Improve quality of rings rendering

[10110111]

Description

Current code that renders planetary rings has some issues:

• Outermost layer has a gap, as noted in Outer ring of Uranus has a gap #2775, and it becomes obvious even on Saturn if we reduce number of "stacks" in the call to sRing() to 3 or 2;
• Rings of Saturn are obviously polygonal when zoomed in to screen size (see left and right sides of the screenshot below);
• Additionally, ring shadow is not quite smooth: sort of aliased.

This PR redoes the rings in the following way:

• Instead of making a polygonal model of the rings, the whole model becomes just a quad, and texture coordinates are computed in the fragment shader to yield an annulus instead of the square.
• Mip mapping is enabled for ring texture.
• Shadow is made behave correctly with mip mapping, finally becoming smooth.

Screenshots (if appropriate):

Old Saturn (viewed from Jupiter):

New Saturn (viewed from Jupiter)

Old Uranus

New Uranus

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• This change requires a documentation update

How Has This Been Tested?

Test Configuration:

• Operating system: Ubuntu 20.04
• Graphics Card: Intel UHD Graphics 620

Checklist:

• My code follows the code style of this project.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (header file)
• I have updated the respective chapter in the Stellarium User Guide
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[github-actions]

Great PR! Please pay attention to the following items before merging:

Files matching src/**/*.cpp:

• Are possibly unused includes removed?

This is an automatically generated QA checklist based on modified files

[gzotti]

Not yet, please! Observe rings from Pan...

or from Saturn:

It seems in perspective projection the rings vanish altogether. Other projections also look weird.

See #435, hopefully you can find a solution that also fixes that?

[10110111]

But this is already much better than what we have in current master:

This PR:

Master:

[gzotti]

Yes, the problem around #435 is that Pan is inside the rings, so the tessellation in all but Perspective projection makes an ugly mess. Perspective is somewhat bearable. It seems that in your solution the observer is now not surrounded by rings.
And look into the strange break in the rings also when observed from Saturn's surface. This works well in master.

EDIT: You see, actually Saturn's shadow on the ring should be below the horizon.

[gzotti]

@10110111 , do you have a working solution at least for this view from Saturn? Else we should push it to 23.1 to not block 1.2.

[10110111]

No, let this go to a next release.

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[github-actions]

Conflicts have been resolved. A maintainer will review the pull request shortly.

[10110111]

So, I have undone the simplification of the ring mesh to a square, and fixed #2775. Only roundedness of the rings (as seen from faraway observers) remains to be the topic of this PR, which shouldn't break the look from inside the rings more than it already is.

[alex-w]

So, I have undone the simplification of the ring mesh to a square, and fixed #2775. Only roundedness of the rings (as seen from faraway observers) remains to be the topic of this PR, which shouldn't break the look from inside the rings more than it already is.

I accept it and approve the PR

[gzotti]

Very good now, thank you!
I also consider #435 solved by this solving of the indexing. The ring is a bit coarse, but no mess.

[github-actions]

Hello @10110111!

The fix has been merged into source code and you may test it via building Stellarium from source code or wait the weekly development snapshot...

[github-actions]

Hello @10110111!

Please check the fresh version (development snapshot) of Stellarium:
https://github.com/Stellarium/stellarium-data/releases/tag/weekly-snapshot

[github-actions]

Hello @10110111!

Please check the latest stable version of Stellarium:
https://github.com/Stellarium/stellarium/releases/latest