Log k

[riclarsson]

I was having a look at porting some of the scattering code (monte carlo) to ARTS3, just to get a feel of the amount of work. It relies on being able to take the logarithm of the transmission - which is easy when you consider purely unpolarized light but gets difficult elsewise. So I add here a rtepack::logK function, which turns a Mueller matrix into a Propagation matrix.

The method is documented in the code. A test is added to catch if it breaks.

While it is of course possible to call this method with any 4x4 matrix, it is not generic but expects to be able to unwind the exponential mechanisms completely. Quick tests shows this method generally performs better than scipy.linalg.logm

[Copilot]

Pull request overview

This PR introduces the rtepack::logK function to compute the logarithm of a Mueller matrix and convert it back to a Propagation matrix. This functionality is needed for porting Monte Carlo scattering code to ARTS3, where computing the logarithm of transmission matrices is essential for polarized light handling.

Key changes:

• Implements rtepack::logK function using Cayley-Hamilton theorem to invert the matrix exponential
• Refactors tran::operator() for improved maintainability by extracting common subexpressions
• Adds utility functions det, tr, and midtr for Mueller matrices
• Includes comprehensive Python test suite with multiple test cases

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
src/core/rtepack/rtepack_transmission.cc	Implements the logK function and refactors tran::operator() for better readability
src/core/rtepack/rtepack_transmission.h	Declares the new logK function signature
src/core/rtepack/rtepack_mueller_matrix.h	Adds det, tr, and midtr utility functions; refactors inv to use det
src/python_interface/py_rtepack.cpp	Exposes logK function and tran class to Python with documentation
tests/python/math/logK.py	Adds test suite covering various propagation matrix configurations

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