"""
.. _ex-morph-volume:

================================
Morph volumetric source estimate
================================

This example demonstrates how to morph an individual subject's
:class:`mne.VolSourceEstimate` to a common reference space. We achieve this using
:class:`mne.SourceMorph`. Data will be morphed based on an affine transformation and
a nonlinear registration method known as Symmetric Diffeomorphic Registration (SDR) by
:footcite:`AvantsEtAl2008`.

Transformation is estimated from the subject's anatomical T1 weighted MRI
(brain) to `FreeSurfer's 'fsaverage' T1 weighted MRI (brain)
<https://surfer.nmr.mgh.harvard.edu/fswiki/FsAverage>`__.

Afterwards the transformation will be applied to the volumetric source estimate. The
result will be plotted, showing the fsaverage T1 weighted anatomical MRI, overlaid with
the morphed volumetric source estimate.
"""
# Author: Tommy Clausner <tommy.clausner@gmail.com>
#
# License: BSD-3-Clause
# Copyright the MNE-Python contributors.

# %%
import nibabel as nib
from nilearn.plotting import plot_glass_brain

import mne
from mne.datasets import fetch_fsaverage, sample
from mne.minimum_norm import apply_inverse, read_inverse_operator

print(__doc__)

# %%
# Setup paths
sample_dir_raw = sample.data_path()
sample_dir = sample_dir_raw / "MEG" / "sample"
subjects_dir = sample_dir_raw / "subjects"

fname_evoked = sample_dir / "sample_audvis-ave.fif"
fname_inv = sample_dir / "sample_audvis-meg-vol-7-meg-inv.fif"

fname_t1_fsaverage = subjects_dir / "fsaverage" / "mri" / "brain.mgz"
fetch_fsaverage(subjects_dir)  # ensure fsaverage src exists
fname_src_fsaverage = subjects_dir / "fsaverage" / "bem" / "fsaverage-vol-5-src.fif"

# %%
# Compute example data. For reference see :ref:`ex-inverse-volume`.
#
# Load data:
evoked = mne.read_evokeds(fname_evoked, condition=0, baseline=(None, 0))
inverse_operator = read_inverse_operator(fname_inv)

# Apply inverse operator
stc = apply_inverse(evoked, inverse_operator, 1.0 / 3.0**2, "dSPM")

# To save time
stc.crop(0.09, 0.09)

# %%
# Get a SourceMorph object for VolSourceEstimate
# ----------------------------------------------
#
# ``subject_from`` can typically be inferred from
# :class:`src <mne.SourceSpaces>`,
# and ``subject_to`` is  set to 'fsaverage' by default. ``subjects_dir`` can be
# None when set in the environment. In that case SourceMorph can be initialized
# taking ``src`` as only argument. See :class:`mne.SourceMorph` for more
# details.
#
# The default parameter setting for *zooms* will cause the reference volumes
# to be resliced before computing the transform. A value of '5' would cause
# the function to reslice to an isotropic voxel size of 5 mm. The higher this
# value the less accurate but faster the computation will be.
#
# The recommended way to use this is to morph to a specific destination source
# space so that different ``subject_from`` morphs will go to the same space.`
# A standard usage for volumetric data reads:

src_fs = mne.read_source_spaces(fname_src_fsaverage)
morph = mne.compute_source_morph(
    inverse_operator["src"],
    subject_from="sample",
    subjects_dir=subjects_dir,
    niter_affine=[10, 10, 5],
    niter_sdr=[10, 10, 5],  # just for speed
    src_to=src_fs,
    verbose=True,
)

# %%
# Apply morph to VolSourceEstimate
# --------------------------------
#
# The morph can be applied to the source estimate data, by giving it as the
# first argument to the :meth:`morph.apply() <mne.SourceMorph.apply>` method.
#
# .. note::
#      Volumetric morphing is much slower than surface morphing because the
#      volume for each time point is individually resampled and SDR morphed.
#      The :meth:`mne.SourceMorph.compute_vol_morph_mat` method can be used
#      to compute an equivalent sparse matrix representation by computing the
#      transformation for each source point individually. This generally takes
#      a few minutes to compute, but can be
#      :meth:`saved <mne.SourceMorph.save>` to disk and be reused. The
#      resulting sparse matrix operation is very fast (about 400× faster) to
#      :meth:`apply <mne.SourceMorph.apply>`. This approach is more efficient
#      when the number of time points to be morphed exceeds the number of
#      source space points, which is generally in the thousands. This can
#      easily occur when morphing many time points and multiple conditions.

stc_fsaverage = morph.apply(stc)

# %%
# Convert morphed VolSourceEstimate into NIfTI
# --------------------------------------------
#
# We can convert our morphed source estimate into a NIfTI volume using
# :meth:`morph.apply(..., output='nifti1') <mne.SourceMorph.apply>`.

# Create mri-resolution volume of results
img_fsaverage = morph.apply(stc, mri_resolution=2, output="nifti1")

# %%
# Plot results
# ------------

# Load fsaverage anatomical image
t1_fsaverage = nib.load(fname_t1_fsaverage)

# Plot glass brain (change to plot_anat to display an overlaid anatomical T1)
display = plot_glass_brain(
    t1_fsaverage, title="subject results to fsaverage", draw_cross=False, annotate=True
)

# Add functional data as overlay
display.add_overlay(img_fsaverage, transparency=0.75)


# %%
# Reading and writing SourceMorph from and to disk
# ------------------------------------------------
#
# An instance of SourceMorph can be saved, by calling
# :meth:`morph.save <mne.SourceMorph.save>`.
#
# This methods allows for specification of a filename under which the ``morph``
# will be save in ".h5" format. If no file extension is provided, "-morph.h5"
# will be appended to the respective defined filename::
#
#     >>> morph.save('my-file-name')
#
# Reading a saved source morph can be achieved by using
# :func:`mne.read_source_morph`::
#
#     >>> morph = mne.read_source_morph('my-file-name-morph.h5')
#
# Once the environment is set up correctly, no information such as
# ``subject_from`` or ``subjects_dir`` must be provided, since it can be
# inferred from the data and used morph to 'fsaverage' by default, e.g.::
#
#     >>> morph.apply(stc)
#
# References
# ----------
# .. footbibliography::
