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All right, at the end of the last exercise, I was showing you how Unsharp Mask has a habit of producing aberrantly colored halos inside of an image. This can happen not only with Unsharp Mask, but any of the sharpening functions inside Photoshop. I want to show you why it happens, and then I want to show you the solution. So what I've done here is, I've gone ahead and opened Simpli-fly.jpg found inside the 15_sharpen folder. Then I've created three more windows into that same image. I did that by the way by going up to the Window menu, choosing Arrange, and then choosing New Window for Simpli-fly.jpg.
The advantage of doing this by the way is that you have multiple views of the exact same image. So in our case, the reason I'm working this way is notice I have the Channels panel up. In this very first version of the image, I'm viewing the entire RGB composite, so the combination of Red, Green, and Blue working together. Then if I switch to the next image, that is this second guy right there. Then I'm just viewing the Red channel. Next, I'm just viewing the Green channel. Finally, I'm just viewing the Blue channel.
That's indicated by the appearance of the eyeball right there. So the eyeball shows up in front of Blue, but not in front of Red or Green. The reason I want to see each one of these three channels is because Photoshop - and this is a hard thing to get used to, or hard thing to wrap your mind around. Photoshop is ultimately a grayscale image editor. Most of Photoshop's functions, something like 90 to 95% of the program works on a single grayscale channel at a time. Then the results are combined in order to create the full color composite.
All right, I'm going to press Shift+Tab to hide the right side panel. So we have a little more room to see our slices of imagery here. Then I'm going to Shift+Spacebar+drag this image, so that I'm scrolling the image inside all of the windows at once. With the RGB version of the image active, I'll go up to the Filter menu. Choose Sharpen, and once again choose Unsharp Mask. I'm going to apply those settings we saw just a moment ago, 250% for the Amount value. Then I'll take that Radius value up to something like 20 pixels, so that we can see.
Sure enough, we've got this blue edge on the inside of the insect's head. So what in the world's happening there? Well, you have to consider what's going on inside each one of the color channels here. Inside the Red channel, the insect is dark and the background is light. So we have a dark halo inside the bug, and a light halo outside the bug. In the Green channel right here, same thing. We've got a dark halo on the inside and a light halo on the outside. However, in the Blue channel notice this, the bug is actually quite light, and the background is very dark, because that yellow background doesn't contain any blue.
As a result, we have a bright highlight on the inside of the bug's head, just a little bit of brightening you can see there, and then something of a dark edge on the outside of the bug's head. That because we have a contribution of blue going on here and we are losing red and green, because we have black in the Red and Green channels. We get this aberrant blue edge inside of the insect's head. Now this is an exaggeration of something that happens all the time when you're sharpening RGB images inside of Photoshop.
You're always getting these aberrant colors. You may not notice them, but they are appearing to some extent or other. So in our case, it just happens to be extremely obvious, because we're working on this low color image. All right, so I'm going to go ahead and apply these settings. So what in the world do we do about it? Clearly, we don't want a weird blue edge, all of a sudden inside of what is otherwise kind of a purple insect. That doesn't make any sense at all. We want it to be nice and dark. Well, what you do, after applying the Filter? Is you go up to the Edit menu, and you choose Fade Unsharp Mask, or you can press Ctrl+Shift+F, Command+Shift+F on the Mac in order to choose that command from the keyboard.
What that allows you to do? This command allows you to take that last pixel-based modification you just applied. In our case Unsharp Mask, and Fade it. That is, blend it with the original version of the image before you applied that last modification. So you're actually blending those two versions of the image together. You can if you want to, just reduce the Opacity value. However, that's just going to back off the Filter. That's not going to get rid of our blue highlight. As you can see, it's still hanging on there even at 48%.
So let's leave that Opacity values set to 100 %, instead, you want to change your Blend mode. We'll see more about how blend modes work in a later chapter. But for now, I'm going to switch it from Normal to the very last Blend mode Luminosity. What that does is it goes ahead and keeps the luminance modification that's been applied, and it blends it with the colors. So the opposite of Luminosity is Color right there. It blends it with the colors from the original version of that butterfly image. So in other words, we're going to get rid of the blue and we are going to replace it with whatever color was already there in the first place.
So I'll go ahead and choose Luminosity, notice that goes ahead and drops out the aberrant colors. We just get the luminance version of the sharpening effect, which is all we want, because it's luminance that carries detail information inside of a digital image. So really on a regular basis, the only thing you want to touch with any of the sharpening filters is Luminosity. So that also means by the way on a regular basis, every time you get unsharpening an image, your next step should be to choose Edit Fade, and then set the Blend mode to Luminosity.
That should be the next thing you do in all cases. Now it's not always necessary, because you don't always notice these weird edges. But if you ever get a hint that those edges are happening, here is how you get rid of them. Anyway, go ahead and click OK in order to apply that modification. And that my friends is why sharpening often delivers aberrantly colored edges. That's how you get rid of those aberrant colors here inside Photoshop.
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