Poly Rigid Registration
=======================

Summary
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Registers a moving image to a fixed image using vertebra-level associations and poly-rigid deformation. Use this for pairwise alignment of two CT (or compatible) images at the spine level; the algorithm can compute or reuse `SpineData` and optimize alignment along spine splines.

See also: :doc:`SpineData <../SpineData>`.

Compatible data
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- Requires: `SharedImageSet` fixed and moving; optionally :doc:`SpineData <../SpineData>` for each.
- Implicit assumptions: consistent geometry/coordinate systems between images; vertebra associations are meaningful for the chosen mode.

Output
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- Registered moving image; updated `SpineData` for fixed/moving as needed.
- Deformation state accessible via the controller/UI.

Key parameters
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- `Association Mode` (enum): Automatic / Classification / SplineBasedRigid / PolyRigid.
- `Current Offset` (0..1): Relative offset along splines for alignment.
- `Num Samples` (int): Samples for similarity optimization.

Workflow
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1) Provide fixed and moving images (optionally spines).
2) Choose an association mode; automatic works in most cases.
3) Generate :doc:`SpineData <../SpineData>` if absent.
4) Optimize similarity; inspect the plot and select the best offset.

Controller / UI
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- The registration controller provides a plot view for similarity across offsets and exposes alignment parameters.

Tips
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- If association seems wrong, switch mode or correct spines before optimizing.
- Ensure both images use consistent geometry for best results.