FIX: Brain lights

[GuillaumeFavelier]

This PR fixes the 3 light system:

PR	master

The patch is just shared to all renderers. Such a function is already implemented on pyvista master so the function update_lighting() will be replaced by enable_3_lights() in the future.

[larsoner]

Perhaps a separate bug, it looks like the lighting is asymmetric:

1. the back/posterior of the left hemisphere and the front/anterior of the right hemisphere are illuminated, and
2. the front/anterior of the left hemisphere and the back/posterior of the right hemisphere are dark

Are the lights set up to illuminate equivalently the left and right? Are they attached to the camera, or to the world?

[GuillaumeFavelier]

Are the lights set up to illuminate equivalently the left and right?

I think we use the _orient_lights() function for that:

mne-python/mne/viz/_brain/_brain.py

Lines 528 to 529 in 2c485d5

	if hemi == 'rh' and hasattr(self._renderer, "_orient_lights"):
	self._renderer._orient_lights()

mne-python/mne/viz/backends/_pyvista.py

Lines 265 to 270 in 2c485d5

	def _orient_lights(self):
	lights = list(self.plotter.renderer.GetLights())
	lights.pop(0) # unused headlight
	lights[0].SetPosition(_to_pos(45.0, -45.0))
	lights[1].SetPosition(_to_pos(-30.0, 60.0))
	lights[2].SetPosition(_to_pos(-30.0, -60.0))

[GuillaumeFavelier]

Are they attached to the camera, or to the world?

Light 0 is a Headlight, Light 1 and Light 2 are CameraLights.

And according to VTK doc:

A Headlight is always located at the camera and is pointed at the camera's focal point. The renderer is free to modify the position and focal point of the camera at any time.

A CameraLight is also attached to the camera, but is not necessarily located at the camera's position. CameraLights are defined in a coordinate space where the camera is located at (0, 0, 1), looking towards (0, 0, 0) at a distance of 1, with up being (0, 1, 0). CameraLight uses the transform matrix to establish this space.

Reference: https://vtk.org/doc/nightly/html/classvtkLight.html#a1e0175706ef066ca837726425bdcc6d7

[larsoner]

Okay here it was on your commit:

And here it is on mine that I just pushed -- the lighting is uniform left/right which I think is important (otherwise you end up with the asymmetry I mention above):

Okay for you @GuillaumeFavelier ?

[GuillaumeFavelier]

Yes, all good 👌

[larsoner]

Thanks @GuillaumeFavelier !