Bevy 2D physics

When to use

Load this when a Bevy app needs simulated 2D bodies, triggers, or layer filtering before choosing a solver. Pair with bevy-character-movement for player motion that should not be a rigid body. Pair with bevy-spatial-queries for casts that also wait on a solver.

ECS shape

• Components own velocity, mass, a circle radius, and a layer mask for props you simulate yourself.
• Resources own gravity.
• Systems integrate on FixedUpdate, then separate overlapping circles. .chain() runs separation after the step.
• Schedule: the fixed step is the clock. A solver plugin can replace the integrator when you need joints and a spatial index. This file does not name that plugin's types. Player locomotion stays on bevy-character-movement.

Coming from Godot

Godot idea	Bevy idiom
Simulated 2D prop	Body2d plus Transform, integrated on FixedUpdate
Player locomotion	bevy-character-movement, not this integrator
Trigger volumes	An Aabb overlap that records occupants
Layers and masks	A LayerMask you define, filtered in the overlap system

Scope

Gravity, velocity, circle overlap, and AABB triggers are ordinary components stepped on FixedUpdate. Joints, continuous collision, and a spatial index belong to a physics solver you add as a plugin. Casts that need that solver's world belong on bevy-spatial-queries.

Examples

[dependencies]
bevy = "0.20"

Integrate velocity under gravity, then push overlapping circles apart when their layer masks intersect. Both systems write Body2d and Transform, so they run as one chain.

#[derive(Component)]
struct Body2d {
    velocity: Vec2,
    mass: f32,
    radius: f32,
}

#[derive(Component)]
struct LayerMask {
    layer: u32,
    mask: u32,
}

#[derive(Resource)]
struct Gravity2d(Vec2);

fn integrate_2d(
    time: Res<Time<Fixed>>,
    gravity: Res<Gravity2d>,
    mut bodies: Query<(&mut Transform, &mut Body2d)>,
) {
    let dt = time.delta_secs();
    for (mut tf, mut body) in &mut bodies {
        body.velocity += gravity.0 * dt;
        tf.translation.x += body.velocity.x * dt;
        tf.translation.y += body.velocity.y * dt;
        if tf.translation.y < body.radius {
            tf.translation.y = body.radius;
            body.velocity.y = body.velocity.y.max(0.0);
        }
    }
}

fn masks_overlap(a: &LayerMask, b: &LayerMask) -> bool {
    (a.mask & b.layer) != 0 && (b.mask & a.layer) != 0
}

fn separate_circles(mut bodies: Query<(Entity, &mut Transform, &mut Body2d, &LayerMask)>) {
    let mut pairs: Vec<(Entity, Entity)> = Vec::new();
    for (a, tf_a, body_a, layer_a) in &bodies {
        for (b, tf_b, body_b, layer_b) in &bodies {
            if a.index() >= b.index() || !masks_overlap(layer_a, layer_b) {
                continue;
            }
            let delta = tf_b.translation.truncate() - tf_a.translation.truncate();
            let min = body_a.radius + body_b.radius;
            if delta.length() < min {
                pairs.push((a, b));
            }
        }
    }
    for (a, b) in pairs {
        let Ok([(_, mut tf_a, mut body_a, _), (_, mut tf_b, mut body_b, _)]) =
            bodies.get_many_mut([a, b])
        else {
            continue;
        };
        let delta = tf_b.translation.truncate() - tf_a.translation.truncate();
        let dist = delta.length();
        let min = body_a.radius + body_b.radius;
        if dist >= min || dist == 0.0 {
            continue;
        }
        let normal = delta / dist;
        let push = (min - dist) * 0.5;
        tf_a.translation.x -= normal.x * push;
        tf_a.translation.y -= normal.y * push;
        tf_b.translation.x += normal.x * push;
        tf_b.translation.y += normal.y * push;
        let rel = (body_b.velocity - body_a.velocity).dot(normal);
        if rel < 0.0 {
            let inv_a = 1.0 / body_a.mass.max(0.001);
            let inv_b = 1.0 / body_b.mass.max(0.001);
            let impulse = -rel / (inv_a + inv_b);
            body_a.velocity -= normal * impulse * inv_a;
            body_b.velocity += normal * impulse * inv_b;
        }
    }
}

pub struct Physics2dPlugin;

impl Plugin for Physics2dPlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(Gravity2d(Vec2::new(0.0, -9.8)))
            .add_systems(FixedUpdate, (integrate_2d, separate_circles).chain());
    }
}

An AABB trigger remembers who is inside. A ray against a circle reports the nearest hit along the segment.

#[derive(Component)]
struct AabbTrigger {
    half: Vec2,
}

#[derive(Component)]
struct Inside(Vec<Entity>);

#[derive(Component)]
struct Body2d {
    velocity: Vec2,
    mass: f32,
    radius: f32,
}

#[derive(Component)]
struct LayerMask {
    layer: u32,
    mask: u32,
}

fn fill_trigger(
    triggers: Query<(Entity, &Transform, &AabbTrigger)>,
    bodies: Query<(Entity, &Transform, &Body2d)>,
    mut occupants: Query<&mut Inside>,
) {
    for (trigger_e, tf, aabb) in &triggers {
        let Ok(mut inside) = occupants.get_mut(trigger_e) else {
            continue;
        };
        inside.0.clear();
        for (body_e, body_tf, body) in &bodies {
            let delta = (body_tf.translation - tf.translation).truncate().abs();
            if delta.x <= aabb.half.x + body.radius && delta.y <= aabb.half.y + body.radius {
                inside.0.push(body_e);
            }
        }
    }
}

fn ray_circle(origin: Vec2, dir: Vec2, center: Vec2, radius: f32) -> Option<f32> {
    let oc = origin - center;
    let a = dir.dot(dir);
    if a == 0.0 {
        return None;
    }
    let b = 2.0 * oc.dot(dir);
    let c = oc.dot(oc) - radius * radius;
    let disc = b * b - 4.0 * a * c;
    if disc < 0.0 {
        return None;
    }
    let t = (-b - disc.sqrt()) / (2.0 * a);
    if t >= 0.0 { Some(t) } else { None }
}

fn spawn_ball(mut commands: Commands) {
    commands.spawn((
        Transform::from_xyz(0.0, 3.0, 0.0),
        Body2d {
            velocity: Vec2::ZERO,
            mass: 1.0,
            radius: 0.4,
        },
        LayerMask { layer: 1, mask: 2 },
    ));
}

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 Godot node script
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 Godot 2D physics node or server types. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No Avian, Rapier, or any other named physics crate as the chosen solver. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No invented collider, layer, or joint APIs. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.0.

Official Documentation

• bevy crate - API index. The 0.20.0 feature list has no physics-engine feature.
• Assets - Official asset catalog. No physics plugin is named from it here.
• Examples - Example tree on the latest branch.
• Quick Start: plugins - Plugin, DefaultPlugins, MinimalPlugins, and third-party add_plugins.
• ECS - Component, systems, Query, Startup, Update, and .chain().
• bevy::time - Fixed and run_fixed_main_schedule (FixedMain) as a clock, not a solver.

Related Skills

Prerequisites

• bevy-master - ECS ownership and the fixed-timestep schedule names.

Complements

• bevy-physics-3d - Simulated 3D bodies and solver choice.
• bevy-character-movement - Player locomotion that should not be a rigid body.
• bevy-spatial-queries - Casts and overlaps. Choose a physics solver before adding them.
• godot-2d-physics - Equivalent Godot skill.

Downstream / consumers

• Combat and shooter skills cover hit detection and shots.

Master

• bevy-master - Bevy router.