Let's take a look at the simpler and older of the two customizable sharpening features and that would be Unsharp Mask, so named because it actually uses a blur function that is masked by the existing edges inside the image in order to produce its sharpening effect. So let's go out to the Filter menu and then choose Sharpen and of course finally choose the Unsharp Mask command right here or if you have loaded D keys and you compress Shift+F5 in order to bring up the Unsharp Mask dialog box.
Now, this dialog box is fairly indicative of the best of the filters in my opinion. It's got an in dialog box preview and a preview see effect in the larger image window as well. So you have a combination of two previews at your disposal in order to keep track of the image. What I tend to do is zoom into the 100% view size inside the larger image window and you can do that even when the dialog box is up on screen. You can go ahead and Ctrl-click or Command-click on the Mac in order to zoom in or Alt-click or Option-click on the Mac in order to zoom out and you can Spacebar+Drag the image round and you can take advantage of the other zoom options as well, Ctrl+ and -, that would be Command+ and - on the Mac.
At any rate, I would basically keep the image zoomed into about 100% and then I will zoom out from the in dialog box preview here by clicking on this - button, so I see the image at the 50% view size. So I can keep track of it at two different zoom levels, which gives me a better opportunity to go ahead and gauge the sharpness that's been applied to the image. Alright, now down here, we have got three slider bars, and that's it, just three numerical values, that's all we have to deal with. It's a little bit tough to come to terms with these options right at first, but once you understand how they work, it's really easy to manipulate them.
The amount value I think is fairly straightforward. The idea is that if you raise the amount, you are going to get more sharpening as you can see here inside of the larger image window, as well as inside the little dialog box preview. And if you lower that amount value, then you get less sharpening, pretty much that straightforward. I am going to go ahead and take this value up to 250%, which is about half of its maximum. The maximum is 500% so I guess it's actually exactly half of its maximum.
Now, the radius value controls the thickness of the edges. I was telling you that Unsharp Mask works by creating softness around the mask edges and that softness manifests itself as Halos which can get pretty darn thick if you raise the radius value or they can stay nice and tight, nice and small if you lower that radius value. And I will show you more about what's going on with radius in the next exercise as it turns out. Then finally, you have the threshold value which allows you to eliminate some neighboring pixels from the sharpening equation.
I am going to go ahead and zoom in even farther on this snake image here, onto its eye, so I am looking at both eyes actually at the 300% zoom ratio and notice the amount of digital noise that's being caught in the background here, in the blue background. Well, the idea behind threshold is you can try to get rid of some of that noise by going ahead and raising the threshold value. And so let's say I take this value up. Notice as I am raising the value, the noise is starting to go away in the background there.
And when I take this value up to 8, which works pretty well for this image as it turns out, I am saying that any two neighboring pixels, if they are at least 8 luminance levels different from each other, then they will get sharpened. If they are less than 8 luminance levels different from each other, then they will not get sharpened. And you recall luminance levels are measured from 0 for black to 255 for white. So, a threshold value of 0 means that everybody gets sharpened because all the neighboring pixels are at least 0, that is none, luminance levels different from each other but as you raise that value, you start to rule out the neighboring pixels that have less differences and you just focus in on the neighboring pixels that have more differences.
Now, you have to be careful with the threshold value. It works very nicely for this image here, especially when it's set to about 8, I find for this image. However, for some images, it's not going to work too well, you are going to get something of a pockmarking effect because it's an on-and-off proposition, either you are sharpening pixels or you are not sharpening pixels according to this threshold value. So keep the value low. I suggest you make sure that the threshold value never exceeds 10 levels, that would be sort of a rule of thumb. Sometimes you are going to violate that rule, but it's just good precaution.
For low noise modern images that you would capture with a digital camera, a threshold value of 2 or 3 is probably your best bet, so very small values there. Alright, so that's basically what's going on. The threshold value doesn't come into to play that often, most of the time you are focusing on amount and radius, and I am going to explore those options in more detail in the next exercise.
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