ClarkVision.com
Experiments with Unsharp Mask for Increasing Apparent Image Sharpness
http://www.clarkvision.com/articles/unsharpmask
http://www.clarkvision.com/articles/unsharpmask
This page is dedicated to experiments in usinng unsharp mask (e.g. in Photoshop), and how it can be used to improve the sharpness of images. The Test target, Figure 1 was blurred using a Gaussian blur with a radius =2.0. This blur is similar to the blur that might be encountered in real images.
Figure 3 shows an attempt to restore the blurred image to the original test target state. It was done by converting from RGB mode to Lab, then running unsharp mask only on the lightness channel. Finally, the image was converted back to RGB. The advantage of using the lightness channel in Lab space is that colored highlights do not become white spots.
Try different combinations yourself. Save the images (I give permission to all to experiment with the images, just not for sale and profit), then try various ways to restore the blurred image to the original. The goal is to not have ringing artifacts (which appear as halos around objects). Figure 3 shows some ringing, especially around the black target. Try to not make the ringing any worse than in Figure 3. If you come up with a better solution, email it to me and I'll add it to this page (with credit to you if you wish).
Soon, I'll add some real images, both originals and unsharp masked for comparison.
Figure 1. Original test target.
Figure 2. Test target blurred with Gaussian blur, radius=2.0.
Figure 3. Test target restored with unsharp mask, parameters indicated on figure.
R. N. Clark Email contact (is encoded to prevent spam):
has the following form: username@qwest.net where
username is rnclark
First published November, 2002.
Last updated November 24, 2002