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

[Partha Bagchi <partha1b gmail com>]

First hit when you google:
http://www.colourphil.co.uk/lab_lch_colour_space.shtml


On Sun, Feb 14, 2016 at 6:33 PM, Elle Stone <ellestone ninedegreesbelow com>
wrote:

> On 02/14/2016 03:56 PM, Sven Claussner wrote:
>
> > Hi,
> >
> > @Elle: you are speaking here of the JCH color model (or space) and
> > mentioned
> > on your website [1] that JAB and JCH have outdated LAB and LCH
> > (which are often considered the high end image editing color
> > spaces/models).
> > 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
> > conditions
> > 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:
> http://rit-mcsl.org/fairchild/PDFs/AppearanceLec.pdf
>
> This page has links to some equations:
> https://en.wikipedia.org/wiki/CIECAM02
>
> This page briefly talks about color appearance models in more
> down-to-earth terms:
> http://www.rit-mcsl.org/fairchild/WhyIsColor/Questions/4-8.html
>
> 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 handprint.com 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:
> http://handprint.com/HP/WCL/water.html,
> http://handprint.com/LS/CVS/color.html
>
> 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
> >
> > Sven
> >
> >
> > [1]
> >
> > http://ninedegreesbelow.com/photography/high-bit-depth-gimp-tutorial-edit-tonality-color-separately.html
> >
> >
> > [2]
> >
> > http://irtel.uni-mannheim.de/pxlab/doc/api/de/pxlab/pxl/ColorSpaceCodes.html#CS_JCh
> >
> >
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