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

[Elle Stone <ellestone ninedegreesbelow com>]

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 Blue    301  [LCh]
sRGB Blue    273  [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