Procedural atmospheric scattering

[ecoskey]

Implement procedural atmospheric scattering from Sebastien Hillaire's 2020 paper. This approach should scale well even down to mobile hardware, and is physically accurate.

Co-author: @mate-h

He helped massively with getting this over the finish line, ensuring everything was physically correct, correcting several places where I had misunderstood or misapplied the paper, and improving the performance in several places as well. Thanks!

Credits

@aevyrie: helped find numerous bugs and improve the example to best show off this feature :)

Built off of @mtsr's original branch, which handled the transmittance lut (arguably the most important part)

Showcase:

For followup

• Integrate with pcwalton's volumetrics code
• refactor/reorganize for better integration with other effects
• have atmosphere transmittance affect directional lights
• add support for generating skybox/environment map

[jnhyatt]

I think there may be an issue with units? I ran the atmosphere example and set the camera transform to Transform::from_xyz(0.0, 10.0, 0.0).looking_to(Dir3::X, Dir3::Y) and here's what I'm seeing:

I assume the altitude is derived from the camera's height? Leaving aside the issue that this won't work for games set in space, 10 meters elevation shouldn't look like this. I'm still reading through the paper and coming to grips with the code, but I'm wondering if for example y=1 is corresponding to "edge of the atmosphere" maybe?
I just found the scene_units_to_m parameter. That explains everything!

[IceSentry]

I really dislike how we have that system in a bunch of different places. Don't need to do anything in this PR but I think there's something a bit weird with the fact that we need it in so many places.

[IceSentry]

I can't really comment on the math, but the results look good and the implementation itself is fine.

I added a few minor comments but none of them are blockers.

[ecoskey]

bevy_example_runner results: https://pixel-eagle.com/project/B25A040A-A980-4602-B90C-D480AB84076D?filter=PR-16314

Nothing seems super out of place

[pcwalton]

Unassigning myself as this has enough reviews already.

[alice-i-cecile]

Agreed, merging :) Thanks!