Spine Poly-rigid Deformation
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Summary
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Initializes a poly-rigid deformation on a CT volume using vertebra-defined rigid regions from a spine annotation (source). Use this to model articulated spine motion for registration/simulation or to pre-initialize deformation from anatomical structures.

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

Compatible data
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- Requires: ``SharedImageSet`` (volume), ``AnatomicalStructureCollection`` (source :doc:`SpineData <../SpineData>`), optional ``AnatomicalStructureCollection`` (destination :doc:`SpineData <../SpineData>`).
- Implicit assumptions: source spine defines vertebra regions consistently; missing vertebrae lead to missing rigid sections.

Output
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- A configured ``PolyRigidDeformation`` with control points/rigid sections, and distance volume(s).

Key parameters
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- ``Chamfer Distance`` (bool): Use chamfer distances when computing distance volumes.
- ``Sphere Mode`` (bool): Sphere approximation for inverse displacement (fast, approximate).
- ``Mode`` (enum): Forward/backward mode for ``GlPolyRigidDeformation``. Forward is recommended for performance.
- ``Inversion Steps`` (int): Iterations for inverse displacement.

Workflow
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1) Provide the volume and the source spine collection (and destination if available).
2) Run the algorithm; it computes distance volumes and initializes rigid sections per vertebra.
3) Adjust mode/inversion steps as needed.