Add Order Independent Transparency

[IceSentry]

Objective

• Alpha blending can easily fail in many situations and requires sorting on the cpu

Solution

• Implement order independent transparency (OIT) as an alternative to alpha blending
• The implementation uses 2 passes
  • The first pass records all the fragments colors and position to a buffer that is the size of N layers * the render target resolution.
  • The second pass sorts the fragments, blends them and draws them to the screen. It also currently does manual depth testing because early-z fails in too many cases in the first pass.

Testing

• We've been using this implementation at foresight in production for many months now and we haven't had any issues related to OIT.

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Showcase

Future work

• Add an example showing how to use OIT for a custom material
• Next step would be to implement a per-pixel linked list to reduce memory use
• I'd also like to investigate using a BinnedRenderPhase instead of a SortedRenderPhase. If it works, it would make the transparent pass significantly faster.

[github-actions]

The generated examples/README.md is out of sync with the example metadata in Cargo.toml or the example readme template. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.

[tychedelia]

Very cool, is a surprisingly straightforward technique to follow along. Just a few minor comments. I tested with multiple cameras/viewports/hdr and everything worked fine, except the crash noted with MSAA enabled.

[tychedelia]

LGTM sans one comment about panic.

[IceSentry]

This PR should fix the CI failure #15629

The big remaining issue is that you can't toggle the OIT in the example because removing the component doesn't propagate to the RenderWorld.

[kristoff3r]

This PR should fix the CI failure #15629

The big remaining issue is that you can't toggle the OIT in the example because removing the component doesn't propagate to the RenderWorld.

This will get fixed in #15582, it's a general problem that shouldn't block this PR

[alice-i-cecile]

Thank you to everyone involved with the authoring or reviewing of this PR! This work is relatively important and needs release notes! Head over to bevyengine/bevy-website#1720 if you'd like to help out.