Rewritten SSIL logic

Adresses Improve SSIL/SSAO

The current state of Godot's SSIL can be seen as quite far behind compared to current state of the art implementations, and so I have rewritten the current SSIL logic to generally match the performance of the current SSIL, but with a more modern, up-to-date approach. I have discussed it quite a bit in the rendering channel.

This approach uses visibility bitmasks, a relatively new method for screen space effects such as AO and GI, that keeps a far more accurate track of occlusion by storing visibility in a 32-bit "bitmask", with each bit representing that sector's visibility. This allows for far more appealing, accurate visuals at a relatively minimal added cost, and was easily shared between AO & GI since it is derived from GTAO-VB logic. This technique has been pretty well documented and I will leave some references I used below.

The most prominent changes made are as such:

• Rewritten SSIL that closely matches this Shadertoy's implementation
• Performance comparable to the current SSIL in-engine
• Optional inclusion of GTAO-VB at little extra cost

Showcase

(intensity 1.0, ao strength 0.5, thickness 0.3, backface rejection enabled, radius 1.6-2.0):


(intensity 1.0, ao strength 0.5, normal rejection 1.0, radius 5.0)

Changes needed

• Rebase PR, get up to code for merging
• Clean up, secure, and optimize logic
• Expose more parameters
• Improve performance
• Skip logic when able to (i.e remove any AO calculations if type is indirect lighting only)
• Gather feedback and make necessary changes based on it
• Fix half size option
• Fix sampling issues (namely "the vortex")

I am going to be quite busy for about a week, so I won't have space to work on this for that time. This is still a draft PR, as its not finished and I have hopes that I can gain feedback to improve this (ideally I'll have made a stupid mistake in code and once I fix it the quality improves ten-fold... lol). To go into detail about the "vortex" issue, I'd suggest using medium quality and aiming the center of the screen at an area with lots of indirect illumination. I don't know what causes it, totally stumped, but it creates a strange vortex-ish shape in the center of the screen. On top of that, at lower quality settings, the gap between slices/samples becomes quite apparent, and so it does not scale well between quality, especially compared to the current SSIL. Some help on those issues would be great!

References/Inspiration

• Screen Space Indirect Lighting with Visibility Bitmask: Improvement to GTAO/SSAO Real-Time Ambient Occlusion Algorithm
• GT-VBGI (uniformly weighted)
• [Godot Forest] SSILVB test, better LOD transitions and optimizations