Added visibility bitmask as an alternative SSAO method

[dragostis]

Early implementation. I still have to fix the documentation and consider writing a small migration guide.

Questions left to answer:

• should thickness be an overridable constant?
• is there a better way to implement Eq/Hash for SSAOMethod?
• do we want to keep the linear sampler for the depth texture?
• is there a better way to separate the logic than preprocessor macros?

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[JMS55]

References for reviewers:

• https://ar5iv.labs.arxiv.org/html/2301.11376
• https://cdrinmatane.github.io/posts/ssaovb-code

[dragostis]

I added a few questions to answer before merging this and also added a linear sampler for the depth texture since I noticed it slightly improves some of the ghosting noticeable in small objects that happens due to the low resolution mip that gets selected for far-away samples.

[JMS55]

Another good reference: https://cybereality.com/screen-space-indirect-lighting-with-visibility-bitmask-improvement-to-gtao-ssao-real-time-ambient-occlusion-algorithm-glsl-shader-implementation/

@dragostis to give you an update, I'd still like to get this merged for Bevy 0.15 if you're interested. It's just going to be a very slow process, as reviewer bandwidth is very limited, and we have a large backlog of rendering PRs. I'll try to test this out in a few weeks when I'll have more free time, and help push it forward.

[dragostis]

@JMS55, I'm happy to get this merged for 0.15!

In it's current form, it only handles AO. Adding GI as well is quite straightforward algorithm-wise, but would require extra textures and a separate denoise pass to look good.

[JMS55]

Yeah lets not do GI in this PR. Feel free to open another after if you're interested though.

To get this PR merged, I want to test this on some more realistic scenes and check how the perf/quality compares. If it's always better than the existing method, then imo we should remove the existing method and maintain just this one. That's basically the only blocker. Unfortunately I don't have a desktop for the next week or two to be able to run Sponza and other scenes.

There's also the opportunity here to change the denoiser/downscale depth/noise patterns that I copied from XeGTAO, and try what the newer vismask impls are using. Up to you on that, we can also leave the existing ones as-is.

[DGriffin91]

The other sampler is called point because it's a nearest sampler. I guess this is still using nearest for the mips.

[JMS55]

The code behind the vismask paper was open sourced (NOTE: Parts, including the denoiser, are from Unity and are not open-source licensed, be careful which files you reference): https://github.com/cdrinmatane/SSRT3

[JMS55]

I'm still quite busy with meshlets, but I need to find some time sometime soon to try this PR out and see how it compares to GTAO.