When using 2D and 3D cameras, moving the 2D UI camera to a nondefault render layer makes shadows flicker

[janhohenheim]

Bevy version

0.16.0

Relevant system information

AdapterInfo { name: "AMD Radeon RX 7900 XTX (RADV NAVI31)", vendor: 4098, device: 29772, device_type: DiscreteGpu, driver: "radv", driver_info: "Mesa 25.0.3", backend: Vulkan }

What you did

I have the following "concepts":

enum CameraOrder {
    World,
    ViewModel,
    Ui,
}

impl From<CameraOrder> for isize {
    fn from(order: CameraOrder) -> Self {
        order as isize
    }
}

bitflags! {
    struct RenderLayer: u32 {
        const DEFAULT = 0b00000001;
        const VIEW_MODEL = 0b00000010;
        const PARTICLES = 0b00000100;
        const GIZMO3 = 0b0001000;
        const UI = 0b0010000;
    }
}

impl From<RenderLayer> for RenderLayers {
    fn from(layer: RenderLayer) -> Self {
        RenderLayers::from_iter(layer.iter().map(|l| (l.bits() >> 1) as usize))
    }
}

Using these terms, my hierarchy is as follows:

• UI camera
• Player camera parent
  • World model camera
  • View model camera

where the relevant code look like this:

UI Camera

Full code: https://github.com/janhohenheim/foxtrot/blob/nondefault-ui-cam/src/ui_camera.rs#L18-L32

(
    Name::new("UI Camera"),
    Camera2d,
    IsDefaultUiCamera,
    Camera {
        // The UI camera order is the highest.
        order: CameraOrder::Ui.into(),
        ..default()
    },
)

World Model Camera

Full code: https://github.com/janhohenheim/foxtrot/blob/nondefault-ui-cam/src/gameplay/player/camera.rs#L98-L125

(
    Name::new("World Model Camera"),
    Camera3d::default(),
    Camera {
        order: CameraOrder::World.into(),
        hdr: true,
        ..default()
    },
    RenderLayers::from(
        RenderLayer::DEFAULT | RenderLayer::PARTICLES,
    ),
    Bloom::NATURAL,
    #[cfg(feature = "native")]
    (
        Msaa::Off,
        ScreenSpaceAmbientOcclusion::default(),
        TemporalAntiAliasing::default(),
        NormalPrepass,
        ShadowFilteringMethod::Temporal,
    ),
)

View Model Camera

Full code: https://github.com/janhohenheim/foxtrot/blob/nondefault-ui-cam/src/gameplay/player/camera.rs#L128-L148

(
    Name::new("View Model Camera"),
    Camera3d::default(),
    Camera {
        // Bump the order to render on top of the world model.
        order: CameraOrder::ViewModel.into(),
        hdr: true,
        ..default()
    },
    // Only render objects belonging to the view model.
    RenderLayers::from(RenderLayer::VIEW_MODEL),
)

The lights all belong to RenderLayer::DEFAULT | RenderLayer::VIEW_MODEL:

Light setup

Full code: https://github.com/janhohenheim/foxtrot/blob/nondefault-ui-cam/src/gameplay/player/camera.rs#L248-L270

fn add_render_layers_to_point_light(trigger: Trigger<OnAdd, PointLight>, mut commands: Commands) {
    let entity = trigger.target();
    commands.entity(entity).insert(RenderLayers::from(
        RenderLayer::DEFAULT | RenderLayer::VIEW_MODEL,
    ));
}

fn add_render_layers_to_spot_light(trigger: Trigger<OnAdd, SpotLight>, mut commands: Commands) {
    let entity = trigger.target();
    commands.entity(entity).insert(RenderLayers::from(
        RenderLayer::DEFAULT | RenderLayer::VIEW_MODEL,
    ));
}

fn add_render_layers_to_directional_light(
    trigger: Trigger<OnAdd, DirectionalLight>,
    mut commands: Commands,
) {
    let entity = trigger.target();
    commands.entity(entity).insert(RenderLayers::from(
        RenderLayer::DEFAULT | RenderLayer::VIEW_MODEL,
    ));
}

This all works fine. Now let's do one little change by moving the UI camera to a nondefault render layer:

fn spawn_ui_camera(mut commands: Commands) {
    commands.spawn((
        Name::new("UI Camera"),
        UiCamera,
        IsDefaultUiCamera,
        Camera {
            order: CameraOrder::Ui.into(),
            ..default()
        },
        // This line causes the issue
        RenderLayers::from(RenderLayer::UI),
    ));
}

What went wrong

The lights flicker uncontrollably (warning for flashing lights!)

Screencast_From_2025-05-10_14-39-17.mp4

Interestingly, this only happens when moving close to my spot lights

Screencast_From_2025-05-10_14-43-52.mp4

Additional information

To see this in action, clone https://github.com/janhohenheim/foxtrot/tree/nondefault-ui-cam (note the branch) and run bevy run.

This has only happened on native, but it's hard to test this on Wasm since there, the lights ignore most of the obstacles in their way anyways:

Instead, I get #19167 on web

[ickshonpe]

The UI doesn't use RenderLayers, UI nodes can only only have a single camera target which you can set by inserting the UiCameraTarget component on the root UI node. There's definitely something wrong though if there any flickering or crashes though.

[janhohenheim]

@ickshonpe I originally did this not to change the rendering of the UI nodes, but to ensure 3D gizmos were not drawn on my UI camera. The solution I landed on in the end was to change the render layer of the gizmos instead.

[janhohenheim]

Maybe related: #19273
Need to check if that fixed it