June 2001
President's
Message
Lindey's Digital Notes - The Unsharp Mask
Lindsey's Digital
Notes -
The Unsharp Mask
- Bill Lindsey
So, what is an unsharp mask? The unsharp mask command is named after a traditional photo lab technique in which two negatives, one normally focused and one blurry, are laid one on top of the other and contact printed together using a long exposure. In Photoshop, this command operates along the same lines, but with much better control. The term "unsharp mask" is misleading to anyone not familiar with the original process and unfortunately named since it does exactly the opposite of what it implies. It makes an image sharp.
OK, enough history, now let's get to the point of this month's article. The Unsharp Mask works by looking at the overall image and comparing neighboring pixels. When it finds adjacent pixels that differ from each other, it considers it an edge. The unsharp masking command increases the local contrast along the sides of the edge, creating a halo effect. If you see the halo after performing unsharp masking, however, you have probably overdone the effect. Normally, you would only be aware of the halo by zooming in on a portion of the image. The unsharp mask dialog box has three confusing options: Amount, Radius, and Threshold. Of the three, Amount has the most obvious function. Turning up the Amount value makes the sharpening effect more intense (as much as 500% can be applied.) It does this by making the dark side of the halo darker and the light side lighter.
The Radius option is less understood, but equally important to understand. The thickness of the halo is determined by the Radius value and measured in tenths of pixels. In practical use, Radius values can also be increased to increase the contrast. This is also a good way to enhance textures.
The third option is Threshold. It controls how great the difference between adjacent pixels must be to be considered an "edge." This effect is easier to describe using an example. Suppose we have a portrait and we want to sharpen some facial details like the eyes and mouth. But, we don't want to sharpen things like freckles and minor variations in skin tone. If the threshold is set to zero, everything will get sharpened, including the skin variations. But if we crank up the threshold, only adjacent pixels that are VERY different in tone will be affected. So, we may be able to sharpen the eyes and mouth selectively because they have this greater difference. Of course, there will be times that threshold will not be selective in the way that you want. Then, you will need to use a local sharpening tool by using the magic wand or make a mask to limit the area where the sharpening will take place.
Most of the above settings
will be by trial and error, remembering that you can always delete everything
you have done and start over again. Take your time implementing these
filters and experiment with images that interest you. This can be a visual
feast of changing image editing. Other image editing programs are also
well suited to this kind of experimentation and you never know what you
will end up with.