Re: [Gimp-developer] Adding better LCH support to GIMP 2.10

On 02/14/2016 03:56 PM, Sven Claussner wrote:

@Elle: you are speaking here of the JCH color model (or space) and
on your website [1] that JAB and JCH have outdated LAB and LCH
(which are often considered the high end image editing color
Searching a while for more information about JCH I found only very few
information, even not on other color management and FOSS graphics devel
mailing lists. Only on the PXLab website [2] I see a short description:

JCH:=The CIE Color Appearance Model (1997) with viewing and scene
to be defined separately.

I'm failing to understand all its implications.
Can you tell us more about JCH and JAB and why you consider it to
be a good choice, please? What about LAB and LCH then?

I can't tell you very much about JCH/JAB because I'm still trying to figure it all out myself.

You might try plowing your way through this PDF:

This page has links to some equations:

This page briefly talks about color appearance models in more down-to-earth terms:

An internet search on terms like Color appearance model, Mark Fairchild, and CIECAM02 will turn up a lot of material. None of it is easy reading.

LAB answers the question "how far apart do colors have to be before the average human observer will say 'those are different colors'". The "home" of LAB was for use with quality control for colors in textiles, printing, and such. LAB wasn't designed to be used as a color space for editing, but it works pretty well for a lot of different editing tasks.

A full understanding of LAB would require understanding the kinds of experiments that were done to map out "when is color X different enough from color Y to be seen as visually different?" I have no idea what kind of experiments were done or how the mathematical model was constructed from the experimental results. But the resulting equations to convert from XYZ to LAB and then LCH are pretty straightforward.

Color appearance models are designed to answer a very different and much more complicated set of questions. They try to answer questions like "How to describe colors?", "Why does the appearance of one color change when juxtaposed next to another color?", and "Why does a surface look the same color even when the light shining on it changes drastically (for example from bright daylight to deep shade or to tungsten lighting, or even in the shadow side of an object)?".

These are complicated questions, being answered using complicated research and complicated resulting models described by complicated sets of equations. Also color appearance models are a very active area of ongoing research, so what's considered really good today might be superceded tomorrow.

My reasons for suggesting that for use in GIMP (1)LAB/LCH is good and (2)JAB/JCH is probably better are so simplistic that you'll all just laugh:

1. Bruce MacEvoy's website on watercolor pigments switched from using LCH to using JCH to give paint pigment colors, and I respect Bruce MacEvoy as an authority on giving useable values for paint pigments:,

2. Mark Fairchild seems to think CIECAM02 is a pretty good color apperance model, though again this is a field of active research, and much of what Mark Fairchild writes goes right over my head.

3. ArgyllCMS and LCMS already incorporate the equations for JAB/JCH in their code, as does RawTherapee, so it has to be not too difficult to code up for GIMP. But my (very limited) efforts to make sense of the ArgyllCMS/LCM/RT code have so far been unsuccessful.

I find the LCH blend modes, color picker, and Hue-Chroma tool to be incredibly useful. The few people who've written to me about using my patched GIMP seem very enthusiastic.

JCH apparently is more accurate than LCH for describing colors. I'm not sure how much difference the "more accurate" would make in the digital darkroom. It would be nice to be able to give JCH a try, but in the meantime I can't imagine going back to editing without LCH.

Does somebody else here know more about it?

No doubt even my short description above needs a lot of corrections!

Thank you in advance




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