Support HCL colors

[ttusar]

Is there any plan to support HCL color spaces? They are perceptually uniform and can be used to create color maps like these:

(from https://hclwizard.org/)

Ideally, a color map (such as Viridis) would first be initialized and then used it in a table to color the background of the cells according to their values.

[u-fischer]

for PDF the colors must at the end be converted into rgb or cmyk. So if you want that, please provide a suitable converter function (with a reliable source as a reference). See https://github.com/latex3/xcolor/issues/25 for an example.

[ttusar]

According to https://www.easyrgb.com/en/math.php, it seems that a CIE-L*uv color can be converted to RGB via CIE XYZ as follows:

*CIE-Luv → XYZ**

//Reference-X, Y and Z refer to specific illuminants and observers.
//Common reference values are available below in this same page.

var_Y = ( CIE-L* + 16 ) /116
if ( var_Y^3  > 0.008856 ) var_Y = var_Y^3
else                       var_Y = ( var_Y - 16 / 116 ) / 7.787

ref_U = ( 4 * Reference-X ) / ( Reference-X + ( 15 * Reference-Y ) + ( 3 * Reference-Z ) )
ref_V = ( 9 * Reference-Y ) / ( Reference-X + ( 15 * Reference-Y ) + ( 3 * Reference-Z ) )

var_U = CIE-u* / ( 13 * CIE-L* ) + ref_U
var_V = CIE-v* / ( 13 * CIE-L* ) + ref_V

Y = var_Y * 100
X =  - ( 9 * Y * var_U ) / ( ( var_U - 4 ) * var_V - var_U * var_V )
Z = ( 9 * Y - ( 15 * var_V * Y ) - ( var_V * X ) ) / ( 3 * var_V )

XYZ → Standard-RGB

//X, Y and Z input refer to a D65/2° standard illuminant.
//sR, sG and sB (standard RGB) output range = 0 ÷ 255

var_X = X / 100
var_Y = Y / 100
var_Z = Z / 100

var_R = var_X *  3.2406 + var_Y * -1.5372 + var_Z * -0.4986
var_G = var_X * -0.9689 + var_Y *  1.8758 + var_Z *  0.0415
var_B = var_X *  0.0557 + var_Y * -0.2040 + var_Z *  1.0570

if ( var_R > 0.0031308 ) var_R = 1.055 * ( var_R ^ ( 1 / 2.4 ) ) - 0.055
else                     var_R = 12.92 * var_R
if ( var_G > 0.0031308 ) var_G = 1.055 * ( var_G ^ ( 1 / 2.4 ) ) - 0.055
else                     var_G = 12.92 * var_G
if ( var_B > 0.0031308 ) var_B = 1.055 * ( var_B ^ ( 1 / 2.4 ) ) - 0.055
else                     var_B = 12.92 * var_B

sR = var_R * 255
sG = var_G * 255
sB = var_B * 255

XYZ (Tristimulus) Reference values of a perfect reflecting diffuser	Illuminant [Observer]	X2 [2° (CIE 1931)]	Y2 [2° (CIE 1931)]	Z2 [2° (CIE 1931)]	X10 [10° (CIE 1964)]	Y10 [10° (CIE 1964)]	Z10 [10° (CIE 1964)]	Note
A	109.850	100.000	35.585	111.144	100.000	35.200	Incandescent/tungsten
B	99.0927	100.000	85.313	99.178;	100.000	84.3493	Old direct sunlight at noon
C	98.074	100.000	118.232	97.285	100.000	116.145	Old daylight
D50	96.422	100.000	82.521	96.720	100.000	81.427	ICC profile PCS
D55	95.682	100.000	92.149	95.799	100.000	90.926	Mid-morning daylight
D65	95.047	100.000	108.883	94.811	100.000	107.304	Daylight, sRGB, Adobe-RGB
D75	94.972	100.000	122.638	94.416	100.000	120.641	North sky daylight
E	100.000	100.000	100.000	100.000	100.000	100.000	Equal energy
F1	92.834	100.000	103.665	94.791	100.000	103.191	Daylight Fluorescent
F2	99.187	100.000	67.395	103.280	100.000	69.026	Cool fluorescent
F3	103.754	100.000	49.861	108.968	100.000	51.965	White Fluorescent
F4	109.147	100.000	38.813	114.961	100.000	40.963	Warm White Fluorescent
F5	90.872	100.000	98.723	93.369	100.000	98.636	Daylight Fluorescent
F6	97.309	100.000	60.191	102.148	100.000	62.074	Lite White Fluorescent
F7	95.044	100.000	108.755	95.792	100.000	107.687	Daylight fluorescent, D65 simulator
F8	96.413	100.000	82.333	97.115	100.000	81.135	Sylvania F40, D50 simulator
F9	100.365	100.000	67.868	102.116	100.000	67.826	Cool White Fluorescent
F10	96.174	100.000	81.712	99.001	100.000	83.134	Ultralume 50, Philips TL85
F11	100.966	100.000	64.370	103.866	100.000	65.627	Ultralume 40, Philips TL84
F12	108.046	100.000	39.228	111.428	100.000	40.353	Ultralume 30, Philips TL83

[u-fischer]

Sorry but I don't have the time to write a converter function. If you want it, please implement it and make a suitable pull request or at least a small exampl.e

[ttusar]

I'm afraid writing a function similar to the example from #25 is beyond what I am able to do.

[u-fischer]

Then I will close this issue for now. It can be reopened if someone offers some code.

[zauguin]

@ttusar If you are using LuaTeX then you can use https://github.com/zauguin/luaicc for such maps.

[ttusar]

I will have a look, thanks @zauguin for the pointer

[ttusar]

To anyone else stumbling over this issue, there is now an example on Overleaf that builds on the code provided by @zauguin. Hopefully it will be of help to someone.