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Here I have an old image that was scanned from a transparency; it has a very noticeable blue cast. All of the aforementioned color correction techniques that I've discussed in this chapter would work to address that cast, but I think we can do it more precisely using a numerical technique. And what I've done is I've put a sample point down here in this foreground area on this concrete floor. So I need you to buy into the idea that the concrete floor should be gray, concrete is gray.
We are all agreed on that. So when I look at the sample point, we see that I have Red: 149, Green: 140, Blue: 194. Remember, for neutral gray, we need all three numbers the same. So what I'm going to do here is using a curve, I'm going to adjust the green curve and I'm going to bring that up to red to 149. 149 is the number we're aiming for here, I'm using the middle value. Then I'm going to leave the red as it is and I'm going to go to the blue curve and bring that down to 149.
Adjusting the green curve isn't going to make much difference because there is not much in it anyways; only 9 levels difference, but adjusting the blue curve is going to affect a tremendous change to this image. So I'm going to come down here to my adjustment layers and choose Curves where I don't want to work on the RGB curve; I want to work on the green curve. Remember, I'm leaving red where it is, and now what I want to do is I want to sample that exact point onto my curve.
I want to see where it occurs on the curve. To make things a little bit easier, if I were to just come back and click in the same place, chances are I wouldn't get exactly the same spot. So I'm going to press my Caps Lock key which turns my cursor to an accurate cursor and then I can position that over the sample point that I have, and hold down my Command key and my Shift key. If I do Command and Shift, it's going to put this point on all three of my color curves and click.
So there is that point on the green curve. Did I get it exactly right? No, I didn't, doesn't matter. Input, I'm going to change that to my input value, my starting color value right here. My Input is going to be 140, my Output is going to be 149, and you'll see that brings the green curve up. As I said, it doesn't really change much about the appearance of the image. But now when we go to the blue curve, let me see there is the point, the equivalent point on the blue curve.
I want the input to be, that's quite a long way off there, 194, and I want the output to be 149, and we can see that makes a massive difference. Let's take a look at this image before. If I turn off that adjustment layer, there we have the blue cost and the after, you'll notice that at my sample point, I have complete neutrality 149, 149, 149, but more important than that, the image looks a whole lot better.
So this is color correction by the numbers. A variant of this technique and the result tends to be much the same is that you add all three numbers together divided by three and then that becomes the output that you are aiming for, for all three of your color curves. Give it a go, you'll be very impressed.
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