How-To



Tuesday, August 4, 2009

Defringing


How to defeat edge artifacts in composited landscape images

Labels: How-To

This Article Features Photo Zoom

Few things are more frustrating to a photographer than edge fringes or halos that appear in Photoshop after a selected or extracted object has been placed onto a new background. Have faith, for all is not lost, as there are several solutions for handling this dilemma.

defringing

Some of these fixes are straightforward and simple, while others may not be as intuitive but can give excellent results, especially where complicated selections are concerned, such as around hair or foliage. Depending on the nature and complexity of the particular selection, it may be necessary to employ more than one fix. Let’s look at how I attack this problem using Photoshop CS4 on a variety of images, going from simple to more intensive methods.

Defringe
This method is quick and is the one I go to first. It’s particularly useful where there’s not a lot of detail in the selection, such as a distant mountain ridge. Figure 1a is a rather simple shot of Fisher Cap Lake in Glacier National Park, but the sky is pretty boring, so I’d like to spruce it up a little by adding a more interesting sky. The composited image, with new sky background, is shown in Figure 1b. I won’t spend any time discussing how to make a selection or place it over a new background—if you’ve experienced edge fringing, then you already know how to accomplish these tasks. The only thing that I’ll mention is that the method utilized to make the selection seems to have little or no bearing on whether or not a resulting halo will be present.

In this example, the composite looks great, but sure enough on closer inspection, there’s a distracting halo along the mountain spine that makes the image appear unrealistic. Before attempting any fix, I zoom in on the fringed edge so that I can really see the problem and how the solution is working. To remove this edge fringe, the foreground layer is selected (in this example, Layer 1); then, from the main toolbar, choose Layers > Matting > Defringe. An option dialog box will appear to offer a choice of how many pixels to use for Defringing. I accept the default value of 1. (I haven’t noticed any marked difference in using higher values on any images that I’ve worked with, but that shouldn’t deter you from experimenting on your own with other values.) Figure 1c shows a close-up comparison of the mountain ridge before and after the Defringe tool has been used. Figure 1d is the completed composited image after defringing.

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