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

[Øyvind Kolås]

On Mon, Feb 15, 2016 at 12:33 AM, Elle Stone
 wrote:
> 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.

Color Appearance Models are even more in the realm of the subjective
phenomenological experience of color than CIE XYZ / CIE Lab - and less
useful in an attempt at absolutely defining/describing a color outside
the context/scene where it is experiences/observed. With a color
appearance model the squares A and B in the Checker Shadow Illusion (
https://en.wikipedia.org/wiki/Checker_shadow_illusion ) would have
different colors - since they *appear* to have different colors.
Through color constancy this affects not only the apparent luminanace
but also the apparent hue of colors in a scene.

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

[Partha Bagchi]

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 
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 on your website [1] that JAB and JCH have outdated LAB and LCH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
>>
>>
>> 

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

[Elle Stone]

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

[Sven Claussner]

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?
Does somebody else here know more about it?

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

[Elle Stone]

I received an email complaining that it was not true that as located on 
the LCH color wheel, sRGB Blue was blatantly violet, sRGB Red was on the 
orange side of red, sRGB Green was on the yellow side of green, and sRGB 
Yellow as on the green side of yellow.


There is a sliver of truth in the complaint. On the LCH color wheel, my 
statement is true.


However, LAB/LCH was designed to measure color differences, not to serve 
as a color appearance model, even though it often gets used as such. 
Also the LAB color space is particularly bad when it comes to dealing 
with blues and violets.


For comparison, here are JCH and LCH values reported using the ArgyllCMS 
xicclu tool:


 Hue  JCH/LCH
sRGB Blue301  [LCh]
sRGB Blue273  [JCh] (still on the violet side of blue, but not 
blatantly violet)


sRGB Green   134  [LCh]
sRGB Green   140  [JCh] (not as far towards yellow)

sRGB Red  41  [LCh]
sRGB Red  32  [JCh] (not as far towards orange)

sRGB Yellow  100  [LCh]
sRGB Yellow  111  [JCh] (farther towards green)


As you can see from these values, according to JCH model, sRGB Blue is 
still on the violet side of the JCH color wheel. But it's not nearly as 
far on the violet side of the JCH color wheel as it is on the LCH color 
wheel.


For both color wheels, sRGB Green is on the yellow side of green, sRGB 
Red is on the orange side of red, and sRGB Yellow is on the green side 
of yellow.


If you would like to check for yourself:

Here's the handprint.com CIECAM (JCH) color wheel: 
http://www.handprint.com/HP/WCL/cwheel06.html


Here's the handprint.com CIELAB (LCH) color wheel: 
http://www.handprint.com/HP/WCL/labwheel.html


Here's the xicclu commands for checking the LCH/JCH values for sRGB 
colors using an sRGB ICC profile:


xicclu -ir -pJ sRGB-elle-V2-g10.icc
xicclu -ir -pL sRGB-elle-V2-g10.icc

Here's a link to the xicclu documentation:
http://argyllcms.com/doc/xicclu.html

Here's a link to download ICC profiles:
http://ninedegreesbelow.com/photography/lcms-make-icc-profiles.html

It would be very nice if GIMP users had a way to pick colors using the 
LCH color wheel or better yet the JCH color wheel. Either color space is 
a vast improvement over the pathetically inadequate HSV.


Best,
Elle

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

[Elle Stone]

GIMP's new LCH blend modes are wonderful. But trying to use these new 
blend modes without the proper supporting tools is sort of like being 
given a new Aston-Martin with a 35-mile-per-hour governor on the engine.


To make the LCH blend modes truly useful GIMP needs:

* an LCH Hue-Chroma tool.
* LCH color sliders added to the "Change Foreground Color" dialog.
* the Luminance blend mode in addition to the LCH Lightness blend mode.

There are already more or less useable patches for the LCH Hue-Chroma 
tool and LCH color sliders 
(https://bugzilla.gnome.org/show_bug.cgi?id=749902), and for the 
Luminance blend mode (https://bugzilla.gnome.org/show_bug.cgi?id=753163).


Adding the LCH Hue-Chroma tool and LCH color picker sliders for GIMP 
2.10, right alongside the eventually-to-be-deprecated(?) HSV 
Hue-Saturation tool and HSV color picker sliders, would allow GIMP users 
a chance to adapt to LCH before HSV is summarily removed from the UI.


