no worries, I will open a bug report later and now I will keep silent here about that, not to muddy waters on your lovely thread 
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@s7habo I have a couple questions, if you have time. In your videos there were a couple instances where you desaturated red berries using color zones. I didn’t notice you doing a gamut check and finding them out of gamut, so I presume the desaturation was for artistic reasons. Why did you not use lut 3d? In the autumn leaves video, you did not mask the leaves, so when you changed the berries, you also changed the leaves; was there a reason for that?
Tall order: given the new and improved color balance planned, will you make a new and improved version of your Episode 32: demonstration of color balance module?
Asking for a friend. 
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For that module it is not going to be hard to “use” the module in fact it is going to be very powerful to be able to impact color by targeting saturation, or chroma or purity. It will be nice to alter perceived color with out changing luminance or creating hue shifts but understanding how to really use these aspects to edit color in an image is going to require some investment into color theory to see how you might apply them. The modules in DT are actually not all the hard but mastering the color theory on which they are based is another thing…
I’ve looked for books and web pages, but haven’t found something that I felt would prepare me for this module. Aurelien made a book recommendation that I am considering. Do you have any recommendations?
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Its not hugely different from the old one…but now you have the ability to edit saturation, chroma, and purity…Sort out those definitions and you will have an idea of what is possible. Chroma is awesome. When you edit saturation you also affect brightness. Using chroma you can add or remove a color with very little change in brightness. Pure red is 255, 0 0… anything less than 255 is not pure red…by reducing purity you slowly remove the red in colors… This is overly simple but those are the sorts of things you can do now so its just about understanding those types of edits…rather than the module being difficult…
From colorbalancergb: improve the color science, colorbalancergb: improve the color science by aurelienpierre · Pull Request #7909 · darktable-org/darktable · GitHub, created by @anon41087856, I taken this image and text:
To understand the difference between chroma, saturation and purity, in the module, here are some swatches.
The pixel in the center of each swatch is the reference (input) and is not changed.
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We then make saturation vary from left to right (left : negative setting, right : positive setting) and
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purity from bottom to top (top : positive setting, bottom : negative setting).
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Luminance is approximately on the top-left to bottom right diagonal, and
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chroma on the bottom-left to top-right diagonal.
I believe I have it sorted out to give this explanation:
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saturation: increase saturation: increase chroma, decrease luminance
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purity: increase purity: increase chroma, increase luminance
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luminance: increase luminance: increase purity, decrease saturation, decrease chroma
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chroma: increase chroma: increase saturation, increase purity, decrease luminance
What I don’t understand from this is varying brightness. Is that a third dimension?
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Brightness is luminance expressed in perceptual units. Luminance is Y from CIE XYZ, brightness is Jz from JzAzBz or L from CIE Lab or CIE Luv.
Basically, it all falls back to:
- some metric of value : (linear) luminance or (perceptual) brightness,
- some metric of the distance between current chromaticity and the achromatic color having the same value : the chroma, which can be either linear or perceptual.
Luminance/chroma and brightness/chroma are both a set of perpendicular coordinates, only scaled differently.
Now, from brightness/chroma to saturation/purity (or actually, I learned recently that purity is actually called brilliance since the 1950’s, might change the name), we just rotate things :
(from The Difference Between Chroma and Saturation | Munsell Color System; Color Matching from Munsell Color Company)
The lines of equal saturation are the oblique ones, so the saturation setting opens or closes their angle like a flower, while purity moves along those oblique lines.
Changing luminance/chroma to saturation/brilliance changes an important thing: if you “de-chroma” red, you move to grey at same luminance, if you desaturate red, you move to pink. Any kindergarten kid knows that adding white in red paint gives pink, yet it’s an impossible color to get in photography, because any saturation algo actually uses chroma.
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Completely ot, but I never understood (though being a native german speaker), why it is Kindergarten and Baumschule and not Kinderschule and Baumgarten…
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LOL. And in America, why do we drive on a parkway and park on a driveway?
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I admit I am having trouble seeing how the tabs in the new color balance module work together. From your, @anon41087856, explanation, I am thinking you have changed the former slider name from saturation to chroma because that is really what it has always done?
