denis-bz/0-HUSL-lch-is-not-CIELCh.md

## 0-HUSL-lch-is-not-CIELCh.md

      
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              0-HUSL-lch-is-not-CIELCh.md
            
          
    HUSL rgb_to_lch is for CIELuv, not CIELab

Keywords: color-space HUSL HSLuv CIELCh CIELuv CIELab max-chroma everyone-knows
_{Whenever something can be done in two ways, someone will be confused.}

There are two quite different color spaces: 

CIELab aka L*a*b*
is widely used, and fairly uniform 

CIELuv
is used in
HSLuv aka HUSL,
"a human-friendly alternative to HSL". 

These have equivalent versions in polar coordinates,
CIELCh <--> CIELab and CIELCh_uv <--> CIELuv: 

C = radius = sqrt( a² + b² ) or sqrt( u² + u² ) 

hue = angle 0 to 360 degrees = atan2( b / a ) or atan2( v / u ).
CIELCh_uv is not CIELCh

The function rgb_to_lch( rgb ) in
seaborn husl.py
or
nphusl
calculates the CIELCh_uv based on CIELuv,
NOT the CIELCh based on CIELab.
Notes

Does it matter -- for what purpose ? See the excellent
Seaborn Choosing color palettes,
in particular the plot of Luminance and Saturation of 6 standard palettes.
MacEvoy's "modern color models" on
handprint
says

CIELUV has been discarded as the basis for future work".

colorspacious
has a nice graph of color spaces, CIELab <--> XYZ100 <--> sRGB1-linear ...
(Beware: cspace_convert( x, anyspace, "sRGB1" )
can land waaay outside RGB 0 .. 1 -- it's up to you to check.)
colorio

The sRGB gamut is a perfect cube in sRGB space,
and takes curious shapes when translated into other color spaces.
The above images show the sRGB gamut in different color spaces.

Finding CIELuv max_chroma( L, hue ) is fast in nphusl,
but finding CIELab max_chroma seems to be not so easy, see
https://github.com/endolith/complex_colormap .
(What's the "boundary" or "edge" of a discrete point cloud,
e.g. 2^24 RGB points in an oddly-shaped color space ?)
A simple test, corners of the RGB cobe -> Luv and Lab

rgb: (8, 3) 
[[  0   0   0   0 255 255 255 255]
 [  0   0 255 255   0   0 255 255]
 [  0 255   0 255   0 255   0 255]] 

{ nphusl__rgb_to_lch  max 179 at [1 0 0] 
[[  0  32  88  91  53  60  97 100]
 [  0 130 135  72 179 137 107   0]
 [  0 266 128 192  12 308  86  20]]
}

{ cspace_rgb_LCh  max 133 at [0 0 1] 
[[  0  32  88  91  53  60  97 100]
 [  0 133 119  50 104 115  96   0]
 [  0 306 136 196  40 328 103 302]]
}

{ cspace_rgb_JCh  max 111 at [1 0 0] 
[[  0  21  80  84  47  54  96 100]
 [  0  90 106  59 111  97  80   3]
 [  0 258 137 198  32 330 106 211]]
}

cheers 

-- denis-bz-py@t-online.de  25 March 2022