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In this exercise I'm going to provide a detailed analysis of what's going on with the Radius and Threshold options inside the Unsharp Mask dialog box. This is one case where it really pays to know what's going on under the hood. That way you'll get more effective results out of Unsharp Mask. So in addition to this Macro butterfly .jpg image here, I've gone ahead and created a stripped down version of the image called Simpli-fly.jpg, also found inside the 15_sharpen folder. In this case, I've gone ahead and distilled the image down into just a few interacting colors here.
That provides us with some very obvious edges inside the image, that the Radius value can glom onto. Notice also, if I zoom in on this image, that I've added a synthetic paper texture here using a couple of filters. We'll see how that works in the future chapter. That will allow us to examine what's going on with Threshold. So I'm going to go up to the Filter menu, choose the Sharpen command, and choose Unsharp Mask, or press Shift+ F5 if you've loaded dekeKeys. What I want to do is move my dialog box over a little bit, and click on this back portion of the wing right here in order to center it inside the in dialog box Preview, just so we can see what we're doing.
I'll raise the Amount value to 250%, which is half of your maximum value inside this dialog box. You can take it as high as 500. Then I'll tab down to the Radius value. I want you to see what's going on here inside the dialog box in particular. Notice as I raise this value here, I'm increasing the thickness of these halos. I'm actually drawing halos along the dark side of the edge, and the bright side of the edge. So this is an edge at this location, because it's an area of rapid luminance transition.
We're going from bright to dark very quickly over the course of the very few pixels. As I raise this Radius value, I'm drawing a bright halo on one side, and a dark halo on the other side, and the thickness of the halo is dictated by this Radius value. It also drops off very softly. Notice this, because the halo is actually produced by a filter called Gaussian Blur inside of Photoshop. So it is a true blur that's going on here. The larger your Radius value, the thicker that halo that you're drawing.
So as a result over time, you're distributing the sharpening effect. So it looks less sharp, and it looks more like you're just emphasizing the edges inside the image. So if you want to create a more volumetric image where you're really enhancing the shadows and the highlights and so on, a low Amount value combined with a high Radius value can really do the trick. I'll go ahead and reduce this Amount value down to 50% for example, and I'll send this Radius file you to 50 pixels. Now notice the difference.
If I turn off the Preview check box, so keep an eye on the image in the background here. If I turn off the Preview check box, this is the original version of the image. Turn on Preview; this is the punched up, higher clarity version of the image. Thanks to once again, a low Amount value, and a high Radius value. However, that doesn't really look all that sharp. If you want sharp edges, tactile edges that is to say. Then you go with the high Amount value, maybe not as high as 250%, but I'm trying to make a point here. Then dropdown to the Radius value and enter a low Radius value, something like 3 pixels or even lower, 3 pixels is pretty good for print work.
Lower values such as 0.5, you can go very low with this value, are useful for screen work. We'll discuss that in more detail. But for now, just know, low Amount, high Radius gives you lots of clarity. High Amount, low Radius gives you lots of sharpness. All right, I'm going to take this Radius value up to something like 20 pixels for now, something very high. So we're combining high Amount and high Radius, just so that we can see the difference that Threshold provides. The whole idea here with this Threshold value is that we're trying to eliminate any of the background junk.
So for example, we're trying to get Unsharp Mask to ignore the noise inside the image, or the paper texture, or this artificial texture that's going on in our case. What I would recommend that you do when you're using this value is just go ahead and click inside of it, and then press the Up Arrow key in order to nudge that value upward, until your background texture disappears. Now if you're trying to omit noise, you're trying to make sure that you don't sharpen noise inside of an image, then you want to keep this Threshold value very low, something like one to three levels.
This is luminance levels between neighboring pixels. Recall from the previous chapter, that you've got 256 luminance levels in all; 0 for black, 255 for white. So three luminance levels difference between two neighboring pixels is not very much. However, when you start to go higher than that, let's say, I take this value up to 40, so that I'm only sharpening pixels that are at least 40 luminance levels different from each other that is neighboring pixels. Then I end up getting this pockmarked effect that we're seeing right there.
Now I'll go ahead and click in the animal's head, so that we can see that some pixels are all of a sudden getting sharpened, and neighboring pixels are not getting sharpened. That's because this is an either/or proposition. Either two neighboring pixels are less than 40 luminance levels different from each other and they don't get sharpened, or they're 40 or more luminance levels different from each other and they do get sharpened, and there is no variance in between. So all of a sudden, out of the blue, you'll have a pixel that will get sharpened. That makes for pockmarks. Never use this option in order to omit sharpening details in somebody's face, because when you're working with portrait shots, high Threshold settings actually make the effect that much worse.
So watch out for that. Anyway, for most of my work, I keep this Threshold value set to 0. But as I say, if you want to avoid noise, then you can raise this value to something between one and three. So go ahead and experiment with that. Now after everything I've told you here, there is one big question mark I would have, if I were looking at this detail of this bug's head right here. The question I would have is, okay Deke, so we're drawing light halos on the light side of an edge, and we're drawing dark halos on the dark side of the edge.
Why in the world do we have blue halos inside of this bug's head? I'll explain what's going on there, and the solution to this problem in the next exercise.
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