viz.plot_alignment() doesn't seem to respect user-specified alpha values

[hoechenberger]

MWE:

import os.path as op
import mne

data_path = mne.datasets.sample.data_path()
subjects_dir = op.join(data_path, 'subjects')
raw_fname = op.join(data_path, 'MEG', 'sample', 'sample_audvis_raw.fif')
trans_fname = op.join(data_path, 'MEG', 'sample',
                      'sample_audvis_raw-trans.fif')
raw = mne.io.read_raw_fif(raw_fname)
trans = mne.read_trans(trans_fname)

kwargs = dict(info=raw.info, trans=trans, subject='sample',
              subjects_dir=subjects_dir)

fig = mne.viz.plot_alignment(surfaces={'head-dense': 0.1},
                             **kwargs)

fig = mne.viz.plot_alignment(surfaces={'head-dense': 0.9},
                             **kwargs)

On my and @SophieHerbst's systems, both figures look the same: translucent on Linux, and opaque on macOS. We're using MNE-Python main.

cc @GuillaumeFavelier

Platform:      macOS-11.4-x86_64-i386-64bit
Python:        3.9.4 | packaged by conda-forge | (default, May 10 2021, 22:13:15)  [Clang 11.1.0 ]
Executable:    /Users/hoechenberger/miniforge3/envs/mne/bin/python
CPU:           i386: 8 cores
Memory:        16.0 GB

mne:           0.24.dev0
numpy:         1.20.3 {blas=NO_ATLAS_INFO, lapack=lapack}
scipy:         1.6.3
matplotlib:    3.4.2 {backend=module://ipykernel.pylab.backend_inline}

sklearn:       0.24.2
numba:         Not found
nibabel:       3.2.1
nilearn:       0.8.0
dipy:          1.4.1
cupy:          Not found
pandas:        1.2.4
mayavi:        4.7.2
pyvista:       0.31.1 {pyvistaqt=0.5.0, OpenGL 4.1 INTEL-16.4.5 via Intel(R) Iris(TM) Plus Graphics 655}
vtk:           9.0.1
PyQt5:         5.12.3

[GuillaumeFavelier]

I observed the same with @agramfort a moment ago. And surprisingly, it works when just 'head' is used.

I'll take a look and see what I can do 👍