Refine registration

This script runs with ``python run_system.py [config] --refine``. In ``[config]``,
``["path_dataset"]`` should have subfolders ``fragments`` which stores fragments
in ``.ply`` files and a pose graph in a ``.json`` file.

The main function runs ``local_refinement`` and ``optimize_posegraph_for_scene``.
The first function performs pairwise registration on the pairs detected by
:ref:`reconstruction_system_register_fragments`. The second function performs
multiway registration.

Fine-grained registration

.. literalinclude:: ../../../examples/python/reconstruction_system/refine_registration.py
   :language: python
   :linenos:
   :pyobject: update_posegraph_for_scene
   :lineno-match:

This script uses the technique demonstrated in :ref:`/tutorial/pipelines/multiway_registration.ipynb`. Function ``update_posegraph_for_scene`` builds a pose graph for multiway registration of all fragments. Each graph node represents a fragment and its pose which transforms the geometry to the global space.

Once a pose graph is built, function ``optimize_posegraph_for_scene`` is called
for multiway registration.

.. literalinclude:: ../../../examples/python/reconstruction_system/optimize_posegraph.py
   :language: python
   :linenos:
   :pyobject: optimize_posegraph_for_scene
   :lineno-match:

Main registration loop

The pose graph optimization outputs the following messages:

.. code-block:: sh

    [GlobalOptimizationLM] Optimizing PoseGraph having 14 nodes and 35 edges.
    Line process weight : 789.730200
    [Initial     ] residual : 1.208286e+04, lambda : 1.706306e+01
    [Iteration 00] residual : 2.410383e+03, valid edges : 22, time : 0.000 sec.
    [Iteration 01] residual : 8.127856e+01, valid edges : 22, time : 0.000 sec.
    [Iteration 02] residual : 8.031355e+01, valid edges : 22, time : 0.000 sec.
    Delta.norm() < 1.000000e-06 * (x.norm() + 1.000000e-06)
    [GlobalOptimizationLM] total time : 0.001 sec.
    [GlobalOptimizationLM] Optimizing PoseGraph having 14 nodes and 35 edges.
    Line process weight : 789.730200
    [Initial     ] residual : 8.031355e+01, lambda : 1.716826e+01
    Delta.norm() < 1.000000e-06 * (x.norm() + 1.000000e-06)
    [GlobalOptimizationLM] total time : 0.000 sec.
    CompensateReferencePoseGraphNode : reference : 0

There are 14 fragments and 52 valid matching pairs between fragments. After 23
iterations, 11 edges are detected to be false positives. After they are pruned,
pose graph optimization runs again to achieve tight alignment.