LCH hue doesn't round-trip

[marcinwrochna]

Hi! I'm trying to extract the LCH hue from a color, but the values I get are off. Am I missing something?

const hue = 200.0;
const newHue = ColorTranslator.toLCHObject(`lch(50% 50% ${hue})`).H;
const degHue = ColorTranslator.toLCHObject(`lch(50% 50% ${hue}deg)`, { anglesUnit: "deg" }).H;
console.log(hue, newHue, degHue);

const newHue = ColorTranslator.toLCHObject(`lch(50% 100% ${hue})`).H;
const degHue = ColorTranslator.toLCHObject(`lch(50% 100% ${hue}deg)`, { anglesUnit: "deg" }).H;
console.log(hue, newHue, degHue);

colortranslator v5.0.0 prints 200 210.769587 210.769587 and 200 235.818193 235.818193.

[elchininet]

Hi @marcinwrochna,
I need to add that to the documentation. What is happening is that LCH doesn't support an exact round trip. The result will be similar to the eye but not exact. This is the trip that conversion follows LCH -> RGB -> CIELab -> LCH, and as there are several steps, the round trip is not exact. You can try with Atmos to notice a similar behaviour. If you find a paper or a code snippet that does this conversion more accurately, I'll be happy to test it and replace the current conversion if it does a better job.

[marcinwrochna]

Ok, so the difference is because (as far as I understand) all operations crop to the sRGB gamut. Once I use a color in the sRGB gamut it does round-trip (up to numerical precision).

Fixing that would require avoiding RGB in such transformations altogether, which I guess is unrealistic. And it probably doesn't change the displayed color at all in most settings. So I'm only leaving this open in case you want to update the docs.

[elchininet]

Yes, basically because the CIELab conversion is based in fixed RYB/XYZ matrices (specifically the sRGB one) and passing through this conversion forward and back doesn't keep the exact values.

So I'm only leaving this open in case you want to update the docs.

Yes, but not only the docs. One thing that could be done without changing the conversion calculations is to avoid making the conversion process if a round trip is detected.

[elchininet]

Hi @marcinwrochna,
Try with the latest version of the library. In this version, when a round trip is performed, the input color is parsed but not converted, just formatted to the desired final output.

With your example code, this is the current return:

200 200 200
200 200 200