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Okay so by now you probably have a pretty rough understanding of how these sliders work, the amount value controls how much sharpening you will apply from the Unsharp Mask dialog box. The threshold value, we are skipping one here the threshold allows you to avoid sharpening grain inside the image so you raise that value in order to skip the grain and focus in on the details. And then there is a radius value which allows you to determine the thickness of the edges.
Of the three it's the trickiest to understand. So let's go ahead and cancel out of the dialog box for a moment in order to return to the original version of the snake and I am going to go ahead and make this darn navigator palette a little smaller here and switch over to the Channels Palette so that we can see our Red Green Blue channels inside of the image and bare in mind that Photoshop when it's working through the RGB image is actually sharpening each color channel by the exact same amount.
So when you are operating on the RGB channel Photoshop is actually evaluating the red channel green channel and blue channels independently of each other and applying a passive Unsharp Mask to each of the three. Alright I want you to go ahead and switch to the red channel because of the three it has the highest degree of contrast. We have a very light snake against a very dark background here and I am going to go ahead and zoom out to the 50% zoom size so that we can see the entirety of the snake here and so with just the red channel selected just for demonstrational purposes here I am now going to go back to the Filter menu and once again choose Sharp and Unsharp Mask to display the Unsharp Mask dialog box here and I am going to go ahead and scroll over to the head of the snake but there is another way to do this.
Notice that I am trying to scroll inside of the in dialog box preview. There is another way to make this happen. You can notice when you move your cursor outside the dialog box it appears as a little square. If you click on a location inside of the image then you center that point inside the in dialog box preview. I am going to go ahead and raise the amount value to 250 once again just so that we can easily see what's going on and now check this out I am going to raise the radius value and I am going to do it in whole number increments by pressing shift up arrow and notice what happens I keep pressing shift up arrow here watch the screen see how we are getting a thick black edge around the snake.
And I will go ahead and take it up a little more quickly I am going to take this value eventually up to 30 pixels as you can see here so I have a radius of 30 and notice what that means. It means that we have a thick black edge going around the snake and outside of the snake and a thick white edge tracing the inside of the snake. The idea being that Photoshop is making the lighter edge the lighter side of the edge even lighter and the darker side of the edge even darker.
And as a result it's creating halos with thick GUI blurry as it turns out halo according to the radius values so this actually turns out to be a Gaussian Blur halo for what it's worth and we will see the Gaussian Blur filter in the next chapter as it turns out but for now I just know that that's what's going on. It is the Blur function that's tracing around the image and it's masked by the image itself, hence Unsharp Mask. So that's what going on with the radius value I just want you to see it there.
It is tracing black on one side on the dark side and white on the light side of the image. Again I am going to go ahead and cancel up but before I do I do want to show you that you have a preview checkbox here so that you can turn off the preview and then turn it back on in order to do it before and after out here in the larger image window. If you want to do a before and after inside this little in dialog box preview then click and hold the preview like so in order to see the before version and then release in order to see the after version.
So it's just something else to bear in mind. Alright I am going to go ahead and cancel out of this dialog box. Now we have a sense of what's going on with that radius value thanks to our look at it inside of the high contrast red channel. I want you to go back to the RGB channel and let's actually see how we would actually sharpen this specific actual image in the next actual exercise.
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