I don’t grasp the global saturation sliders. Why not a (real) saturation slider to go below each hue slider? Or am I missing the point of luminance and chroma sliders?
I don’t get what you don’t get. What is a real saturation slider ? Or do you refer to the 4-ways saturation ? This one is the intensity of the color shift you define for each way : pick the hue of the deviation, and push it more or less along that hue with saturation, at constant luminance. While the global saturation is just making the pixel more or less colorfull at constant hue.
Yes, basically any “saturation” thingy in photo editing apps have always been massaging chroma and naming it wrongly.
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Thanks for your answer. I’ve stolen the thread long enough. I apologize, Boris.
This is a great site with lots of visual demonstrations. The Dimensions of Colour, modern colour theory
For example this one showing the effect of looking a squares with the same chroma in proportional light where they all appear to be the same color however all of them against white will apprear differently …all the basic color concepts are here…
Looking in a similar way at saturation to demonstrate that…
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I want to move off this post, so I will post on rebooting color balance - #8 by aurelienpierre.
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Given the following from Aurélien (at add a new colorbalancergb module by aurelienpierre · Pull Request #7347 · darktable-org/darktable · GitHub):
That’s confusing because 99% of image processing applications call “saturation” a chroma adjustment (as did the previous color balance).
So, the previous color balance had saturation sliders, but, as indicated above, they were chroma operations, if I understand correctly. In that case, the awesomeness is not new. 
In fact, Aurélien goes on to say:
Saturation actually helps getting deeper colours because they don’t wash to grey but to white, which is more like painting.
[…]
The saturation adjustment gives deeper and more natural color than the chroma.
So, could we say that purity / brilliance is a kind of saturation-preserving ‘brightness’; that is, increasing purity will make the affected area brighter, preserving saturation as long as we stay within the gamut (sliding further away from black along one of the oblique ‘isosaturation’ (as in ‘isobar’) lines); decreasing it will make it less bright, sliding towards black.
In that case, purity might not be the most appropriate term (so I’d vote for the brilliance version), as a ‘pure red’, at least to me, is one that shows no contamination from other colours (such as an orange or purple tinge).
And also (from add a new colorbalancergb module by aurelienpierre · Pull Request #7347 · darktable-org/darktable · GitHub)
Saturation actually helps getting deeper colours because they don’t wash to grey but to white
So, increasing / decreasing saturation will darken / brighten the area (keeping the same distance from black, increasing / decreasing the angle of the oblique line)?
I never really though about those….I used them to dial in the strength of autopicker hue selections more often than not. They were called saturation but I never really though about them that way just more like strength sliders… 
I was talking more about the global chroma slider and this is clearly doing a chroma correction and not saturation ….the old color balance input and output saturation sliders were I believe doing saturation corrections as the colors clearly deepen and darken or lighten and brighten or what ever terminology you want to use…
Basically to my eyes using global chroma pull out some color and doesn’t alter the luma or the impression of luma too much where saturation has a major effect…
Exactly.
With the ‘old’ color balance module, input and output saturation, I think, does not change perceived brightness, which is consistent with what Aurélien says (‘That’s confusing because 99% of image processing applications call “saturation” a chroma adjustment (as did the previous color balance).’). Chroma adjustment is at constant brightness.
Neutral:
‘old’ color balance at 0%
Color balance RGB at -100%
‘old’ color balance at 150% (neutral is 100%)
Color balance RGB at 100% (cannot be pushed further)
Overdrive
‘old’ color balance output at 200%
Note how the overlapping part of the histogram stays in place.
Compare with:
Perceptual saturation at 0% (neutral):
At 50% (becomes darker)
At -50% (becomes lighter):
Purity at +50% (becomes brighter):
Purity at -50% (becomes darker):
Finally, a purity vs exposure comparison:
Purity (sorry, I reset the history stack by mistake, so not the same as above) at 50, exposure at +0.5EV:
Purity at 0%, exposure at +1.35EV, in split view with the purity@50%, exposure@+0.5EV above:
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