Global illumination V2

[Try]

Starting new issue, for new and better GI, successor to #510

Concept:

• Irradiance cache points, similar to offline rendering

Goals:

• Improve quality (multi bounce)
• Allow scaling to low-end most hardware

Key building blocks

1. Irradiance points for final gather
2. RayQuery (if supported), for tracing rays
3. ??? for world-space irradiance cache

Prototype (https://github.com/Try/OpenGothic/tree/gi-rendering-2)

(only sun is pathtraced, 64 paths per IC)

First bounce (working)

GBuffer driven hole filling on empty-tile.
Iterative hole filling on tiles with healthy cache.

Second bounce (idea)

On primitive storage for cache-points. uvec2(instanceID, primitiveID) form a key to hash table, with multiple records for cache-points.
Spawn of secondary surfels restricted by remaining path energy.

Reference materials:

DreamWorks:

https://www.tabellion.org/et/paper/siggraph_2004_gi_for_films.pdf
https://cgg.mff.cuni.cz/~jaroslav/gicourse2010/giai2010-04-eric_tabellion-slides.pdf
Very game-friendly setup, with min/max surfel radius accounted in interpolation math.

Disney(Cache Points for Many-Lights)

https://cdn.disneyanimation.com/uploads/publications/DIGIPRO2024_cachepoints_paper_final.pdf
https://disneyanimation.com/publications/cache-points-for-production-scale-occlusion-aware-many-lights-sampling-and-volumetric-scattering/
https://media.disneyanimation.com/uploads/production/publication_asset/177/asset/a.pdf
Not today tech for a game, but maybe valid future-looking idea. Maybe some particular building blocks can be of use.

Extended:
https://www.cs.umd.edu/~zwicker/publications/RadianceCachingParticipatingMedia-TOG08.pdf
https://dcgi.fel.cvut.cz/wp-content/wpallimport-dist/publications/pdf/publications-2006-krivanek-egsr-mricp-paper.pdf
https://sgvr.kaist.ac.kr/~sungeui/SGA08/course_note/IC.pdf
https://studios.disneyresearch.com/wp-content/uploads/2019/03/Practical-Hessian-Based-Error-Control-for-Irradiance-Caching-2.pdf
https://cgg.mff.cuni.cz/~jaroslav/papers/2008-irradiance_caching_class/03-greg-ic.pdf
https://people.mpi-inf.mpg.de/alumni/d4/2016/rherzog/Papers/herzogEG09.pdf

Reference materials (rendering):
Micro-rendering:
https://jankautz.com/publications/microrenderingSIGAsia09.pdf

Today probably, no point to bother with surfel-clouds. Instead same concept can be implemented with nanite-like visibility buffer. Every micro-framebuffer is ~24x24 pixels, making any polygon effectively micropoly that can be rasterized with atomicMin

[Try]

added a command-line toggle:
-gi 2
once solution is more stable - will replace old -gi 1 completely, or ideally become part of default setup

[Try]

Some interesting research from Utah, on blue-noise based allocation of VPL (fits surfels just fine):
https://graphics.cs.utah.edu/research/projects/virtual-blue-noise-lighting
https://graphics.cs.utah.edu/research/projects/virtual-blue-noise-lighting/Virtual_Blue_Noise_Lighting-HPG2022-Talk-Notes.pdf