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:

[mate-h]

This fix has a similar theme to this PR #23406. Upon cross checking they are separate fixes, both needed. the other one is for fog AABB traversal / far-plane intersection.
this one fixes per-sample view position which affects both clustering and the shadow cascades sampled. so worth testing the shadows as well to see.

[mate-h]

I wonder if flipping these loops around between for each cluster and for each step introduces any performance regression. while it's definitely the right approach, this requires recomputing the cluster each step meaning more work per fragment. I wonder how/if we can save this. Optimization could be left for later anyway in a later PR.

[mate-h]

This looks like the correct fix compared to the directional path above. Same discrete Beer–Lambert extinction per step as before, the atmosphere-side is more analytic along the actual path, which could be a separate improvement.

[mate-h]

looks unused, can remove this

[mate-h]

Oh and also thanks for posting this PR, must have been a tough one to track down!