Hello,
I started using Darktable again for some months now, and I’m still struggling with some darktables vocabulary.
So here is today question : in color calibration module, there is a tab named colorfulness. I suppose it is to make the image more colorful…
How is it different from saturation that I can see in other modules ? Does colorfulness makes something that saturation doesn’t ? (or vice versa).
Thanks 
Colourfulness, saturation chrominance, hue, chroma… All deal with the same subject: colour and all of them are (subtly) different from each other.
@anon41087856 is the colour expert and he can tell you exactly and in many details what it is all about, but to give you a running start I’ll provide you with some articles that you might find interesting:
Hue, Saturation, Chroma and Other Ways of Understanding Color [changingminds]
The Difference Between Chroma and Saturation [munsell]
TL;DR: There are subtle differences between them and using the correct terms when talking about them is important. That being said: Often times these terms are (wrongfully) mixed and matched when talked about.
Hi Jade,
Thanks for the links. I see that wording is important. That’s the reason of the question, why use colourfulness term or not in darktable. And what impact it has or not
The short answer would be: Because that is what that slider influences in this specific case and not saturation.
Why the developer (Aurelien) implemented it this way I cannot tell you. You’ll have to wait 'till he shows up and elaborates on that.
If you look at the darktable docs (color calibration) you’ll notice the following being said (emphasis by me):
The brightness and colorfulness (color saturation) of pixels in an image can also be adjusted based on […]
I’m not sure why this is stated the way it is. It either changes saturation or it changes colourfulness. The difference might be so trivial in the end though that it really doesn’t matter all that much.
I’ll leave it up to Aurelien to explain that one.
There are many nuances to this…here is a pretty good explanation…
As the article says : “This exploration is becoming quite a tongue-twister as well as a brain-teaser!” 
Thanks for the link.
If you want to really geek out…this is the site… http://www.huevaluechroma.com/
And as there is a new color balance module being worked on you might be interested as some of these discussions around chroma saturation etc came up here
I like the huevaluechroma site, but it doesn’t nail the difference between saturation, chroma and colorfulness in a way that I can understand. Nor does any other site I have seen.
For example, I want to see two colour patches A and B, where A has higher chroma but lower colorfulness than B. And I would like that explained in words and maths.
And if these terms have different definitions from different bodies or in different colorspaces, that’s okay, but I would like to see explanations.
Not sure about the math…I did stumble on this
But given this example you could imagine a patch that had higher chroma and less colorfulness
File:Brightness and colorfulness.png - Wikipedia I shoudl say my eyes can imagine it but not sure its true…
This was an interesting thread…
I find these things confusing for sure. Quoting David Briggs…
" The correct word for strength of colour is chroma when talking about objects, and colorfulness when talking about light/overall appearance, as in, a high-chroma red ball presents a colorful appearance/gives off colorful light when it is brightly lit, and presents a less colorful appearance/gives off less colorful light when it is sitting in shadow."
I think if I understand it chroma is a physical property and colorfulness is more about perception.
It is explained similarly here but with some simple math (correctness not implied by me)
http://changingminds.org/explanations/perception/visual/colourfulness.htm
So I am assuming that the colorfulness sliders are an attempt to modify saturation or chroma and perhaps luminance to simulate or produce the perception of increased color??
This quote seems to imply (possibly unintentionally) that it makes no sense to talk about chroma in photography. Based on the quote, a red ball may have strong chroma, even in a completely dark room. But both photography and our eyes detect the light reflected from the red ball, and that is affected by the amount of light that reflects off the ball, and that’s what we are (normally) attempting to capture. We may attempt to exaggerate the ball or alter the hue, but the general notion is to do something based on the appearance of the ball given the light reflected from it. From that quote, we’d be dealing with colorfulness rather than chroma.
Writing definitions for these words can be tricky!
I believe he is a professor of color science so I could only take the comment as delivered. I’m still not sure of the nuance that you are referring to but that is my ignorance…I think what you are saying is more or less what he has said. For now I have to stick with chroma being a physical entity relative to grey and colorfullness something describing how we perceive it in the current illumination that the object lives in……
Thanks, @priort. Interesting links. Definitions (like standards) are great because there are so many to choose from.
There is, of course, a difference between the reflectivity of a red ball, and the light emitted from a red ball (which also depends on the illumination), and the perceived colour of the red ball. When we are dealing with images, we care about the pixels, which depends only on the light that hits the sensor.
The CIE originated in concepts of illumination rather than images, and still retains that perspective.
The Hasler and Susstrunk paper quantifies “overall colourfulness” of a picture, rather than of a single colour. A picture might be a solid colour, so standard deviations are zero, whch simplies the calculation. So we can compute the coloufulness of each pixel in an image.
All commands use ImageMagick in Windows BAT scripts.
magick ^
%SRC% ^
-channel RGB -separate +channel ^
( -clone 0,1 ^
-compose MinusSrc -composite ^
-evaluate Pow 2 ^
) ^
( -clone 0-2 ^
-poly "0.5,1,0.5,1,-1,1" ^
-evaluate Pow 2 ^
) ^
-delete 0-2 ^
-compose Plus -composite ^
-evaluate Pow 0.5 ^
hs.jpg
From Colorfulness, Saturation, Chrominance and Other Ways of Understanding Color
magick ^
%SRC% ^
-channel RGB -separate +channel ^
( -clone 0-2 -evaluate-sequence Max +write mpr:MAX +delete ) ^
( -clone 0-2 -evaluate-sequence Min +write mpr:MIN +delete ) ^
-delete 0-2 ^
( mpr:MAX mpr:MIN -compose Plus -composite ) ^
( mpr:MAX mpr:MIN -compose MinusSrc -composite ) ^
-compose Multiply -composite ^
mpr:MAX -compose DivideSrc -composite ^
mm.jpg
This isn’t a million miles away from the previous result.
Let’s try 2 * Saturation * Lightness:
magick ^
%SRC% ^
-colorspace HSL ^
-channel 1,2 -separate +channel ^
-compose Multiply -composite ^
-evaluate Multiply 2 ^
sl.jpg
This is very different. Ah, well.
Clear as mud
I’m not actually making any statement regarding the correctness of what he says. I’m battling with some of the terminology too, especially when I see comments suggesting that many of the words are commonly misused. The result of that is, whenever you see a term, you need to figure out what the individual using a term thinks the term means before you can really process what that person means.
I’m just observing that if what he says is correct, then chroma becomes kind of a “who cares?” word, at least in cases where the object is not itself a light source. We see the light reflected off the object, and our cameras detect the light reflected off the object.
Ain’t that the truth!
I thought I asked a simple question… what have I done? 
Are they not both parts of the equation…I think that is the exact premise of a Munsell chart …it takes a hue and gives you patches for a chroma vs lightness leaving you with patches of colorfullness?? Perhaps that is way to simplified…Your most important point I think is everyone needs to be able to have a common understanding of what somebody means when they use a term…I just assume I don’t get it
100%!
Sometimes definitions serve well as specifications, but don’t help much with understanding. I’ve been wrestling with this too, and here’s a quote from Wikipedia that might help:
“ * Colorfulness is the “attribute of a visual perception according to which the perceived color of an area appears to be more or less chromatic[clarification needed]”.[1][2] The colorfulness evoked by an object depends not only on its spectral reflectance but also on the strength of the illumination, and increases with the latter unless the brightnessis very high (Hunt effect).”
My takeaway is that it’s a matter of perception as well as physical attributes, ie, a red ball is perceived to be more colorful in the sun than it is in the shade. Not sure that that helps.