Transparent base_color_texture with AlphaMode::Mask should make shadow casters transparent

[Nathan-Fenner]

In the following scene, a simple mesh is constructed from a basic plane mesh:

commands
    .spawn_bundle(PbrBundle {
        mesh: meshes.add(Mesh::from(shape::Plane { size: 1.0 })),
        material: materials.add(StandardMaterial {
            base_color_texture: Some(chara_texture.clone()),
            double_sided: false,
            alpha_mode: AlphaMode::Mask(0.5),
            ..Default::default()
        }),
        transform: Transform::from_xyz(0.0, 0.5, 0.0),
        ..Default::default()
    });

The mesh itself is transparent ("cut out" where the alpha of the texture is more than 0.5) but the entire plane casts a shadow. This means that it casts a shadow in a different shape than the object itself.

What problem does this solve or what need does it fill?

Meshes textures with transparency using AlphaMode::Mask can cast shadows that match their shape in space.

What solution would you like?

When a StandardMaterial has AlphaMode::Mask, it should automatically cast shadows using the same mask for transparency. If this is infeasible, a built-in ShadowTextureMask{ ... } component could be an alternative solution.

What alternative(s) have you considered?

In some cases, the shape of the object could be constructed out of geometry instead of using textures, or a custom alternative shadow pass could be built. This would be a lot of work to match the probably "expected" behavior.

Additional context

It's not entirely clear how shadows should behave for transparency with AlphaMode::Blend, so that's out-of-scope for this issue. Using a separate component (even optionally) could make this clearer, by allowing different strategies for the shadow and the mesh in the standard pipeline (e.g. blend transparency in the main pipeline, while using AlphaMode::Mask(0.1) for shadows).

[superdump]

I haven’t looked into any they about how this should be done but I suspect it’s as simple as having to implement opaque and alpha mask shadow phases where the alpha mask phase has a fragment shader that discards similar to the pbr shader. I haven’t yet had to implement a fragment shader that doesn’t return anything. All this has to do is check the alpha for the fragment and discard or not. I don’t know if that is a valid fragment shader.

[nicholasc]

+1 I would love to have this

[Anti-Alias]

Second. Objects like foliage make heavy use of this and is a must-have.