Vulkan: StandardMaterial3D with any transparency does not cast shadows

[jasonallen]

Godot version

v4.0.alpha8.official [917fd65]

System information

Mac OSX 12.4, M1 Pro

Issue description

I'm unable to get a material to render a shadow when I set any transparency other than "Disabled":

This seems related to #50344 but that seems fixed.

Steps to reproduce

1. Create a scene with a directional light and flat terrain that accepts shadows
2. Create a Mesh3D with a quad, give it a StandardMaterial3D and a texture with alpha in it.
3. Position it so you can see the shadow casted on the flat terrain.
4. Now set the material "Transparency" to anything other than "Disabled"

==! The shadow disappears.

Minimal reproduction project

shadow_test.zip

[TokisanGames]

On GD4 Alpha13 Win10/64, I can get a shadow using depth prepass mode or alphascissors. Not with Alpha hash or alpha.

[TokisanGames]

Also ProximityFade (part of the transparency pipeline) has no shadow, only self-shadows. No PF on the right. The rock looks weird because of #63675.

@Calinou Can this be added to the gd4 roadmap and material tracker #56333?

[Calinou]

Also ProximityFade (part of the transparency pipeline) has no shadow, only self-shadows.

Alpha blending is used for materials with proximity fade enabled, so this is expected. Using proximity fade with alpha scissor probably wouldn't make much sense (except with dithering/alpha hash, I guess).

It might be possible to set the depth draw mode to Opaque prepass to improve sorting and make it cast shadows again, but this may come at a performance cost.

[TokisanGames]

@Calinou Can this be added to the gd4 roadmap and material tracker #56333?

[Calinou]

I can't reproduce the original issue anymore on 4.0.alpha13 (Linux, NVIDIA 515.57):

The alpha hash issue is already tracked in #61882.

Please continue the discussion about the proximity fade issue in #63675.

[mournguard]

The other proximity fade issue (#63675) is about z-ordering, and this issue is about shadows on different transparency modes, but this still leaves the fact that proximity fade doesn't have shadows at all, even though it can properly be used to fade with modes that have shadows such as alpha scissor.