11.40.  Retinex

Revision History
Revision $Revision: 2442 $ 2007-10-21 ude

These options call for notions that only mathematicians and imagery engineers can understand. In actual practice, the user has to grope about for the best setting. However, the following explanations should help out the experimented GIMP user.

Level

Here is what the plug-in author writes on his site [PLUGIN-RETINEX]: “To characterize color variations and the lightor, we make a difference between (gaussian) filters responses at different scales. These parameters allow to specify how to allocate scale values between min scale (sigma 2.0) and max (sigma equal to the image size)”...

Uniform

Uniform tends to treat both low and high intensity areas fairly.

Low

As a rule of thumb, low does “flare up” the lower intensity areas on the image.

High

high tends to “bury” the lower intensity areas in favor of a better rendering of the clearer areas of the image.

Scale

Determines the depth of the Retinex scale. Minimum value is 16, a value providing gross, unrefined filtering. Maximum value is 250. Optimal and default value is 240.

Scale division

Determines the number of iterations in the multiscale Retinex filter. The minimum required, and the recommended value is three. Only one or two scale divisions removes the multiscale aspect and falls back to a single scale Retinex filtering. A value that is too high tends to introduce noise in the picture.

Dynamic

As the MSR algorithm tends to make the image lighter, this slider allows you to adjust color saturation contamination around the new average color. A higher value means less color saturation. This is definitely the parameter you want to tweak for optimal results, because its effect is extremely image-dependent.