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Sharpening with an Edge Mask - or
Creating an Edge Mask in Photoshop
category: digital photography and image editing
If the image to be sharpened is noisy or grainy, create an edge mask for sharpening that, when inverted, acts as a perfect mask for using the Gaussian Blur or Median filters to clean up the noise in smooth areas without softening the edges.
Duplicate the channel with the most detail and the least noise. This will probably be the green channel. Rename the copied channel Edge Mask. Now choose Filter, Stylize, and then Find Edges. Your image window should now be edges in black against a dirty white background.
OK, press Ctrl+I to invert the image, or choose Image, Adjustments, and then Invert. This is your mask. To clean it up a little, so it looks black and white with very little gray or snow, use a Levels adjustment.
We don't want to push this cleanup too far, so choose Window, Arrange, and then New Window. Tile these windows in the same menu, and then set one window to 200% view looking at some edges and the other zoomed out so you can see the whole image. Press Ctrl+L, or choose Image, Adjustment, and Levels.
Now, drag the input sliders for white and black toward the center, a little at a time, until most of the gray and snow in the zoomed out view of the image turns black, but the close up view of the edges, now white, do not lose their definition. If you're not sure, cancel and start over. By the way, if you press the Alt key, the cancel button changes to Reset to save you some time.
When you're happy with the Levels adjustment, choose Filter, Blur, and then Gaussian Blur. The idea here is to blur the edge of the edges, just a little, to smooth the transition between sharpened and unsharpened areas. Sometimes, even with a Radius of one or two, the entire edge gets gray. That's OK, we can fix that; right now we're worried about the transition blur.
When you're happy with the blur, you can brighten up the centers of the edges by returning to Levels; Press Ctrl+l, and this time move only the center slider, the Gamma slider, to the left until the center of the edge lines are white again, but the blur on the edge of the edges is not defeated. You're done; just Ctrl-Click on your new mask to convert it into a selection and sharpen away. If the moving selection lines are in the way, press Ctrl+H to make them invisible.
Further sharpening input, with or without a mask, can be found at If Images Look Oversharpened.
He said on 05.25.05 @ 03:19 PM CST
