Nilearn: Add "linewidths" arg for nilearn.plotting.plot_surf_contours

[alpinho]

Similarly to other functions in Nilearn, the function plot_surf_contours should have a linewidths argument that changes the boundary thickness of the contours.

[jeromedockes]

@mjboos WDYT?

[mjboos]

That's a nice idea, but it's currently not possible. Background is that matplotlib did not allow for explicitly setting zorder in Axes3D objects (although that seems to be fixed in the meantime) and therefore we could not actually draw the contours as lines and instead colored the edges on the outside of the region. The only way to make this line thicker would be to include more edges - in principle possible but IMO even more hacky and linewidth would then correspond to the integer number of edges to use.

TL;DR: I think it only makes sense to tackle this if functionality of surface contours is changed from a coloring of edges in Poly3DCollection (current behavior) to be actually drawn as a Line3DCollection - which might now be possible using matplotlib 3.4 (but wouldnt work for all earlier matplotlib versions).

[jeromedockes]

Thanks for this explanation! coloring 2 or more rows of faces might still be useful in cases where the resolution of the mesh is so big that the lines become hard to see, but matplotlib would have a hard time plotting these anyway. I agree it's probably not worth investing time in very hacky solutions that would result in a clumsy interface, and wait for plotly plots in nilearn or for the z order of Line3DCollection to become available in matplotlib to add this feature.

curious to see what other people think, though

[jeromedockes]

ok it was a good idea but closing for now; hopefully if we add plotly surface plots this kind of thing will be easier to do! (#2793)