Deformable Spine-to-Spine Registration
======================================

Summary
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Deformably aligns a moving ``SpineData`` to a fixed ``SpineData`` by registering their meshes. Use this to align a patient to a template or perform longitudinal alignment; the algorithm transforms associated features (keypoints, splines, point clouds, other meshes) and can transfer face labels to the fixed mesh.

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

Compatible data
----------------
- Requires two :doc:`SpineData <../SpineData>` inputs: one fixed and one moving.

Input
-----
- Fixed :doc:`SpineData <../SpineData>`, Moving :doc:`SpineData <../SpineData>`.
- Must have sufficient overlap in anatomical structures between fixed and moving.

Output
------
- Registered moving `SpineData`. Only structures present in both inputs are included.
- Optionally, moving SpineData content transferred to the fixed SpineData.

Key parameters
--------------
- `Pre-Alignment using keypoints` (bool): Use keypoints to initialize alignment.
- `Perform registration` (bool): Enable the deformable step; turn off to transfer labels only.
- `Transfer to fixed mesh` (bool): Transfer face labels/features from moving to fixed mesh.
- Demons smoothing: `Half Kernel Size` (double).
- Volume-based mesh registration: `Distance field cut-off` (double), `resolution` (double).

Workflow
--------
1) Select fixed and moving :doc:`SpineData <../SpineData>`.
2) (Optional) Enable keypoint pre-alignment.
3) Run registration; verify alignment of vertebrae and sacrum/ilium.
4) (Optional) Enable transfer-to-fixed to propagate labels/features.