Bevy Camera Systems Skill for Cursor

Bevy camera systems

When to use

Load this when a view must render 2D or 3D. Camera, Camera2d, Camera3d, and the two_passes example are the sources for the names below. The library pin is 0.20.

ECS shape

• Components: Camera is the viewpoint (projection and render target). A Camera with no render graph errors at runtime. Camera2d enables the 2D graph and requires Camera, Projection, and Frustum. Camera3d enables the main 3D graph and requires Camera and Projection. 3D space is right-handed: X right, Y up, Z back, so forward is -Z.
• Spawn: the example uses commands.spawn((Camera3d::default(), Transform::from_xyz(...).looking_at(Vec3::ZERO, Vec3::Y))). A second camera sets Camera { order: 1, clear_color: ClearColorConfig::None, ..default() } so it draws on top without clearing.
• Query: systems may take (&Camera, &GlobalTransform) with With<Camera2d>. Camera::viewport_to_world_2d maps a viewport position to a 2D world position for an orthographic camera along Z.
• Schedule: spawn on Startup. Follow, split viewports, and trauma offset are your components on Update.

fn spawn_cameras(mut commands: Commands) {
    commands.spawn((
        Camera3d::default(),
        Transform::from_xyz(0.0, 6.0, 10.0).looking_at(Vec3::ZERO, Vec3::Y),
    ));
    commands.spawn((
        Camera2d,
        Camera {
            order: 1,
            clear_color: ClearColorConfig::None,
            ..default()
        },
    ));
}

#[derive(Component)]
struct FollowTarget;

#[derive(Component)]
struct FollowCam {
    offset: Vec3,
    stiffness: f32,
}

fn follow_target(
    time: Res<Time>,
    targets: Query<&Transform, (With<FollowTarget>, Without<FollowCam>)>,
    mut cameras: Query<(&FollowCam, &mut Transform)>,
) {
    let Ok(target) = targets.single() else {
        return;
    };
    let dt = time.delta_secs();
    for (follow, mut transform) in &mut cameras {
        let desired = target.translation + follow.offset;
        let t = 1.0 - (-follow.stiffness * dt).exp();
        transform.translation = transform.translation.lerp(desired, t);
        transform.look_at(target.translation, Vec3::Y);
    }
}

use bevy::camera::Viewport;

fn split_view(mut cameras: Query<&mut Camera, With<Camera3d>>) {
    let mut iter = cameras.iter_mut();
    if let Some(mut left) = iter.next() {
        left.viewport = Some(Viewport {
            physical_position: UVec2::new(0, 0),
            physical_size: UVec2::new(640, 720),
            depth: 0.0..1.0,
        });
    }
    if let Some(mut right) = iter.next() {
        right.order = 0;
        right.viewport = Some(Viewport {
            physical_position: UVec2::new(640, 0),
            physical_size: UVec2::new(640, 720),
            depth: 0.0..1.0,
        });
    }
}

fn cursor_to_world(
    windows: Query<&Window>,
    cameras: Query<(&Camera, &GlobalTransform), With<Camera2d>>,
) {
    let Ok(window) = windows.single() else {
        return;
    };
    let Ok((camera, transform)) = cameras.single() else {
        return;
    };
    let Some(cursor) = window.cursor_position() else {
        return;
    };
    let _world = camera.viewport_to_world_2d(transform, cursor);
}

#[derive(Component)]
struct Trauma(f32);

fn apply_trauma(time: Res<Time>, mut cameras: Query<(&mut Trauma, &mut Transform), With<Camera3d>>) {
    for (mut trauma, mut transform) in &mut cameras {
        trauma.0 = (trauma.0 - time.delta_secs()).max(0.0);
        let shake = trauma.0 * trauma.0;
        transform.translation.x += (time.elapsed_secs() * 31.0).sin() * 0.15 * shake;
        transform.translation.y += (time.elapsed_secs() * 27.0).cos() * 0.1 * shake;
    }
}

[dependencies]
bevy = "0.20"

View choices

| Need | Bevy approach |

| --- | --- |

| Active view | An entity with Camera2d or Camera3d |

| Camera pose | Transform on that entity |

| Draw order / overlay | Camera.order and ClearColorConfig::None |

| Screen point to 2D world | viewport_to_world_2d with GlobalTransform |

Scope

• Follow, look-at, trauma offset, and split Viewports are the systems above. A Camera still needs its render graph: spawn Camera2d or Camera3d so Bevy inserts CameraRenderGraph.
• A texture target is a RenderTarget::Image component on the camera entity. The split example keeps the window target and sets Camera::viewport.

Expert Knowledge Delta

Target: Bevy 0.20+.

Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their APIs. Systems run in parallel unless .chain() orders the ones that conflict.

Who owns state

| Piece | Owner | Who may mutate | Schedule |

| --- | --- | --- | --- |

| Data on one entity | Component | One system via Query | Update or FixedMain |

| One value for the whole app | Resource | One system | Startup to insert, then the schedule that reads it |

| Spawn | Commands | The spawning system | Startup for the initial world; later schedules for runtime spawn |

| Order that must not race | .chain() | The chained systems only | Same schedule |

| Plugin boundary | Plugin | build registers systems and resources | Before the app runs |

Borrow rule: two systems that both mutably query the same component must be ordered (.chain()) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API.

Decision

| If you need | Do this | Do not |

| --- | --- | --- |

| Per-entity data | Component + Query | A script field copied onto the entity |

| App-wide config or clock | Resource | Copy the value onto every entity |

| Stable simulation step | FixedMain and the Fixed clock | Tie simulation to raw frame Update |

| Startup spawn | Commands in Startup | Spawn from a render system |

| Cross-system notice that is not shared state | Message or observer slot in bevy-events-messaging | A new event type invented in this file |

| An API outside the documented set | Check the official docs first | A made-up crate or API |

Load references/ecs-ownership.md for the ownership table.

NEVER

• Do not pin /0.20.0/ in docs URLs. WHY: links stay on latest; the 0.20 target is stated once in prose.
• Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless .chain() orders them.
• Do not invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.

• No scene-tree camera node types in Bevy code. WHY: Camera2d and Camera3d are the documented Bevy pieces.
• No claim that Bevy ships a follow, spring-arm, or shake component. WHY: those rigs are the FollowCam and Trauma components in this skill, written as ordinary ECS.
• No third-party camera crates. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

Official links stay on bevy.org. The library pin on bevy-master stays 0.20.0.

Official Documentation

• Examples - Example tree on the latest branch.
• ECS - Component, systems, Query, Startup, Update, and .chain().

Related Skills

Prerequisites

• bevy-master - Who owns which piece of state.
• bevy-ecs-composition - Spawn tuples and systems.

Complements

• bevy-input-handling - Mouse motion a camera system may read.
• bevy-character-movement - Actor the view may track.
• godot-camera-systems - Same topic on the Godot track.

Downstream / consumers

• bevy-asset-scenes - Scenes that include a camera entity.

Master

• bevy-master - Bevy router.