Changing a Material

2023-10-06

When you load a scene from a glTF and spawn it multiple times, the material handles in each instance of the scene point to a shared material asset.

If you want to modify the material properties of only one of these instances, you can clone it, modify it, and add it to the asset server.

Bevy 0.15

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, (setup_scene, setup))
        .add_observer(decorate_red_fox)
        .run();
}

/// A marker component that we will add to the particular fox that we want to be red.
#[derive(Component)]
struct RedFox;

fn decorate_red_fox(
    trigger: Trigger<SceneInstanceReady>,
    instances: Query<&SceneInstance, With<RedFox>>,
    spawner: Res<SceneSpawner>,
    mut material_handles: Query<&mut MeshMaterial3d<StandardMaterial>>,
    mut material_assets: ResMut<Assets<StandardMaterial>>,
) {
    let Ok(instance) = instances.get(trigger.entity()) else {
        return;
    };

    for instance_entity in spawner.iter_instance_entities(**instance) {
        let Ok(mut original_material_handle) =
            material_handles.get_mut(instance_entity)
        else {
            continue;
        };

        let Some(original_material_asset) =
            material_assets.get(&*original_material_handle)
        else {
            continue;
        };

        // This creates a new material asset every time this code runs, so
        // potentially multiple new distinct material assets for every `RedFox`
        // you spawn.
        //
        // This can be a performance concern, so you may want to find a way to
        // reuse these materials.

        let mut new_material = original_material_asset.clone();
        new_material.base_color = basic::RED.into();

        *original_material_handle = material_assets.add(new_material).into();
    }
}

fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
    let fox_handle = asset_server.load("models/animated/Fox.glb#Scene0");

    commands.spawn((
        SceneRoot(fox_handle.clone()),
        Transform::from_scale(Vec3::splat(0.02)).with_translation(Vec3::NEG_X),
        RedFox,
    ));

    commands.spawn((
        SceneRoot(fox_handle),
        Transform::from_scale(Vec3::splat(0.02)).with_translation(Vec3::X),
    ));
}