer this question is because if you Google it, you are going to find a lot of people saying yes, and it's just not true and knowing what it really is it's very helpful in anticipating how Linear Burn works.
So for those of you who are curious stay tuned otherwise move along, and by the way, I have gone ahead and saved my changes so far as this document right here, Bright gradient.psd. Now we are going to visit this command that we won't be seeing for a while, we won't be seeing until we talk about masking, but it's this command right there Calculations, and what it lets you do is it let blend two independent channels with each other using a blend mode. So instead of blending layers together as we can here inside the Layers palette, we are blending channels, such as the red and green channels, which is what I'm going to use this in an example here.
So go ahead and choose the Calculations command. Now it looks pretty gnarly at first, but it's something of a teddy bear once you get used to it, which of course you probably aren't. So what I suggest is that you just sit back and watch me for a moment here. We started things off with Source 1 and Source 2 and those are our channels, the channels that we are going to be blending together here, and we specify the channels, because you can blend channels from different open documents if you want to. You have to start by saying which document you are working inside of, so in both cases I want to set it to Bright gradient.psd, my document up here of course. And you could also mix channels from different layers if you want to.
So I'm going to set both layers, both layer options that is, to Merged. So, by default, the first one is set to Merged already and now I'm setting the second one at Merged and then I'm going to change the second channel to Green. That way we are mixing red with green and we are currently mixing the two channels together using the Multiply mode. All right let's go ahead and switch to Linear Dodge (Add) right there and I want you to know we have a very bright effect of course, also we don't have any extra options down here in this section of the dialog box, I just want you to see that for a moment.
Now let's check out (Add), the guy in parenthesis that's a separate blend mode it's down here, these modes do not exist in the blend mode pop-up menu here in the Layers palette. But you should also notice that here we are missing some of the layers blend modes. We are missing Hue Saturation, Color and Luminosity, all of which require color to work. And when we are working with channels we are just seeing grayscale information. So I'm going to go ahead and choose Add and notice watch the effect on the screen, watch the image window. No difference, it's exactly the same effect. So long as these new options that have just popped up here, are set to their defaults, which are 0 for Offset and 1 for Scale, very important.
Now let's check out, so sure enough Linear Dodge is add, exactly the same, except for Add includes some extra options right there which are quite helpful once you get to know how to use them. Let's check out Linear Burn. This is what Linear Burn looks like. Very dark, a lot of the image is getting clipped to black, fair enough, versus Subtract where all of the image is getting clipped to black. In the case, of this particular mix that we have got going and the reason is we are subtracting the luminance of one pixel from the luminance of another pixel and in doing so the two merged channels here are so similar to each other that they are basically canceling each other out.
All right, so this by the way is the case as long as Offset and Scale are set to 0 and 1 respectively as by default. What then is Linear Burn? Well it turns out to be the Add mode minus white. So let me go ahead and show that you. We'll switch to the Add mode, and then what do I mean when I say subtract white? well Offset allows you to add or subtract luminance levels and if you say -255 it means subtract 255 and which one is 255? White.
You may recall back from our levels discussion, 0 is back, 255 is white, white is a heck of lightening agent if you get rid of the minus sign and you add it, my gosh then the entire image turns white on us. But if you subtract it, it becomes a heck of a darkening agent. Think about if you add sunlight, things brighten up, but if you were to subtract sunlight, you get a black hole. That's might theory anyway, I don't know if it least but accurate. So that's what Add minus White looks like. Now let me show you what Linear Burn looks like exactly the same guy.
So the moral of the story is that Linear Dodge is the luminance level of one pixel added to the luminance level of the pixel below whereas Linear Burn is that same thing, Add the luminance level to the one below and then subtract white and you end up getting a very, very dark intense effect indeed and that's what they call blend mode magic here inside of Photoshop. In the next exercise if anybody is still is with me, we are going to check out overlay through hard mix, the contrast modes, and I promise not to hurt your brain with nearly so much discussion of the word math.
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