Fixed cluster sampling issues for volumetric point and spot lights

[JeroenHoogers]

Objective

• Fixes SpotLight / PointLight artifacts in FogVolume #18371
• Fixes sampling volumetric sampling issues when camera is outside of Fog Volume

Solution

• The problem was that only the point / spot lights that overlap with the first z-slice of the cluster grid were considered in the ray marching algo. In the updated code, I moved the ray marching to the outer loop. Then for each sample, I get the cluster index from the sample's view position, with the index I find all lights contributing to this cluster and add up their light contributions.
• Another issue I found while looking into this issue is that the ray origin for the volumetric tracing was the camera origin, even when the camera was outside the FogVolume. This is now fixed in both directional light and the point / spotlight implementations.

Testing

I tested using the code given in the bug report and adapted it to the latest version of bevy:

use bevy::{
    color::palettes::css::RED,
    core_pipeline::{tonemapping::Tonemapping},
    light::{
        FogVolume, VolumetricFog, VolumetricLight,
    },
    post_process::bloom::Bloom,
    prelude::*,
};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .insert_resource(GlobalAmbientLight::NONE)
        .add_systems(Startup, setup)
        .add_systems(Update, move_camera)
        .run();
}

fn setup(mut commands: Commands) {
    commands.spawn((
        Transform::from_xyz(-2., 0., 0.),
        SpotLight {
            intensity: 500_000.0, // lumens
            color: RED.into(),
            shadow_maps_enabled: true,
            ..default()
        },
        VolumetricLight,
    ));

    commands.spawn((
        Transform::from_xyz(2., 0., 0.),
        SpotLight {
            intensity: 500_000.0, // lumens
            color: RED.into(),
            shadow_maps_enabled: true,
            ..default()
        },
        VolumetricLight,
    ));

    commands.spawn((
        FogVolume::default(),
        Transform::from_scale(Vec3::splat(35.0)),
    ));

    commands.spawn((
        Camera3d::default(),
        Camera { ..default() },
        Transform::from_xyz(0., 0., 5.),
        Tonemapping::TonyMcMapface,
        Bloom::default(),
        VolumetricFog {
            ..default()
        },
    ));
}

fn move_camera(
    mut query: Query<&mut Transform, With<Camera3d>>,
    input: Res<ButtonInput<KeyCode>>,
    time: Res<Time>,
) {
    for mut transform in query.iter_mut() {
        if input.pressed(KeyCode::ArrowUp) {
            transform.translation.z -= 10. * time.delta_secs();
        }

        if input.pressed(KeyCode::ArrowDown) {
            transform.translation.z += 10. * time.delta_secs();
        }

        if input.pressed(KeyCode::KeyZ) {
            transform.translation.z = 5.;
        }

        if input.pressed(KeyCode::KeyX) {
            transform.translation.z = 130.;
        }
    }
}

---

Showcase

After changes (using code from: #18371):

volumetric_spotlights.mp4

Note: The flickering of the light is due to the regular sampling interval, adding more samples or jittering the ray origin minimizes this problem.

Ray marching when camera is outside of FogVolume (Before):

After: