3D map plotting

[dstansby]

This is a very very rough draft of code to enable 3D plots to be made with Map. This works by:

1. Calculating the coordinates of all the corners of the pixels in a map
2. Converting these to HeliocentricInertial coordinates (an arbitrary, but fixed choice of coordinate frame).
3. Using the x, y, z coordinate values in these cooridnates to define the corners of mesh quadrilaterals
4. Creating a pyvista (a python interface to VTK) mesh from these mesh coordinates
5. Plotting in pyvista

This is super useful when combining visualisation of multiple objects (e.g. adding magnetic field lines, or adding a LASCO map and an AIA map as in #3997 (comment))

If you checkout this branch and run test.py hopefully you should get something like:

All feedback welcome!

[pep8speaks]

Hello @dstansby! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2020-12-20 10:53:15 UTC

[Cadair]

Maybe this would be better as a submodule in sunpy.visualization?

[hayesla]

This is super cool 🌞

[dstansby]

😢 readthedocs doesn't support a headless display, so I don't think pyvista examples will run on RTD (readthedocs/readthedocs.org#1951 (comment))

[nabobalis]

😢 readthedocs doesn't support a headless display, so I don't think pyvista examples will run on RTD (readthedocs/readthedocs.org#1951 (comment))

Given this, what about skipping the example just so people know it's possible?
We can leave a note saying why its static.

[dstansby]

Yep, that's what I was planning. Could also possibly embed a .gif or a .png with the output run locally.

[Cadair]

It is technically possible to compile VTK in headless mode so it doesn't need an X server...

[Cadair]

Another (easier idea) is maybe to spin this up in the conf.py: https://pypi.org/project/xvfbwrapper/

[Cadair]

It is technically possible to compile VTK in headless mode so it doesn't need an X server...

There is supposed to be a conda-forge vtk variant which is compiled with osmesa (off screen mesa GL), but it seems to have been disabled for the newest builds (despite me seemingly being able to use it with a rc of vtk 9.0) so it's possible there is a decent solution to this not that far away.

[dstansby]

Another (easier idea) is maybe to spin this up in the conf.py: https://pypi.org/project/xvfbwrapper/

Hmm, I gave this a go, but VTK doesn't seem to be detecting/using the xvfb wrapper and is just spinning up it's own window for me locally.

[dstansby]

I'm wondering about spinning this off into a separate package, so I can more rapidly develop and test it without putting it all in what would be a mega PR. I think there's enough things different (needing pyvista/vtk installed, and really need a way to do image comparison tests with pyvista, which is possible, but I think worth trialing elsewhere first) to warrant this.