PR: Implement support for *HyCH* large color difference

[mesvam]

Summary

Preflight

Code Style and Quality

• Unit tests have been implemented and passed.
• Pyright static checking has been run and passed.
• Pre-commit hooks have been run and passed.
• New transformations have been added to the Automatic Colour Conversion Graph.
• New transformations have been exported to the relevant namespaces, e.g. colour, colour.models.

Documentation

• New features are documented along with examples if relevant.
• The documentation is Sphinx and numpydoc compliant.

[mesvam]

I think I'm going to need a bit of help with HyCH since it's a bit more complicated than HyAB, and I don't work in this field so I'm not confident in my knowledge.

So first, I'm not sure what's the best way to implement this. If I'm interpreting the equation in the paper correctly, I think HyCH would duplicate the majority of code in delta_E_CIE2000. Should both delta_E_CIE2000 and delta_E_HyCH both call a helper function then to avoid duplication?

Also, Equation 6 for CD4 is notated using L*C*H* in the paper, but I'm seeing L'C'H' used in other sources and also in this package. I'm assuming when the paper uses ΔC*, they mean ΔC' because of the k_C * S_C denominator, but I'm not confident in this. And also since the paper doesn't provide any examples, I can't find any sample data to compare results to verify my implementation.

[KelSolaar]

Hey @mesvam,

I squashed your commits from the previous branch so you would need to create a new branch from develop, cherry-pick 21823ba, and force-push that branch back to mesvam:feature/HyCH.

I will take a look at the paper but reading what you wrote LCH should really just be Lab converted to LCHab. We have it there colour.models.Lab_to_LCHab BUT it is programmatically generated thus not visible explicitly in the codebase:

colour/colour/models/__init__.py

Line 470 in 363145a

for _Jab, _JCh in COLOURSPACE_MODELS_POLAR_CONVERSIONS:

[KelSolaar]

I will need to add an intermediate definition so that we do not copy-paste the entire CIE 2000 code here.

[KelSolaar]

Hello,

I added a new colour.difference.delta_e.intermediate_attributes_CIE2000 definition that should have all the required attributes and avoid code duplication.

Let us know how it goes!

Cheers,

Thomas

[KelSolaar]

Thanks a lot, I will merge in a staging branch and doctor the history a bit.

[mesvam]

Also, Equation 6 for CD4 is notated using LCH* in the paper, but I'm seeing L'C'H' used in other sources and also in this package. I'm assuming when the paper uses ΔC*, they mean ΔC' because of the k_C * S_C denominator, but I'm not confident in this. And also since the paper doesn't provide any examples, I can't find any sample data to compare results to verify my implementation.

So I contacted the authors, who kindly provided some test data to compare. I verified that this implementation matched their results to within numerical precision, with the exception of an optional hue rotation term.

You can cherry-pick this mesvam@2730318 if you want to add it to the test suite