The name

croak stands for Complete Retrieval by Optimisation and Adjoint Kernels.

Complete refers to what is retrieved and what is modelled: the full complex field, amplitude and phase, through a forward model that includes the measurement physics (propagation in the nonlinear medium, beam geometry and collection) rather than a thin-medium idealisation of it. Adjoint kernels names the machinery: each interaction (SHG, SD, PG) contributes a kernel to the forward map, and each carries a matching hand-derived Wirtinger adjoint that yields exact gradients for the cost of one extra forward pass.

Why it used to be called toad

For as long as it was an internal tool the package was called toad — Trace Optimisation through Automatic Differentiation. That acronym described what was then the headline feature: the forward model is differentiable end to end, so a JAX twin of every solver can be built and used to check the analytic gradients against automatic differentiation.

The name could not be kept for the public release, because toad was already taken on the Python Package Index. Rather than decorate it — toad-frog, pytoad, and similar — the package was renamed outright.

Automatic differentiation is, if anything, more central now than when it named the package: geometric smearing, the focal and collection models, the fitted medium thickness and delay-zero, the B-spline phase basis and the Laplace covariance all depend on it. What changed is that it no longer covers the whole package, which grew a hand-derived Wirtinger adjoint for the core model, a complete measurement-to-result workflow and uncertainty quantification. Adjoint kernels covers both routes: each interaction contributes a kernel to the forward map, and its derivative is available either in closed form or through AD.

The mascot

The mascot was not renamed. The wizard’s icon is still a toad, which — given what toads are known for — remains approximately correct.

Note

Nothing in the public API ever carried the old name, so there is no compatibility shim to be aware of: the package is imported as croak, the console scripts are croak and croak-gui, and saved result files record a croak_version header. Files written by the pre-release toad package are not read by this one.