too many point lights behave strangely

[robtfm]

Bevy version

main

Operating system & version

Windows 10

What you did

Create a scene with more than 256 point lights.

What you expected to happen

Rational degradation of the lights in the scene.

What actually happened

Odd behaviour as some clusters get more light than expected, and some get less / none.

Additional information

Example code to see the issue (most noticeable after ~10 seconds when the CLUSTER_COUNT_MASK limit is exceeded for the middle cluster):

use bevy::{prelude::*, diagnostic::{FrameTimeDiagnosticsPlugin, LogDiagnosticsPlugin}, window::{WindowMode, PresentMode}};

fn main() {
    App::new()
        .insert_resource(WindowDescriptor{
            width: 1280.0,
            height: 720.0,
            mode: WindowMode::Windowed,
            present_mode: PresentMode::Immediate,
            ..Default::default()
        })
        .add_plugins(DefaultPlugins)
        .add_plugin(FrameTimeDiagnosticsPlugin)
        .add_plugin(LogDiagnosticsPlugin::default())
        .add_startup_system(setup)
        .add_system(move_camera)
        .run()
}

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    // some quad
    commands.spawn_bundle(PbrBundle{
        mesh: meshes.add(shape::Quad{size: Vec2::splat(100.0), flip: false}.into()),
        material: materials.add(Color::WHITE.into()),
        transform: Transform::from_translation(Vec3::new(0.0, 0.0, -5.0)),
        ..Default::default()
    });

    // some bright offscreen lights
    for x in 0..25 {
        for y in 0..10 {
            commands.spawn_bundle(PointLightBundle {
                point_light: PointLight {
                    color: Color::RED,
                    intensity: 10.0,
                    range: f32::sqrt(10.0 * 10.0 / (4.0 * std::f32::consts::PI)),
                    ..Default::default()
                },
                transform: Transform::from_translation(Vec3::new(20.0 + 10.0 * x as f32, y as f32 * 10.0 - 50.0, -4.9)),
                ..Default::default()
            });
        }
    }

    // grid of lights
    for x in 1..=18 {
        for y in 1..=18 {
            commands.spawn_bundle(PointLightBundle {
                point_light: PointLight {
                    color: Color::BLUE,
                    intensity: x as f32 * y as f32 * 0.01,
                    range: f32::sqrt(x as f32 * y as f32 * 0.01 * 10.0 / (4.0 * std::f32::consts::PI)),
                    ..Default::default()
                },
                transform: Transform::from_translation(Vec3::new(x as f32 * 0.2 - 2.0, y as f32 * 0.2 - 2.0, -4.9)),
                ..Default::default()
            });
        }
    }

    //camera
    commands.spawn_bundle(PerspectiveCameraBundle::new_3d());

    //action
}

fn move_camera(
    mut q: Query<&mut Transform, With<Camera>>,
    time: Res<Time>,
) {
    for mut t in q.iter_mut() {
        t.translation.z += time.delta_seconds();
    }
}