User login

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

click an image below to view slideshow

In the High Pass layer, apply the High Pass filter located in >Filter>Other. Set the Radius value to a relatively low number to isolate only the edges for sharpening. I would suggest experimenting a bit until you get the feel for the technique. The basic starting point for me is a Radius Value of 1. The higher the value, the more detail that will be sharpened. Clicking OK applies the High Pass filter and converts the High Pass layer to a grayscale mask (Figure 2).

Now set the Blending method for the High Pass layer to Overlay and adjust the opacity setting to blend the mask with the Background layer. You can visualize how much sharpening is applied. You’re adjusting a duplicate layer, so the original image remains untouched and undamaged.

A close variation of the High Pass filter is the use of the Find Edges filter as a layer mask to further enhance the selectivity of the sharpening. Begin this sequence by opening the RGB image. Add a new blank channel to the image. With the RGB channels active and visible, Select All>Copy>Paste into the new channel. This yields a grayscale copy of the full-color image into the Alpha Channel you created.

With only the Alpha Channel visible and active, apply the Find Edges filter. It is located in >Filter>Stylize>Find Edges. This will convert the AlphaChannel to an image where the edges are emphasized and the detail is greatly reduced. Invert this channel (>Image>Adjustments>Invert) and adjust the Contrast of this channel as shown in Figure 3 (>Image>Adjustment>Brightness/Contrast), enter a value of about 75, and click OK. Finally, choose >Filter>Blur>Gaussian Blur, set the Radius to 1, and click OK. This sequence effectively widens the edge areas and softens the transition of the edges to the image. In the bottom of the Layers pallet, click on the Add a Layer Mask icon (example shown) to link this channel to the duplicated layer. Select the Background Copy layer (not the mask) to make it active. Choose >Filter>Other>High Pass, enter a Radius Value of about 2-3, and click OK. Using the same Blending Mode of Overlay, adjust the opacity until you achieve the look you want.

 

When and how to use it

Experience tells me to apply sharpening at the beginning of the digital workflow. USM works by changing color and tonal values. Critical color corrections can often be lost or altered when sharpening is applied after the corrections have been made. By sharpening at the beginning, you can correct any color or tone shifts that occur as a result of the USM and the other adjustments you make.

As I mentioned earlier, USM can easily be overdone. You have a fine line to walk here. Some over-sharpening is necessary, but too much can have a very destructive effect on an image. It’s very easy to make an image appear grainy or textured. If you have acquired your image from a photograph or transparency, the High Pass method will keep the film grain from becoming too noisy and degrading the image. The same is true if you’ve added noise to the image in an attempt to smooth tonal gradation and reduce banding in rasterized vector conversions.

The more work I do that requires sharpening, the more I like the High Pass and High Pass/Find Edges approaches. These methods are especially useful because they’re reversible and editable. Try these two useful techniques the next time you have the need to sharpen an image. I think you’ll also find the results very effective.

 

Mark A. Coudray is president of Coudray Graphic Technologies, San Luis Obispo, CA. He has served as a director of the Specialty Graphic Imaging Association Int’l (SGIA) and as chairman of the Academy of Screenprinting Technology. Coudray has authored more than 250 papers and articles over the last 20 years, and he received the SGIA’s Swormstedt Award in 1992 and 1994. He covers electronic prepress issues monthly in Screen Printing magazine. He can be reached via e-mail at coudray@coudray.com.

 


Terms:

Did you enjoy this article? Click here to subscribe to the magazine.