Haze removal

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This tutorial was last tested for the Gimp 2.2.13

The basis of this technique was taken from the Gimp Guru here.

Overview

Various man made and natural factors contribute to a photo having "haze", these include water vapor, smoke and pollution, etc. Digital cameras can also contribute to the problem by not capturing the full dynamic range.

The first image here is the cropped original, the second one has had this technique applied.


Original	Final

I will be using the original photo on the left than can be downloaded here.

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Prerequisites

None

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Steps

The first step is to improve the compressed dynamic range of the image.

Go to <Image> -> Layer -> Colors -> Levels (or select the Levels tool and click on the image)
Adjust the lower limit to 50.
- As you can see with a crop of the original beside the same photo with just the levels adjustment it is an improvement but more can still be done.
Go to <Image> -> Filters -> Enhance -> Unsharp mask
The settings I use as a starting point are:
1. Radius = 60
2. Amount = 0.40
3. Threshold = 0
Depending on the size of your photo The GIMP and the grunt your computer has under the hood, may take a while to process the image. You can watch the progress bar at the bottom, I would suggest waiting until that has finished. I have had some weird effects when I ran out of patience and applied other adjustments before that was finished.
The settings you use on your photo will vary, the Radius should be between 50 and 100. The Amount parameter I would always have between 0.35 and 0.50
As you can see the results can be quite good, compare the crop of the levels only and the one of having the Unsharp mask applied to it. As you can see the image on the right has more details in the guys face and the wave area as well as the colours having a lot more punch.
As a final optional step you could tweak the curves of the photo. I am still learning how to do this so won't offer any advice on that.