How-To



Tuesday, August 4, 2009

Defringing


How to defeat edge artifacts in composited landscape images

Labels: How-To

This Article Features Photo Zoom

Expand Selection
To illustrate this method, I selected an early-morning image of the Garden Wall in Glacier National Park (Figure 2a). The sky in this image is uninteresting, so I’d like to add a more exciting background. Again, I made a composited image as in the previous example (Figure 2b). Once more, upon close inspection, it’s apparent that there’s a significant amount of edge fringing or halo along the mountain edge.

To get rid of the halo this time, highlight Layer 1 in the Layers palette and then, from the main toolbar, choose Select > Load Selection. The Load Selection dialog box will appear—check the Invert box and click OK, accepting the default Source and Channel options. You should now see marching ants around the outside of the fringe on the original selection.

Here’s where the fix comes in—Select > Modify > Expand. Another dialog box appears, allowing the number of pixels by which the selection can be expanded to be entered. Generally, a value of 1 or 2 will be more than enough—but again, you may need to experiment. In this example, I used a value of 2. Hit OK, and the marching ants will move from the edge into the image by the number of pixels chosen. If there’s still fringe showing inside the line of marching ants, repeat the process in small increments until it looks right. If you feel that you’ve gone too far, simply back up a step using the History palette or the shortcut Cmd/Ctrl + Z.


Once you’re happy with the expanded selection, hit the Delete key to remove the fringe band. Lastly, Select > Deselect (Cmd/Ctrl + D) to get rid of the marching ants and you’re done. In Figure 2c, the upper image shows a close-up of the edge fringe before any corrections are made, and the bottom image illustrates the result of a 2-pixel edge expansion. The final corrected image is depicted in Figure 2d.

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