Hello,

I was wondering if something like that existed or could be created, it would work like the dodge tool but it would only affect saturation, and possibly with a "preserve brightness" switch like the one I see within color balance tab.

Paint tools and layers have a "saturation" mode.... (bottom of the list)

For a saturation adjustment layer, add a new layer, flood fill it with any colour around mid saturation and put it into saturation mode. You can then run colours/hue-saturation on that layer instead of the original image. If you like, instead of flood filling it, you can paint colour onto the layer where you want it.

Painting with grey on this layer will desaturate while painting with saturated colour will adjust saturation.

...I see, I admit I've never think about using the saturation layer mode, in fact, except 'overlay', I rarely use layer modes anymore, in favor of curves. It does the trick indeed, it's not as intuitive as the tools like dodge and blur, but it does what I wanted to see.

And what about brightness preservation, how would you simulate it, with this method ?

Preserve the brightness of the original ?
duplicate the original layer, put the dup on top in value mode

If you do remember that most of the sharpness derive from the top layer in value mode, so in case of colored noise or too abrupt color transition, you may apply quite a bit of gaussian blur to the layers below the top one without blur in a perceptible way the visible ,the combination of the various layers .

Putting a duplicate of the original on top in value mode doesn't affect it so I imagine the brightness isn't affected by the saturation layer even though it appears to be. However, Edit/copy visible - paste as new layer followed by desaturate produces differing values where the saturation/desaturation is but desaturate changes the values anyway. Using a dupe of the original in value mode on top of the desaturated image recovers the original values.

Desaturated



With value layer

Real brightness should not be affected at all by saturation but apparent brightness may be :

Suppose 2 colors with same "real" brightness, as example 1 yellow and 1 violet :
the yellow would almost always seems lighter and the violet darker (except when totally washed out , and so white, or totally darkened and so black )

And i suppose more saturate are the colors more this become intense and vice versa.

Color BW conversion take care of this with various methods , usually giving different weight to the basic hues to give a greyscale more close to the..."apparent brightness"

It seems I can't get to preserve the brightness.
I don't get the differences between real and apparent brightness, but lets put it aside until we figure out how to solve the apparent brightness distortion caused by saturation.

Here's what I get using Erisian avatar (hope you don't mind):
originals are on the first row.
What I see is that my saturation layer darkened the image, I then ran desaturate-brightness to verify.

I don't mind at all. Try this in order to understand. Open your foreground colour to reveal the colour selection dialogue. Vertical movement gives saturation and horizontal gives brightness in computer terms. As a colour becomes more saturated, it becomes more intense giving the illusion of being darker. The brightness hasn't changed in computer terms but visually it appears darker the more saturated it is. I'm afraid this is an unavoidable bi-product of saturating. What you might want to do is put a duplicate of the original on a value layer on top and run levels or brightness/contrast on it. Hope this helps.

Of course ! I didn't pay attention to the value !
...Increasing saturation doesn't affect value, but viewed from our eyes it decreases value. I understand how I was fooled. But then, the tricky thing is that saturation, from human point of view, contains some kind of value, just not the value as calculated by the computer. So, we would need some computation that could interpret this "false" value loss to counter-balance.

Shortly, something that could manage the apparent brightness side, not the real one.
I'm afraid that intuitive saturation tool I'm thinking about doesn't exist yet, those layers methods truely lack usability for painting.

Oh for a magic wand! ( a real one - not a fuzzy select tool )

I don't know if this is worth playing with. Try painting onto a saturation layer, duplicate it ( not the original ) then put your new layer into value mode and use brightness/contrast to adjust.

Well, it works, but it implies guesses and workarounds that kills the usability, for me.
But maybe it's me, maybe I want my gimp to act fluidly, like a dedicated painting software, which is not the main focus of gimp after all.

It would be nice and I'm sure we all want things made easier but I suppose nothing's perfect. Besides, if it keeps getting easier and easier, eventually it would do everything for us while we make coffee all day. I don't think I like that idea to be honest.