`FogVolume`s silently render nothing when no `VolumetricLight` exists in the scene

[blaind]

Bevy version and features

• v0.19.0 (default features, Linux x86_64, Vulkan)

What you did

Tried to build ambient-only fog (overcast sky, no god rays): a FogVolume, a camera with VolumetricFog, and a directional light without the VolumetricLight marker:

commands.spawn((
    Camera3d::default(),
    Hdr,
    VolumetricFog {
        ambient_intensity: 8.0,
        ..default()
    },
    Transform::from_xyz(0.0, 4.0, 16.0).looking_at(Vec3::new(0.0, 3.0, 0.0), Vec3::Y),
));

commands.spawn((
    FogVolume {
        density_factor: 0.15,
        scattering: 0.6,
        absorption: 0.05,
        ..default()
    },
    Transform::from_xyz(0.0, 5.0, -22.0).with_scale(Vec3::new(100.0, 20.0, 60.0)),
));

// Note: no VolumetricLight anywhere in the scene.
commands.spawn((
    DirectionalLight {
        illuminance: 12000.0,
        shadow_maps_enabled: false,
        ..default()
    },
    Transform::from_rotation(Quat::from_euler(EulerRot::XYZ, -1.0, 0.4, 0.0)),
));

Full runnable reproduction:
https://github.com/blaind/bevy-volumetric-ambient-repro (MODE=nolight)

What went wrong

• Expected: fog lit by the ambient term, per the FogVolume::fog_color docs: "Note that the fog must be lit by a VolumetricLight or ambient light in order for this color to appear." At minimum, extinction.
• Actually: nothing renders at all. No in-scatter, no extinction. The screenshot is byte-identical (same sha256) to a render without working fog.

The gate is explicit in crates/bevy_pbr/src/volumetric_fog/render.rs, extract_volumetric_fog:

if volumetric_lights.is_empty() {
    // TODO: needs better way to handle clean up in render world
    for (entity, ..) in view_targets.iter() {
        commands.entity(entity)
            .remove::<(VolumetricFog, ViewVolumetricFogPipelines, ViewVolumetricFog)>();
    }
    for (entity, ..) in fog_volumes.iter() {
        commands.entity(entity).remove::<FogVolume>();
    }
    return;
}

So ambient-only fog cannot be expressed at all. The workaround is attaching VolumetricLight to a light that contributes nothing (e.g. a shadowless directional light), purely to activate the pass.

Possibly a design decision rather than a plain bug, but at least one of these seems warranted:

1. Run the pass when a FogVolume and a VolumetricFog camera exist, even with zero VolumetricLights (extinction and ambient still apply), or
2. Fix the docs: FogVolume::fog_color currently promises ambient-only lighting works, and the VolumetricFog camera component doc (the natural docs.rs entry point) mentions neither the FogVolume nor the VolumetricLight requirement (it does document the WebGPU-on-Wasm requirement).

[alice-i-cecile]

Definitely a bug, not a design decision.

[blaind]

Definitely a bug, not a design decision.

Ok! Agree, the TODO comment in the source also points into that direction. Fix PR at #25286