Well, it's a new and more complicated way to simulate simple High Pass filtering. ;-)
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Hehe, almost everything in the gimp is more complicated unless you want to write a script .
It is very similar to a high pass filter. However, a slight variation that keeps it from desaturating the image: the guassian blurred image is desaturated and then set to a higher opacity. In a traditional high pass the guassian blurred layer is inverted (but not desaturated) and then set to 50%. If you were to just set the gaussian blurred layer to a higher opacity than 50% in a traditional high pass filter, it would tend to lower saturation. So, you could look at this as a hybrid between a contrast mask and a high pass filter.
I show a comparison of high pass, this tone mapper technique and several others here:
[a href=\"http://gimpaddict.googlepages.com/contrastcomp.html]http://gimpaddict.googlepages.com/contrastcomp.html[/url] (might be slow to load on a dialup connection).
I've been looking more carefully at the lightzone tone mapper. While this process does a decent job of emulating their results, it still is somewhat different.
Note: if you have an image that needs a really strong contrast reduction, you're better off using a full blown tone mapper algorithm like reinhard, drago, or fattal (via pfstools for instance).