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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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To employ this technique, convert your RGB image to L*a*b* under the Mode menu. Turn off the a and b channels so that you are only viewing the L channel and are only writing to the L channel. What you’re seeing is the tonal values of all the colors in the image. Low tonal values show as black, while high tonal values (white, yellow, light blue, etc.) show as much lighter shades of gray. When you apply USM to the L channel, you effectively improve the underlying tonal contrast of the entire image without excessively sharpening detail in skin tones and other sensitive areas. This has the effect of making the image look much cleaner and clearer.

Finally, selective USM is crucial in helping the eye to pick out critical detail in an image. Photoshop’s selection tools (Lasso and Marquee) allow you to isolate specific parts of an image and apply sharpening to them. You can combine this approach with any of the other three approaches just mentioned. The results can be breathtaking. Very mediocre images can become transformed into highly memorable statements.

Unfortunately, USM can have a rather undesirable side effect. Depending on the resolution of the image and the degree (if any) of JPEG compression, USM can introduce significant amounts of surface noise to the image. One of the most powerful techniques to overcome this is to combine high-pass filtering with layer masking (Figure 1). This is a relatively simple, yet very powerful and sophisticated, technique for dramatically improving edge definition without introducing additional noise into the image. While the Smart Sharpening available in Photoshop CS2 and CS3 has a similar purpose, the High Pass filter approach offers more control. When used as a layer mask, the sharpening has the added benefit of being nondestructive to the image and is completely editable and reversible in the event your client wants more or less correction.

To use this technique, make a copy of your image layer in Photoshop. You can call it whatever you wish, but I would suggest something like “High- Pass” or another distinct name. The purpose of the High Pass filter is to target the image edges for sharpening, leaving the less detailed portions of the image unaffected.


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