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Giving credit where credit is due: I did not come up with this method. I adapted it for The GIMP from an article on the Luminous Landscape web site (great photography web site, BTW, I recommend it).
|Here's the original image loaded into the GIMP. It suffers from both types of haze that I described above: a compressed dynamic range (probably a result of the in-camera metering algorithm being fooled, or overwhelmed by the wide contrast in the scene), and haze due to water vapor over the town of Kailua on Oahu, Hawaii.|
The first thing to always check is the histogram. Here I bring
up Levels (Layer/Colors/Levels in GIMP 2.0). Notice how the
upper (right) end of the histogram falls off well before the
right end? I can improve this compressed dynamic range by
bringing the white point slider down a bit to the end of the
histogram, as I have done here. Hitting "Auto" (Auto Levels)
will often produce good results, with the caveat that you lose
the control that you get manually adjusting the sliders.
As you can see, making full use of our available dynamic range improves the overall contrast of the image.
If I zoom in to a 100% view you can see that there is still a
fair bit of "natural" haze left in this photograph. It is
unlikely that I can remove this completely, but there is another
trick that helps out significantly.
Run an Unsharp Mask filter on the image (Filters/Enhance/Unsharp Mask). Set the Radius parameter to a largish, seemingly unreasonable value somewhere between 50 and 100. You'll need to experiment with the Amount parameter, but the idea is generally to have it as a small to moderate value (in this case .40). Leave the Threshold parameter at 0.
What you are doing is applying a local contrast enhancement to the entire image. Unsharp mask works by increasing the contrast between edges in the photos (areas of color or tonal transition), making them appear sharper. Even though they really aren't, your brain registers this increased contrast--technically called "edge accutance"--and the edges appear more defined, thus sharper. What we are doing here is making the pixel Radius big enough so that the filter is not just increasing the contrast along fine lines, but in larger swaths.
As you can see, the result has more punch and a less "gauzy" appearance.
|Zoomed out we see the full effect. To see the before and after effect in your own image, make use of the Undo (Ctrl+Z) and Redo (Ctrl+Y) features to quickly compare before and after at different zoom levels. If you want to try different sharpening parameters (say more or less Amount), just Undo, bring up the last filter applied with Ctrl+Shift+F and tweak the parameters.||
Last modified: Sun Sep 5 22:39:30 HST 2004
|Text and/or images are available for personal or commercial use under certain conditions.|