Self-shadowing on back side of meshes with "double sided lighting" enabled

[rparrett]

Bevy version

bevy main / 0.8

Operating system & version

2022-02-23T00:19:18.020580Z  INFO bevy_render::renderer: AdapterInfo { name: "Apple M1 Max", vendor: 0, device: 0, device_type: DiscreteGpu, backend: Metal }

What you did

Modified 3d_scene to use a quad with the back side rotated towards the camera with culling disabled and double-sided lighting enabled.

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),
        cull_mode: None,
        double_sided: true,
        ..Default::default()
    });
    commands.spawn_bundle(PbrBundle {
        mesh: meshes.add(Mesh::from(shape::Quad::default())),
        material: square_material.clone(),
        transform: Transform::from_xyz(-0.6, 0.5, 0.0),
        ..Default::default()
    });
    commands.spawn_bundle(PbrBundle {
        mesh: meshes.add(Mesh::from(shape::Quad::default())),
        material: square_material,
        transform: Transform::from_xyz(0.6, 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(Camera3dBundle {
        transform: Transform::from_xyz(-2.0, 2.5, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
        ..Default::default()
    });
}

What went wrong

Quads should look the same but one has some funky shadows.