Best,
Elle

How to split tone digital images using LCH and Luminance blend modes
http://ninedegreesbelow.com/photography/split-toning-using-LCH-blend-modes.html



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

[Alexandre Prokoudine]

On Sun, Feb 7, 2016 at 10:39 PM, Elle Stone wrote:
> GIMP's new LCH blend modes are wonderful. But trying to use these new blend
> modes without the proper supporting tools is sort of like being given a new
> Aston-Martin with a 35-mile-per-hour governor on the engine.
>
> To make the LCH blend modes truly useful GIMP needs:
>
> * an LCH Hue-Chroma tool.
> * LCH color sliders added to the "Change Foreground Color" dialog.
> * the Luminance blend mode in addition to the LCH Lightness blend mode.
>
> There are already more or less useable patches for the LCH Hue-Chroma tool
> and LCH color sliders (https://bugzilla.gnome.org/show_bug.cgi?id=749902),
> and for the Luminance blend mode
> (https://bugzilla.gnome.org/show_bug.cgi?id=753163).
>
> Adding the LCH Hue-Chroma tool and LCH color picker sliders for GIMP 2.10,
> right alongside the eventually-to-be-deprecated(?) HSV Hue-Saturation tool
> and HSV color picker sliders, would allow GIMP users a chance to adapt to
> LCH before HSV is summarily removed from the UI.

Personally, I find darktable's Color Zones module [1] a far better
alternative. It's more flexible, for instance.

So _maybe_ it would be advisable to implement a new similar 'tool'
that would use LCH from the very beginning. The dt's implementation
already uses LCH [2].

[1] https://www.darktable.org/usermanual/images/darkroom/modules/colorzones.jpg
[2] https://github.com/darktable-org/darktable/blob/master/src/iop/colorzones.c

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

[Elle Stone]

On 02/07/2016 06:06 PM, Alexandre Prokoudine wrote:

Personally, I find darktable's Color Zones module [1] a far better
alternative. It's more flexible, for instance.


I don't mean any disrespect to you or to darktable. That looks like a 
nice tool built on top of LCH. Personally I'd probably never use it, but 
that's a matter of editing style.


But in what universe is that tool more flexible than giving users direct 
control of color through LCH and Luminance blend layers, along with the 
ability to modify Lightness, Hue, Chroma directly using an LCH 
Hue-Chroma tool?


How will that tool allow the user to sample LCH colors to see what 
colors are actually in an image? As an aside, a bunch of RGB values is 
singularly unhelpful information unless the user intends to do a bunch 
of RGB to LCH conversions at the command line.


How will that tool allow the user to choose paint pigments colors from a 
LCH pigment color wheel?


Elle

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

[Alexandre Prokoudine]

On Mon, Feb 8, 2016 at 2:19 AM, Elle Stone wrote:

> I don't mean any disrespect to you or to darktable. That looks like a nice
> tool built on top of LCH. Personally I'd probably never use it, but that's a
> matter of editing style.
>
> But in what universe is that tool more flexible than giving users direct
> control of color through LCH and Luminance blend layers, along with the
> ability to modify Lightness, Hue, Chroma directly using an LCH Hue-Chroma
> tool?

I _wasn't even referring_ to LCH-based blend modes in the first place.

In _my_ universe it allows to e.g. selectively change
saturation/lightness for any ranges of hues. I can see how this would
irritate people who want numeric inputs for everything though.

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

[Alexandre Prokoudine]

On Mon, Feb 8, 2016 at 2:40 AM, Elle Stone
 wrote:
> On 02/07/2016 06:28 PM, Alexandre Prokoudine wrote:
>>
>> I_wasn't even referring_  to LCH-based blend modes in the first place.
>
>
> Hmm, my post was about LCH blend modes and how to be truly useful the user
> also needs the Luminance blend mode plus LCH Hue-Chroma plus LCH sliders in
> the "Choose foreground color" tool. So I assumed you were talking about
> replacing some or all of the above with a tool like the darktable tool.
>
> What were you actually referring to?

To quote your email:

"* an LCH Hue-Chroma tool."

Alex
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