StandardMaterial's double_sided field does not prevent backface culling

[rparrett]

Bevy version

main (cb2ba19)

Operating system & version

Macos 12.0.1. (M1)

What you did

Modify the 3d_scene example in the following way

use bevy::prelude::*;

fn main() {
    App::new()
        .insert_resource(Msaa { samples: 4 })
        .add_plugins(DefaultPlugins)
        .add_startup_system(setup)
        .run();
}

/// set up a simple 3D scene
fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    // plane
    commands.spawn_bundle(PbrBundle {
        mesh: meshes.add(Mesh::from(shape::Plane { size: 5.0 })),
        material: materials.add(Color::rgb(0.3, 0.5, 0.3).into()),
        ..Default::default()
    });
    // square
    let square_material = materials.add(StandardMaterial {
        base_color: Color::rgb(0.8, 0.7, 0.6),
        double_sided: true,
        ..Default::default()
    });
    commands.spawn_bundle(PbrBundle {
        mesh: meshes.add(Mesh::from(shape::Quad::default())),
        material: square_material,
        transform: Transform::from_xyz(0.0, 0.5, 0.0)
            .with_rotation(Quat::from_rotation_y(std::f32::consts::PI)),
        ..Default::default()
    });
    // light
    commands.spawn_bundle(PointLightBundle {
        point_light: PointLight {
            intensity: 1500.0,
            shadows_enabled: true,
            ..Default::default()
        },
        transform: Transform::from_xyz(4.0, 8.0, 4.0),
        ..Default::default()
    });
    // camera
    commands.spawn_bundle(PerspectiveCameraBundle {
        transform: Transform::from_xyz(-2.0, 2.5, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
        ..Default::default()
    });
}

What you expected to happen

Square and shadow

What actually happened

Just shadow

Additional information

@superdump mentioned on discord that this is a bug and double_sided should be affecting culling. It looks like there's some code in the shaders that's flipping normals, but as far as I can tell, there's no machinery that might affect culling.

I was able to fix this by adding specialization for cull_mode, and hooking double_sized up to it, but something is slightly off with the lighting so I may not have done that correctly.

I'm also not sure whether it's more appropriate to use 2 bits to represent every possible cull mode, or just one to enable/disable culling.

If I have time later though, I'll disentangle the code from another idea I was working on and throw a PR up, but this should be a pretty quick one for someone who actually knows what they're doing in bevy's renderer.

[aevyrie]

Piggybacking to note that we were using double_sided and noticed this regression when upgrading to 0.6.

This statement was incorrect, the issue I was seeing in that case was caused by something other than the 0.6 upgrade.

However, I was able to replicate the issue using the replication steps above.

[rparrett]

It turns out that the "quad flipping" that I was using to demonstrate this was perhaps not actually meant to flip the normals (see #3741).

I replaced the example above with one that just rotates the default quad 180 degrees. (result is the same)