Implement alpha to coverage (A2C) support.

[pcwalton]

Alpha to coverage (A2C) replaces alpha blending with a hardware-specific multisample coverage mask when multisample antialiasing is in use. It's a simple form of order-independent transparency that relies on MSAA. "Anti-aliased Alpha Test: The Esoteric Alpha To Coverage" is a good summary of the motivation for and best practices relating to A2C.

This commit implements alpha to coverage support as a new variant for AlphaMode. You can supply AlphaMode::AlphaToCoverage as the alpha_mode field in StandardMaterial to use it. When in use, the standard material shader automatically applies the texture filtering method from "Anti-aliased Alpha Test: The Esoteric Alpha To Coverage". Objects with alpha-to-coverage materials are binned in the opaque pass, as they're fully order-independent.

The transparency_3d example has been updated to feature an object with alpha to coverage. Happily, the example was already using MSAA.

This is part of #2223, as far as I can tell.

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Changelog

Added

• The AlphaMode enum now supports AlphaToCoverage, to provide limited order-independent transparency when multisample antialiasing is in use.

[superdump]

The MIP-level scaling, which could be a preprocessing thing, can wait I suppose.

The article also mentions an issue with Unity's cascaded shadow mapping that I'm wondering if we are also affected by and need to address. But that can also be done in a follow-up. I would appreciate confirmation of whether it is an issue for us though. :)

[pcwalton]

@superdump I've changed the logic to fall back to alpha mask with a cutoff of 0.5 if MSAA is off.

As for the Unity bug: Translated to Bevy concepts, the Unity issue is that if you use ExtendedMaterial<StandardMaterial, ...> with alpha to coverage on, shadows won't show up because Unity's shadow mapping has a fragment shader that leaves the alpha value at 0.0, and alpha to coverage is enabled during the shadow pass. Bevy is unaffected by this for two reasons. First, Bevy doesn't enable alpha to coverage during the shadow pass. Second, Bevy disables the fragment shader entirely during the shadow mapping pass. Section 26.4 of the Vulkan spec requires that, when the fragment shader is disabled, alpha to coverage doesn't modify the coverage mask, which prevents the Unity bug.

[IceSentry]

In the transparency example, when the alpha blended cube has an alpha of 0 the shadows disappear, but they remain there for the A2C cube. I think that should be handled the same way for both

[pcwalton]

@IceSentry The shadow behavior for A2C now matches blend.

[IceSentry]

Alright, just one final tiny nitpick, in the example, on the system that updates the alpha of all meshes. There should be a short description of what happens to a2c meshes.

It should probably mention the thing you replied to me in discord about the limitation of MSAA samples

[pcwalton]

@IceSentry Done.