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Using High-Pass Filtering to Improve Image Clarity

(September 2007) posted on Tue Sep 04, 2007

Unsharp masking is a useful method for enhancing digital images. Learn about different ways to apply this tool and other Photoshop features to bring clarity and vibrancy to your graphics.


By Mark A. Coudray

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Another reason to sharpen has nothing to do with the mechanics of the printing process, but with how the human eye perceives detail, tone, and color. Unfortunately, photogand illustrators often miss this aspect of their work. The result is that the image we are asked to print, such as a graphic intended to sell a product, often confuses and contradicts the message we’re trying to get across. Therefore, we often need a way to isolate and enhance the critical parts of the image to improve our ability to sell the product. USM is an excellent method to accomplish that.

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Unsharp masking can be applied in at least four ways: by color, channel, luminosity, and selective sharpening. You can sharpen by color only, which is very handy if, for example, you have a picture of a model. In this case, you want to improve the edge definition of the image, but not create undesirable texture in the skin tone. By selecting colors and sharpening the color break, you improve the edge detail, but do not hurt the skin tones.

Sharpening an entire channel can be a very, very effective way to improve a poor scan. Assuming that you have acquired your image in an RGB format, you will selectively sharpen in the green and red channels only. The reason for this is that the blue channel typically suffers from a high degree of noise. This is a condition in which the image’s digital information looks grainy or has a sand-like texture. If you slightly blur or soften the blue channel and sharpen the green and red channels, you will greatly improve both the tonal values and the color values of the image.

Another very useful sharpening technique is to use the L channel of the L*a*b* Mode to sharpen. This is one of my favorite tricks . Photoshop sharpens the underlying tonal values of the image without changing any of the color values. This is very important if you have critical colors in your image that might be altered during sharpening.


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