Bevy Animation Graph Agent Skill

Bevy animation graph

When to use

Load this when clips must blend, add, or mask. Single-clip playback stays on bevy-animation.

ECS shape

• Assets: AnimationGraph describes how clips blend. add_clip parents a clip node. add_blend parents a blend node. add_clip_with_mask stores a u64 mask (bone groups you assign; 0 means no mask).
• Components: AnimationGraphHandle is a handle to the graph used by the AnimationPlayer on the same entity. AnimationTransitions crossfades between nodes.
• Weights: ActiveAnimation::set_weight on the player, after play, drives a node that is already in the graph.
• Systems: evaluation still runs through the animation player systems. Do not add a second player.

#[derive(Resource)]
struct LocomotionNodes {
    graph: Handle<AnimationGraph>,
    walk: AnimationNodeIndex,
    run: AnimationNodeIndex,
}

fn build_locomotion_graph(
    mut commands: Commands,
    asset_server: Res<AssetServer>,
    mut graphs: ResMut<Assets<AnimationGraph>>,
) {
    let mut graph = AnimationGraph::new();
    let blend = graph.add_blend(0.5, graph.root);
    let walk = graph.add_clip(
        asset_server.load(GltfAssetLabel::Animation(0).from_asset("models/fox.glb")),
        1.0,
        blend,
    );
    let run = graph.add_clip_with_mask(
        asset_server.load(GltfAssetLabel::Animation(1).from_asset("models/fox.glb")),
        0b0011,
        1.0,
        blend,
    );
    let handle = graphs.add(graph);
    commands.insert_resource(LocomotionNodes {
        graph: handle,
        walk,
        run,
    });
}

#[derive(Resource)]
struct LocomotionNodes {
    graph: Handle<AnimationGraph>,
    walk: AnimationNodeIndex,
    run: AnimationNodeIndex,
}

fn attach_player(
    mut commands: Commands,
    nodes: Res<LocomotionNodes>,
    mut players: Query<(Entity, &mut AnimationPlayer), Without<AnimationGraphHandle>>,
) {
    for (entity, mut player) in &mut players {
        commands.entity(entity).insert((
            AnimationGraphHandle(nodes.graph.clone()),
            AnimationTransitions::new(),
        ));
        player.play(nodes.walk).repeat();
        player.play(nodes.run).repeat();
    }
}

#[derive(Resource)]
struct LocomotionNodes {
    graph: Handle<AnimationGraph>,
    walk: AnimationNodeIndex,
    run: AnimationNodeIndex,
}

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

fn sync_blend(mut players: Query<(&mut AnimationPlayer, &RunBlend)>, nodes: Res<LocomotionNodes>) {
    for (mut player, blend) in &mut players {
        if let Some(active) = player.animation_mut(nodes.run) {
            active.set_weight(blend.0.clamp(0.0, 1.0));
        }
        if let Some(active) = player.animation_mut(nodes.walk) {
            active.set_weight(1.0 - blend.0.clamp(0.0, 1.0));
        }
    }
}

use std::time::Duration;

#[derive(Resource)]
struct LocomotionNodes {
    graph: Handle<AnimationGraph>,
    walk: AnimationNodeIndex,
    run: AnimationNodeIndex,
}

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

fn fade_to_run(
    nodes: Res<LocomotionNodes>,
    mut query: Query<(&mut AnimationPlayer, &mut AnimationTransitions, &RunBlend)>,
) {
    for (mut player, mut transitions, blend) in &mut query {
        if blend.0 > 0.5 {
            transitions.play(&mut player, nodes.run, Duration::from_millis(180));
        }
    }
}

[dependencies]
bevy = "0.20"

Graph choices

| Need | Bevy approach |

| --- | --- |

| Blend clips | AnimationGraph plus AnimationGraphHandle |

| Gameplay mode | App or entity states stay on bevy-state-machines. The graph is not that state type. |

| Additive layer | AnimationGraph::add_additive_blend inserts AnimationNodeType::Add. Clip weights still go through ActiveAnimation::set_weight. |

Scope

• Crossfades use AnimationTransitions::play with a Duration, as in fade_to_run.
• Persisting a graph is SerializedAnimationGraph plus ron. A 1D or 2D blend-space type is not part of AnimationGraph; blend with node weights.

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 AnimationTree, BlendSpace, or scene-tree state-machine types in Bevy code. WHY: AnimationGraph, AnimationGraphHandle, and AnimationTransitions are the documented Bevy pieces.
• No invented APIs or unnamed third-party crates as the chosen solver. 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

• Assets - Official asset catalog. No third-party crate was selected.
• Examples - Example tree on the latest branch.
• ECS - Component, systems, Query, Startup, Update, and .chain().
• Plugins - Plugin, DefaultPlugins, MinimalPlugins, and add_plugins.

Related Skills

Prerequisites

• bevy-master - Hub and schedules.
• bevy-ecs-composition - Components and systems.
• bevy-animation - Player and clips the graph drives.

Complements

• bevy-state-machines - Gameplay mode is not an animation graph.
• godot-animation-tree-mastery - Same topic on the Godot track.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.