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@xxxxxxxxxxxxxxxxxxxx> 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 on your website [1] that JAB and JCH have outdated LAB and LCH 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 >> >> >> _______________________________________________ >> gimp-developer-list mailing list >> List address: gimp-developer-list@xxxxxxxxx >> List membership: >> https://mail.gnome.org/mailman/listinfo/gimp-developer-list >> List archives: https://mail.gnome.org/archives/gimp-developer-list >> >> > _______________________________________________ > gimp-developer-list mailing list > List address: gimp-developer-list@xxxxxxxxx > List membership: > https://mail.gnome.org/mailman/listinfo/gimp-developer-list > List archives: https://mail.gnome.org/archives/gimp-developer-list > _______________________________________________ gimp-developer-list mailing list List address: gimp-developer-list@xxxxxxxxx List membership: https://mail.gnome.org/mailman/listinfo/gimp-developer-list List archives: https://mail.gnome.org/archives/gimp-developer-list
