More on LCH and JCH

[Elle (Elle Stone)]

@Carmelo_DrRaw

On the other hand, the CIELCh colorspace has been designed to be perceptually uniform, and to approximate a constant visual saturation for a given C value across different Hue values.
[. . .]
The bottom line: CIELCh is a better representation than HSV/HSL if you want to edit colors in an intuitive and device-independent way. For example, you can fix C and change h in order to make colors warmer or cooler without affecting the resulting “perceived saturation”.

What @Carmelo_DrRaw says is of course correct, except for the word “saturation”. “Saturation” in image editing has far too many definitions:

• We all use the word “saturation” loosely speaking to refer to “more or less colorful colors”.

• We also use it technically to refer to “Saturation” calculated using HSL, HSI, etc. Let’s put HSL/HSI/etc to one side as not relevant to a discussion of LCH - as @Carmelo_DrRaw notes, these HSL/HSI/ect saturation values change every time you change the RGB working space, and this includes changes to the color space TRC as well as changes to the RGB primaries.

• The word “saturation” has a precise definition in color appearance models such as CIECAM02, and that definition is different than the definition of “chroma”. This page by Mark Fairchild presents some nice definitions of colorfulness, saturation, and chroma as used in color appearance models:

http://www.rit-mcsl.org/fairchild/WhyIsColor/Questions/4-8.html

Here’s the TOC for the whole series of “WhyIsColor”, which strives to hit the “explain it to me as if I were five” sweet spot, but I still find the explanations of chroma, colorfulness, and saturation difficult to visualize:

http://www.rit-mcsl.org/fairchild/WhyIsColor/map.html

LAB/LCH is not a color appearance model (it’s a “color difference” model), so “colorfulness” technically doesn’t apply to LCH. That doesn’t keep people from coming up with equations for colorfulness in LAB/LCH. This Wikipedia page gives a nice equation for calculating “colorfulness” - or maybe its really “saturation” - in the LAB/LCH color space. The page also has some very confusing (to me) sections on other topics:

Colorfulness

In the LCH color space, LCH blend modes and color pickers allow to keep Chroma constant while changing Lightness or Hue. But keeping the appearance of colorfulness/saturation constant is a different matter, requiring some workarounds. In GIMP, if you want to change tonality of an image and also keep colorfulness constant (or is it saturation? the definitions are not easy for me to visual!), use Luminance blend to blend the new tonality with the original image colors.

OK, putting all this technical stuff to one side, what does any of this mean for actual image editing? Here’s an example using GIMP-2.9’s Lightness and Luminance blends to show the difference between LCH Chroma and colorfulness/saturation (apparently you have to click on the image to all of it - the “4. Luminance blend” version is at the bottom):

What you can’t tell from the above image is that some of the colors in the umbrellas in images #1 and #4 (but not in #3) are out of gamut with respect to the sRGB color space. One could use a mask to blend in some of the result of the Lightness blend, to bring the Luminance blend colors all back into gamut. The good news is that GIMP 2.9 (default, not yet my patched version) now has a really nice clip warning for floating point images.

If anyone wants to experiment with color appearance model terminology by changing the various parameters, the RawTherapee CIECAM02 module allows you to do just that, which might help quite a lot in acquiring a practical understanding of the difference between saturation, chroma, and colorfulness, and various other color appearance model terms. Plus the sliders and curves in the RT CIECAM02 model are just plain fun to experiment with.