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.