Colors>Components>Decompose>LAB
This option decomposes image into three greyscaled layers, layer “L” for Luminance, layer “A” for colors between green and red, layer “B” for colors between blue and yellow.

The LAB Decomposing is a color model of the Luminance-Color family. A channel is used for the Luminosity while two other channels are used for the Colors. The LAB color model is used by Photoshop.

thanks Rod, by googling I found the initial ideas (that you already synthesized)
I post here what I found, for some other novice like me on LAB

When any image is converted to 'Lab' it will look exactly the same. Because the 'Lab' contains all possible colours there can be no deterioration in the image quality as all colours translate unaltered. But when the separate 'Lab' channels are looked at they can seem a little odd at first. An RGB image is easily understood as there are three logical colours. But 'Lab' has a mix of one channel with no colour (L), plus two channels with a dual colour combination that have no contrast (a+b).

The 'L' channel, or Lightness, is the easiest to understand as it is a Greyscale. It has no colour value at all; it just contains the contrast between the lightest and darkest points in the image. On the other hand the 'a' and 'b' channels need a little explanation. The 'a' is the colour balance between Green and Magenta, and the 'b' is the colour balance between Blue and Yellow. .... Wherever the channel is mid grey there is a balance between the dual colours.

NOTE: While the 'L' stands for Lightness, the 'a' and 'b' are arbitrary names; they could just as easily have been labelled 'α' and 'β', or '1' and '2' instead.

I actually tried using LAB as an algorithm for the Rodilius filter for GIMP.
But motion blurring (and rotating) the "L" channel posed a problem. I couldn't get it to work. Luckily David wrote us one for G'MIC.

I should actually get back to work on that problem some day.

Have a look at both this tutorial and the G'MIC method in the second post.
Subject: Using the LAB color space to adjust an image

Excellent!



Under Colors>mixer-lab now though.

thanks Odin!
(being a bit lazy I went immediately to the Gmic method!...)
nice results (a Gmic filter I never considered sofar, being ignorant about LAB)

Just tried out the GMiC>color>lab and it works great.