Intent

itk::bridge holds convenience wrappers over third-party numerical solvers, currently Eigen. They exist to ease the initial migration of code away from the deprecated VNL algorithms: a call site that used vnl_svd can be moved to itk::bridge::Math::SVD as a mechanical, low-risk edit, without the caller first learning Eigen's API.

Not part of the core ITK mission

These are a migration aid, not a numerics library. ITK does not undertake to maintain, validate, optimize, or preserve the API or ABI of anything in itk::bridge to the standard it applies to the rest of the toolkit. Specifically:

The interfaces may change or be removed between releases without the deprecation cycle that core ITK API changes receive.
The numerical behavior is that of the underlying backend. ITK does not independently validate accuracy, conditioning, or convergence, and does not guarantee bit-for-bit stability across backend versions.
Coverage is limited to what ITK's own migration needed. Absent functionality will not necessarily be added.

Guidance for downstream users

Prefer calling Eigen directly. Downstream projects should depend on Eigen (or another numerical library of their choosing) rather than on the API or ABI of these convenience wrappers. Using itk::bridge as a transitional step while retiring VNL is reasonable; treating it as a permanent dependency is not.

Code that needs a long-lived, supported numerical interface should call the backend directly, where the API contract, versioning policy, and documentation are those of the numerical library itself.

See also: itk::bridge::Math::SVD; itk::bridge::SymmetricEigenDecomposition