Initial Sky Atmosphere

[AMZN-alexpete]

Initial integration of open source unreal sky atmosphere based on https://github.com/sebh/UnrealEngineSkyAtmosphere into Atom/feature/common

Features

1. Realistic atmospheric scattering
2. Limited shadowing and ground albedo support
3. Fast sky optimization for when inside atmosphere
4. Sun orientation using own transform or optional sun entity component property
5. Primitive sun radius, falloff, and color controls
6. Multiple atmosphere supported

Implementation

The atmosphere shader samples along a ray for each pixel in a full-screen shader. To speed this up, a transmission LUT and sky view LUT are calculated, so when the full pipeline for an atmosphere is active you have the following passes

1. Transmittance LUT writes the transmittance (based on optical depth) into a small LUT 256x64 in a compute shader
2. A 192x108 SkyView LUT is rendered from inside the atmosphere using the Transmittance LUT
3. The ray marching full screen pass runs it branches based on
   1. if the current pixel contains sky only, sample the SkyView LUT and we're done
   2. if the current pixel is in the atmosphere and hits something, perform a variable number of samples along a ray from the camera to the world space position of the pixel using the depth buffer. If shadowing is enabled we'll check the shadow state of points along the ray.

There are other minor bits like taking ground albedo into account and some features in the original implementation haven't been integrated yet like the multi-scattering and fast-aerial perspective, which add additional LUTs. Also, we may be able to disable updating the SkyView LUT every frame and only update it when other LUTs change or the sun position changes.

Known issues

1. Fast aerial perspective not yet integrated
2. Multi-scattering not yet integrated
3. Gnarly banding and poor performance with high samples
4. LUT sizes are hard-coded

Testing

1. PC only in editor and game using various atmosphere and sun configurations
2. Ran AtomSampleViewer tests and verified they passed except the parallax tests which have not reliably pass on my machine.

[Adi-Amazon]

Should this not be in its own Gem?

[AMZN-alexpete]

It can easily be moved to a gem, but I thought that if it would be eventually replacing the existing physical sky implementation then perhaps it makes sense to put in the same place.

[Adi-Amazon]

Is this macro required? it is also declared 3 times and probably should be removed

[AMZN-alexpete]

It was added to avoid shader compilation issues that occurred because all three passes (raymarching, transmittence, skyview) use the same integration function so some needed the o_shadowsEnabled on and some need it off. I can probably think of a way to ensure I only turn that shader variant on for the raymarching shader.

[antonmic]

Awesome work! Sorry for the delayed review

[gadams3]

Unless someone else chimes in with other objections, I'm improving to unblock development of this feature after addressing @antonmic comments.

Personally, even if this is enabled by default with atom, I strongly feel like this should be in a separate gem. It's a large enough change, an isolated feature, and a fantastic example of how to integrate something new into the engine. Keeping it separate will make it easier for others to dissect and identify any obvious gaps that other features will need.

[DoItForGrandpa]

@gadams3 I agree with this should become a gem, mainly because this engine is known for that capability, and thinking of the future, when/if VR support gets added you do have to keep in mind that having the ability to basically turn off features that are not needed to gain performance and space on the finished project is in high demand.
I am all for adding this straight into the engine right now, and at a future date have another PR for switching it over. This would be a good scenario to point people to after the fact to show how you can turn a full feature in O3DE into a gem that I believe injects into the render pipeline.