Dynamic Lightmap GI

[mrjustaguy]

I've been looking at Lightmap GI/Baked Lightmap and it gave me an interesting idea that might be worth exploring.

Currently, this method is baking the end result color, forcing lights that are contributing to be static, and this principle is really old and simple, and performant..
However what if you didn't store the Light in the Lightmap? What if instead, you stored references...?
What do I mean by that? Simple.. Store UV Coordinates to where the ray would deflect to (the first place it hits)..

This could work because the Lightmap essentially creates a Big texture atlas and all of the triangles of the meshes that are a part of the baked lightmap are assigned values on the texture atlas, which means you could instead of storing final colors, one could instead create a feedback loop by storing UV coordinates in the texture atlas, encoded in RGBA, that would just point to a UV coordinate of said atlas, that would point to some other point on the same atlas, and do so as many times as the user defines (for better quality, at the cost of performance)

Now How would this help make Dynamic Lights with GI Possible?

1. Determine which are the initial Surfaces hit by the Light (the UV coordinates)
2. Read said UV coordinates to determine which UV coordinates you are pointed to, and Compute Indirect Light for that Point
3. Repeat 1 and 2, but instead of initial surface, the surface that you're determining is the one you've been pointed to in 2nd step. This step is for Extra Bounces

Implementation Considerations:

1. 16k Shadow map requires 28 bits (14x,14y) to be covered precisely
2. Calculating Indirect Light needs distance between the two 3D Points of the two UV coordinates (reflecting light and getting light) for attenuation. This could be baked in, but would require more data

Other Considerations:

1. This would Only support Static Objects getting "proper" GI, and it is unclear what sort of mechanics would be required for Dynamic objects to benefit from this GI, however to my knowledge Lightmap GI does offer some sort of Light probes that could do the trick, maaybe.
2. It is unclear if this approach would suffer from issues similar to SDFGI Is Splotchy On Large Uniform Surfaces godot#50773 but for different reasons (such as the possible issue with noise getting generated as is the case with Lightmap GI but combated by denoising, but that type of denoising probably wouldn't work here), and how to combat the issues if so.