# Objective
- Fix the atmosphere LUT parameterization in the aerial -view and
sky-view LUTs
- Correct the light accumulation according to a ray-marched reference
- Avoid negative values of the sun disk illuminance when the sun disk is
below the horizon
## Solution
- Adding a Newton's method iteration to `fast_sqrt` function
- Switched to using `fast_acos_4` for better precision of the sun angle
towards the horizon (view mu angle = 0)
- Simplified the function for mapping to and from the Sky View UV
coordinates by removing an if statement and correctly apply the method
proposed by the [Hillarie
paper](https://sebh.github.io/publications/egsr2020.pdf) detailed in
section 5.3 and 5.4.
- Replaced the `ray_dir_ws.y` term with a shadow factor in the
`sample_sun_illuminance` function that correctly approximates the sun
disk occluded by the earth from any view point
## Testing
- Ran the atmosphere and SSAO examples to make sure the shaders still
compile and run as expected.
---
## Showcase
<img width="1151" alt="showcase-img"
src="https://github.com/user-attachments/assets/de875533-42bd-41f9-9fd0-d7cc57d6e51c"
/>
---------
Co-authored-by: Emerson Coskey <emerson@coskey.dev>
Implement procedural atmospheric scattering from [Sebastien Hillaire's
2020 paper](https://sebh.github.io/publications/egsr2020.pdf). 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
---------
Co-authored-by: Emerson Coskey <56370779+EmersonCoskey@users.noreply.github.com>
Co-authored-by: atlv <email@atlasdostal.com>
Co-authored-by: JMS55 <47158642+JMS55@users.noreply.github.com>
Co-authored-by: Emerson Coskey <coskey@emerlabs.net>
Co-authored-by: Máté Homolya <mate.homolya@gmail.com>
